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Le dépistage prénatal de la trisomie 21 a beaucoup évolué ces dernières années avec l’arrivée des tests de dépistage prénatal non invasif par analyse de l’ADN libre circulant dans le sang maternel. Si l’intérêt de ces tests pour le dépistage des trisomies communes, impliquant les chromosomes 13, 18 et 21, n’est plus à remettre en question aujourd’hui, l’évolution de l’étendue des anomalies détectables ainsi que les contraintes liées à l’origine de l’ADN analysé requièrent deux exigences : une bonne connaissance des limites du test pour en éviter les écueils ; un travail collégial en amont pour encadrer au mieux les pratiques, dans l’intérêt des femmes enceintes. Prenatal screening for trisomy 21 has considerably evolved in recent years thanks to the advent of non-invasive prenatal screening tests based on cell free DNA analysis. Today, the interest of these tests for the screening of common trisomies, involving chromosomes 13, 18 and 21, is no longer to be questioned. However, the technical and analytical developments allowing for the detection of additional chromosomal aberrations along with the constraints due to the origin of the analyzed DNA require, first, a good knowledge of the limits of the test in order to avoid its pitfalls, and second, collective discussions to elaborate best practice recommendations in the interest of pregnant women.

Overgrowth syndromes are a large group of rare disorders characterized by generalized or segmental excessive growth. Segmental overgrowth syndromes are mainly due to genetic anomalies appearing during the embryogenesis and leading to mosaicism. The numbers of patients with segmental overgrowth with an identified molecular defect has dramatically increased following the recent advances in molecular genetic using next-generation sequencing approaches. This review discusses various syndromes and pathways involved in segmental overgrowth syndromes and presents actual and future therapeutic strategies.Les syndromes de surcroissance segmentaire et les stratégies thérapeutiques.Les syndromes de surcroissance sont un groupe de pathologies caractérisées par une croissance excessive généralisée ou segmentaire. Les syndromes de surcroissance segmentaires sont principalement dus à des anomalies génétiques apparaissant durant l’embryogenèse et aboutissant à un mosaïcisme. Le nombre de patients atteints d’un syndrome de surcroissance avec une mutation identifiée a fortement augmenté grâce à des avancées récentes en génétique moléculaire, en utilisant le séquençage de nouvelle génération (NGS). Cette revue détaille les différents syndromes de surcroissance segmentaire ainsi que les voies moléculaires impliquées et les options thérapeutiques envisageables.

Many types of genodermatosis exist, with numerous modes of transmission. The development of molecular genetic methods, in particular the most recent sequencing techniques, can be used to identify an increasing number of genes involved in these forms of genodermatosis while providing confirmation or more details regarding clinical diagnosis. Thanks to this approach, it is possible to determine risk of recurrence and to formulate an antenatal strategy. These technologies have led to improved molecular definition and to a better understanding of the physiopathological mechanisms involved in different genodermatoses such as bullous epidermolysis, keratinisation disorders, pigmentation disorders, potentially tumoral conditions, and epidermal and pilar dysplasia. The large amount of information provided by high-throughput sequencing makes it possible to study modifying genes as well as genotype-phenotype correlations. However, this genetic information in its turn poses problems of interpretation and of control of the resulting data. The use of genetics in dermatology for the purposes of diagnosis or research requires a consultation to provide patients with information regarding the genetic tests involved and the potential consequences thereof for them and their families. Furthermore, with pangenomic approaches there is a higher probability of fortuitous discovery of abnormalities such as variants associated with risks predisposing to cancer or neurodegenerative disease. Collaboration between dermatologists and geneticists enables optimisation of patient management in terms of diagnosis and genetic counselling in the event of such rare diseases. Therapeutic applications are beginning to be developed. The scope of therapeutic application includes gene therapy, replacement therapy (enzyme therapy) and targeted therapy.

Fragile X syndrome is the most usual cause of hereditary intellectual deficiency. Typical symptoms combine intellectual deficiency, social anxiety, intense emotional vigilance, and a characteristic facial dysmorphy. This is subsequent to a complete mutation of the FMR1 gene, considering a semidominant transmission linked to the unstable X. The expansion of the CGG triplet greater than 200 units combined with a high methylation pattern lead to a transcriptional silence of the FMR1 gene, and the protein product, the FMRP, is not synthesized. This protein is involved in synaptic plasticity. Brain MRI can show an increased volume of the caudate nucleus and hippocampus, combined with hypoplasia of the cerebellar vermis. Fragile X Associated Tremor Ataxia Syndrome (FXTAS) syndrome is a neurodegenerative disorder occurring in carriers of the premutation in FMR1. Brain MRI shows an increased T2 signal in the middle cerebellar peduncles. This syndrome is linked to a premutation in the FMR1 gene. We report here the case of two brothers presenting a typical fragile X symptomatology. Brain MRI showed hyperintensities of the middle cerebellar peduncles. Such MRI findings support the assumption of a genetic mosaicism.

Neurofibromatosis type 2 (NF2) is a rare dominantly inherited disease. Its clinical presentation can be completely different in children and adults and early diagnosis is often difficult. The NF2 gene molecular analysis can help for diagnosis, but its result can be negative in case of NF2 mosaicism.We report the case of a 43-year-old man who had developed a severe phenotype with bilateral vestibular schwannomas at 19 years of age. His son presented a retinal hamartoma with loss of vision in his right eye at 2 months of age. At 9 years of age, asymptomatic schwannomas of the cranial nerves were discovered: cranial nerves X (left), XI (left), and VIII (bilateral). Partial constitutional NF2 deletion (from exons 2-7) was detected in his son. The deletion was not detectable in the DNA blood of his father and we strongly suspect a mosaic form of NF2.Ophthalmological manifestations can be the initial sign of NF2 in childhood. These features must be actively sought during the first year of life in individuals at risk of NF2. NF2 mosaicism is often described as a mild form of NF2 with a very low risk of transmission to the carrier's children. We show that NF2 mosaicism can sometimes develop severe NF2 symptoms and we confirm that the transmission risk to the offspring depends on the proportion of zygotes carrying the mutation. NF2 remains a life-limiting and life-spoiling condition. Early diagnosis is necessary to prevent complications and the follow-up of NF2 patients must be organized throughout life in specialty centers.

Chromosomal abnormalities are common in patients with oligozoospermia or azoospermia. We report the case of a 32-year patient, with male phenotype, and without hormonal or morphological abnormalities, with a severely reduced spermatogenesis. It was revealed a 45,X/46,XY gonadal dysgenesis. We have reviewed the various problems inherent in the discovery of this rare gonadal dysgenesis, including genetic, cancer and fertility risks.

Retinoblastoma is the most common intraocular childhood cancer and occurs when both alleles of the RB1 gene are inactivated in the retina. In patients without genetic predisposition, the two mutations occurred in a single unique retinal cell. In subjects with a genetic predisposition to retinoblastoma, the first RB1 mutation is found in the germline and the second appears as a somatic mutation. Germline carriers usually develop bilateral or multifocal tumors and the diagnosis is earlier. However, some rare families exhibit low penetrance and variable expressivity of the disease because bilaterally affected, unilaterally affected, and unaffected mutation carriers are known to coexist. The existence of genetic modifiers in retinoblastoma therefore appears highly probable and must be considered. The lack of penetrance and the variable expressivity could be the sum of three independent causes. The presence of a mosaic can affect the phenotype, the nature of the mutation can drive low penetrance and particular phenotype like psychomotor delay in case of large genomic deletions and genetic modifier factors could modulate the phenotype. These three major keys have been studied in order to highlight the relations between the phenotype and the genotype. Firstly, the consequences of mosaicism have been illustrated by a prenatal diagnosis concerning a couple with a bilateral retinoblastoma-affected male patient who requested five successive prenatal diagnoses and in whom RB1 mutation mosaicism had important implications. Implications of mosaicism in genetic counseling have been discussed and taken into consideration in order to limit bias in the two following genotype/phenotype studies. Secondly, the association between whole germline monoallelic deletions of the RB1 gene and psychomotor delay was studied by a high-resolution CGH array focusing on RB1 and its flanking region. Comparative analysis detected a four megabase critical interval including a candidate gene, protocadherin 8 (PCDH8). PCDH8 is thought to function in signaling pathways and cell adhesion in a central nervous system-specific manner, making loss of PCDH8 one of the probable causes of psychomotor delay in RB1-deleted patients. Thirdly, a candidate gene approach based on partners that are necessary for the development of the tumor attempted to find possible genetic modifiers. MDM2, which increases p53 and pRB catabolism, was therefore a prominent candidate. The minor allele of MDM2 that includes a 309T>G transversion (SNP rs2279744) in the MDM2 promoter is known to enhance MDM2 expression. In family-based association analyses performed in 70 retinoblastoma families, the MDM2 309G allele was found to be statistically significantly associated with incidence of bilateral or unilateral retinoblastoma among members of retinoblastoma families under a recessive model (Z = 3.305, two-sided exact P = .001). The strong association of this genotype with retinoblastoma development designates MDM2 as the first modifier gene to be identified among retinoblastoma patients; Le rétinoblastome est une tumeur rare qui touche la rétine du jeune enfant. L’inactivation bi-allélique du gène RB1 est à l’origine du développement tumoral. RB1 est le premier gène suppresseur de tumeur qui ait été identifié et la prédisposition au rétinoblastome constitue un véritable paradigme de la prédisposition aux cancers. Dans les formes non prédisposées génétiquement, les deux mutations apparaissent dans une cellule rétinienne unique ; le rétinoblastome est alors unilatéral. Dans les formes à prédisposition génétique, la première mutation est constitutionnelle et la deuxième est somatique. La mutation constitutionnelle est une néo-mutation pré- ou post- zygotique dans les formes sporadiques, alors qu’elle est héritée dans les formes familiales. Dans les formes avec prédisposition génétique, le diagnostic est plus précoce que dans les formes sans prédisposition et la bilatérisation du rétinoblastome est généralement la règle. Néanmoins, de rares familles présentent une pénétrance réduite et une variabilité phénotypique se traduisant par la coexistence de patients atteints de rétinoblastome bilatéral ou unilatéral, d’apparentés porteurs sains et d’apparentés présentant des rétinomes. Les mécanismes responsables de la variabilité phénotypique intrafamiliale sont inconnus et l’existence de facteurs génétiques modulant le phénotype tumoral est probable.L’origine de la variabilité de l’expression phénotypique du rétinoblastome peut être la résultante (i) de l’existence de mutations en mosaïque, (ii) de mutations de RB1 et (iii) de facteurs modificateurs génétiques indépendants du locus de RB1. Trois axes distincts et originaux basés sur ces origines possibles de variabilité phénotypique ont été développés pour caractériser les relations génotype/phénotype dans le rétinoblastome. Premièrement, les conséquences d’une mosaïque somatique ont été illustrées grâce à l’étude d’une famille ayant bénéficié de cinq diagnostics prénatals. Dans ces familles, certains fœtus porteurs de l’allèle à risque identifié par une approche indirecte basée sur l’étude de microsatellites au locus de RB1, n’étaient pas porteurs de la mutation du parent atteint, lui-même atteint d’un rétinoblastome bilatéral. Ainsi, nous avons démontré la présence d’une mosaïque somatique et gonadique chez ce parent lourdement atteint. La conséquence de l’existence de patients présentant une mosaïque dans le cadre du conseil génétique a été discutée. La suite de nos travaux a pris en compte ces résultats afin de limiter les biais que pourrait induire la présence de mutations en mosaïque dans des études de corrélation génotype/phénotype dans le rétinoblastome. Deuxièmement, l’association de grandes délétions emportant RB1 avec des retards psychomoteurs chez des patients atteints de rétinoblastome a été étudiée. Une approche de CGH hautement résolutive, ciblée sur le locus de RB1, a été mise en place afin de caractériser le rôle des gènes contigus de RB1 dans ce syndrome. Ainsi, cette approche a permis de définir une zone à risque de retard psychomoteur que nous proposons comme seuil d’alerte pour le généticien. Cette zone définit un gène, PCDH8 d’expression cérébrale exclusive, comme un excellent candidat au retard psychomoteur. Enfin, troisièmement, une approche « gène candidat » reposant sur l’étude du SNP309 du promoteur de MDM2, a été mise en œuvre.

Retinoblastoma is the most common intraocular childhood cancer and occurs when both alleles of the RB1 gene are inactivated in the retina. In patients without genetic predisposition, the two mutations occurred in a single unique retinal cell. In subjects with a genetic predisposition to retinoblastoma, the first RB1 mutation is found in the germline and the second appears as a somatic mutation. Germline carriers usually develop bilateral or multifocal tumors and the diagnosis is earlier. However, some rare families exhibit low penetrance and variable expressivity of the disease because bilaterally affected, unilaterally affected, and unaffected mutation carriers are known to coexist. The existence of genetic modifiers in retinoblastoma therefore appears highly probable and must be considered. The lack of penetrance and the variable expressivity could be the sum of three independent causes. The presence of a mosaic can affect the phenotype, the nature of the mutation can drive low penetrance and particular phenotype like psychomotor delay in case of large genomic deletions and genetic modifier factors could modulate the phenotype. These three major keys have been studied in order to highlight the relations between the phenotype and the genotype. Firstly, the consequences of mosaicism have been illustrated by a prenatal diagnosis concerning a couple with a bilateral retinoblastoma-affected male patient who requested five successive prenatal diagnoses and in whom RB1 mutation mosaicism had important implications. Implications of mosaicism in genetic counseling have been discussed and taken into consideration in order to limit bias in the two following genotype/phenotype studies. Secondly, the association between whole germline monoallelic deletions of the RB1 gene and psychomotor delay was studied by a high-resolution CGH array focusing on RB1 and its flanking region. Comparative analysis detected a four megabase critical interval including a candidate gene, protocadherin 8 (PCDH8). PCDH8 is thought to function in signaling pathways and cell adhesion in a central nervous system-specific manner, making loss of PCDH8 one of the probable causes of psychomotor delay in RB1-deleted patients. Thirdly, a candidate gene approach based on partners that are necessary for the development of the tumor attempted to find possible genetic modifiers. MDM2, which increases p53 and pRB catabolism, was therefore a prominent candidate. The minor allele of MDM2 that includes a 309T>G transversion (SNP rs2279744) in the MDM2 promoter is known to enhance MDM2 expression. In family-based association analyses performed in 70 retinoblastoma families, the MDM2 309G allele was found to be statistically significantly associated with incidence of bilateral or unilateral retinoblastoma among members of retinoblastoma families under a recessive model (Z = 3.305, two-sided exact P = .001). The strong association of this genotype with retinoblastoma development designates MDM2 as the first modifier gene to be identified among retinoblastoma patients; Le rétinoblastome est une tumeur rare qui touche la rétine du jeune enfant. L’inactivation bi-allélique du gène RB1 est à l’origine du développement tumoral. RB1 est le premier gène suppresseur de tumeur qui ait été identifié et la prédisposition au rétinoblastome constitue un véritable paradigme de la prédisposition aux cancers. Dans les formes non prédisposées génétiquement, les deux mutations apparaissent dans une cellule rétinienne unique ; le rétinoblastome est alors unilatéral. Dans les formes à prédisposition génétique, la première mutation est constitutionnelle et la deuxième est somatique. La mutation constitutionnelle est une néo-mutation pré- ou post- zygotique dans les formes sporadiques, alors qu’elle est héritée dans les formes familiales. Dans les formes avec prédisposition génétique, le diagnostic est plus précoce que dans les formes sans prédisposition et la bilatérisation du rétinoblastome est généralement la règle. Néanmoins, de rares familles présentent une pénétrance réduite et une variabilité phénotypique se traduisant par la coexistence de patients atteints de rétinoblastome bilatéral ou unilatéral, d’apparentés porteurs sains et d’apparentés présentant des rétinomes. Les mécanismes responsables de la variabilité phénotypique intrafamiliale sont inconnus et l’existence de facteurs génétiques modulant le phénotype tumoral est probable.L’origine de la variabilité de l’expression phénotypique du rétinoblastome peut être la résultante (i) de l’existence de mutations en mosaïque, (ii) de mutations de RB1 et (iii) de facteurs modificateurs génétiques indépendants du locus de RB1. Trois axes distincts et originaux basés sur ces origines possibles de variabilité phénotypique ont été développés pour caractériser les relations génotype/phénotype dans le rétinoblastome. Premièrement, les conséquences d’une mosaïque somatique ont été illustrées grâce à l’étude d’une famille ayant bénéficié de cinq diagnostics prénatals. Dans ces familles, certains fœtus porteurs de l’allèle à risque identifié par une approche indirecte basée sur l’étude de microsatellites au locus de RB1, n’étaient pas porteurs de la mutation du parent atteint, lui-même atteint d’un rétinoblastome bilatéral. Ainsi, nous avons démontré la présence d’une mosaïque somatique et gonadique chez ce parent lourdement atteint. La conséquence de l’existence de patients présentant une mosaïque dans le cadre du conseil génétique a été discutée. La suite de nos travaux a pris en compte ces résultats afin de limiter les biais que pourrait induire la présence de mutations en mosaïque dans des études de corrélation génotype/phénotype dans le rétinoblastome. Deuxièmement, l’association de grandes délétions emportant RB1 avec des retards psychomoteurs chez des patients atteints de rétinoblastome a été étudiée. Une approche de CGH hautement résolutive, ciblée sur le locus de RB1, a été mise en place afin de caractériser le rôle des gènes contigus de RB1 dans ce syndrome. Ainsi, cette approche a permis de définir une zone à risque de retard psychomoteur que nous proposons comme seuil d’alerte pour le généticien. Cette zone définit un gène, PCDH8 d’expression cérébrale exclusive, comme un excellent candidat au retard psychomoteur. Enfin, troisièmement, une approche « gène candidat » reposant sur l’étude du SNP309 du promoteur de MDM2, a été mise en œuvre.

Fanconi anemia (FA) is a rare genetic disease, transmitted in an autosomal recessive mode. The clinical phenotype is very broad and heterogeneous, related to the wide range of genes involved in this pathology. The classical triad of short height, physical abnormalities, and bone marrow failure is suggestive. The main physical abnormalities found involve the limbs, spinal column, skin, kidneys and urinary tract, and the ORL zone. Recent progress in molecular biology has identified 15 genes whose mutation causes FA chromosomal instability. FA is diagnosed by cytogenetic examination, then specified by molecular analysis. As FA patients may present multiorgan abnormalities and a high risk for neoplasia development, their medical follow-up has to be multidisciplinary and prolonged throughout life. The main challenges of the follow-up are patient information and education. Bone marrow failure, appearing during the first decade, requires close hematological monitoring and for severe cases requires hematopoietic stem cell transplantation, major and specific care with frequent serious complications and high mortality, but this is the only curative treatment in FA. Extrahematological care consists in screening for organ abnormalities and defects as well as monitoring precancerous lesions and tumors.

Le développement sexuel est un processus complexe qui dépend de nombreux gènes, une mutation pouvant entraîner un développement sexuel anormal. Par ailleurs, des anomalies chromosomiques peuvent avoir des répercussions importantes sur la détermination gonadique, surtout lorsqu'il s'agit du chromosome Y puisqu'il porte le gène clé du développement sexuel masculin. Premièrement, nous avons identifié par cytogénétique moléculaire le point de cassure chez 5 patients avec une translocation X;Y et 10 patients avec un chromosome Y isodicentrique. Nous avons ainsi démontré que certaines régions sont plus à risque d'être remaniées, notamment lorsqu'elles contiennent des palindromes ou d'autres séquences répétées. Nous avons également établi une relation entre la distance séparant le centromère et le point de cassure et l'instabilité des chromosomes Y isodicentriques lors des divisions cellulaires. Deuxièmement, nous avons étudié en cytogénétique les gonades de 22 patients avec un chromosome Y normal ou remanié et présentant un développement sexuel anormal. Nous avons mis en évidence la perte du chromosome Y remanié dans une majorité de cellules gonadiques des 10 patients étudiés, expliquant leur phénotype sexuel anormal. Cependant, chez 11 des 12 patients avec un chromosome Y normal, aucun mosaïcisme expliquant clairement leur détermination gonadique anormale n'a été retrouvé. Finalement, nous avons analysé par immunohistochimie les gonades dysgénésiques de 30 patients avec une anomalie du développement sexuel et un chromosome Y normal ou remanié. Nos travaux ont montré la présence de cellules germinales immatures au sein de cordons sexuels primitifs sous forme de tissu gonadique indifférencié dans 15 gonades, dont 9 ont évolué en tumeur gonadique. Dans 13 autres gonades, ces cellules germinales immatures avaient disparues par apoptose. Dans l'ensemble, notre recherche met en évidence la susceptibilité du chromosome Y à subir des remaniements et à être instable lors des divisions cellulaires, et indique que le mosaïcisme peut avoir des répercussions sur la détermination gonadique. Nos travaux montrent également que le tissu gonadique indifférencié peut évoluer vers deux entités, une tumeur gonadique ou une bandelette suite à l'apoptose des cellules germinales, mettant en lumière la nécessité d'analyser le tissu gonadique des patients XY avec dysgénésie gonadique dont les gonades sont laissées en place., Sexual development is a complex process which depends on numerous genes, mutations possibly resulting in an abnormal sexual development. Furthermore, chromosome abnormalities can have important repercussions on gonadal determination, especially when it comes to the Y chromosome since it carries the master gene of male sexual development. First, we identified by molecular cytogenetics the breakpoint in 5 patients with an X;Y translocation and 10 patients with an isodicentric Y chromosome. We were thus able to show that some regions are more at risk of being rearranged, especially when they contain palindromes or other repeated sequences. We were also able to establish a relationship between the distance separating the centromere from the breakpoint and instability of isodicentric Y chromosomes during cell divisions. Second, we studied by cytogenetics the gonads of patients with a normal or rearranged Y chromosome and presenting an abnormal sexual development. We demonstrated loss of the rearranged Y chromosome in a majority of gonadal cells of the 10 analyzed patients, explaining their abnormal sexual phenotype. On the other hand, in 11 of the 12 patients with a normal Y chromosome, no mosaicism clearly explaining their abnormal gonadal determination was found. Finally, we also analyzed by immunohistochemistry the dysgenetic gonads of 30 patients with an abnormal sexual developement and a normal or rearranged Y chromosome. We showed the presence of immature germ cells in primitive sex cords as undifferentiated gonadal tissue in 15 gonads, including 9 that evolved in a gonadal tumor. In 13 other gonads, these immature germ cells had disappeared through apoptosis. Altogether, our research demonstrates that the Y chromosome is susceptible to rearrangements and can be unstable through cell divisions, and that mosaicism may have repercussions on gonadal determination. Our work also shows that undifferentiated gonadal tissue can evolve in two entities, a gonadal tumor or a streak following apoptosis of germ cells, thus emphasizing the necessity of studying the gonads of XY patients with gonadal dysgenesis when gonads are left in place.

We report three cases of a peculiar rash with mixed clinical features of both epidermal nevus and acanthosis nigricans. Their characteristics have been compared to those of very rare but similar cases found in the medical literature.Two young adults (one male, one female) and a 7-year-old boy consulted for hyperchromic asymptomatic plaques located respectively on the right scapula, the left upper arm and the right frontotemporal area of the face. None had any noticeable familial medical history. None were overweight or had a personal history of diabetes or endocrine dysfunction. The plaques had appeared soon after puberty in the adults and at the age of 4 years in the child and they had remained stable for years. They were polycyclic in shape, sharply demarcated, and with a linear distribution. Their surface was slightly papular, with a velvety appearance and texture. No other skin or mucous membrane lesions were observed elsewhere. The physical examination was otherwise normal. Skin biopsy specimens showed mild acanthosis with papillomatosis, hyperorthokeratosis with elongated rete ridges and slight thinning of the roof of the dermal papilla.The striking clinical features of this rash rule out confusion with any other epidermal nevus (because of age of onset, shape and texture) or with acanthosis nigricans (because of its topography and the lack of associated neoplastic or endocrine disease). A dozen similar case reports can be found in the medical literature, mostly under the term "nevoid acanthosis nigricans". Such a rash may be linked to postzygotic mosaicism with the same mutations of the FGFR3 gene. However, since the physiopathology of this rash remains hypothetical, we propose to name it "RAVEN", for "rounded and velvety epidermal nevus", which is simply a descriptive acronym and nothing more.

Although specifically male, the human Y chromosome may be observed in female karyotypes, mostly in women with Turner syndrome stigmata. In women with isolated gonadal dysgenesis but otherwise normal stature, the testis determining factor or SRY gene may have been removed from the Y chromosome or may be mutated. In other women with Turner syndrome, the karyotype is usually abnormal and shows a frequent 45,X/46,XY mosaicism. In these cases, the phenotype depends on the ratio between Y positive and 45,X cell lines in the body. When in mosaicism, Y chromosomes are likely to carry structural abnormalities which explain mitotic instability, such as the existence of two centromeres. Dicentric Y isochromosomes for the short arm (idic[Yp]) or ring Y chromosomes (r[Y]) are the most frequent abnormal Y chromosomes found in infertile patients and in Turner syndrome in mosaic with 45,X cells. Although monocentric, deleted Y chromosomes for the long arm and those carrying microdeletions in the AZF region are also instable and are frequently associated with a 45,X cell line. Management of infertile patients carrying such abnormal Y chromosomes must take into account the risk and the consequences of a mosaicism in the offspring.

Turner's syndrome (TS) affects about 1/2500 female infants born alive. The syndrome results from total or partial absence of one of the two X chromosomes normally present in females. We report the results of a retrospective analysis of 89 cases of TS observed during a six-year period (2000-2005). The patients' age ranged from two days to 51 years at the time of this analysis. Most patients were adults (48%). The aim of this study is to ascertain the principal clinical features leading to a request for a karyotype, searching for a possible relationship between chromosomal anomalies and clinical expression of TS. Pediatric patients were referred for statural retardation or dysmorphic features, while reproduction anomalies were the main indication for karyotyping in patients aged over 20 years. Mosaicism was prevalent (47%), whereas the homogeneous karyotype 45,X was found in only 32% of the patients; structural anomalies were found in 21%. Regarding the advanced age of our patients, we established a relationship between chromosome anomalies and the clinical expression of TS, based on an analysis of stature and reproduction disorders. Short stature and primary amenorrhea were correlated with total deletion of one chromosome X or imbalanced gene dosage due to structural X anomalies. Whereas cases of infertility, recurrent miscarriages and secondary amenorrhea were associated with a mosaic karyotype pattern (45,X/46,XX or 45,X/46,XX/47,XXX ...), with a slight mosaicism in most cases. Thus, chromosome investigations should be performed in cases of reproduction failure even for women with normal stature.

Bietti's crystalline corneoretinal dystrophy is a tapetoretinal degeneration, characterized by the presence of refringent crystals in the corneal limbus and the retina with sclerosis of choroidal vessels. We report the clinical and angiographic features of an 8-year-old girl affected with Bietti's crystalline dystrophy.This 8-year-old girl was a sporadic case, born of consanguineous parents. She was referred to our hospital for intermittent strabismus. Her visual acuity was 4/10 at the right eye and 3/10 at the left eye. Biomicroscopy revealed very fine crystals in the limbal area bilaterally. Mydriatic funduscopic examination showed bilateral macular pigment mottling and depigmentation, numerous tiny refractile yellow dots scattered throughout the posterior pole and the mid-periphery associated with diffuse retinal pigment epithelial atrophy and pigment accumulation. Fluorescein angiography revealed retinal pigmentary epithelium alterations. Indocyanine green (Infracyanin) angiography showed areas of choroidal atrophy. The electroretinogram noted a reduction in the number of both types of photoreceptors.Ophthalmological lesions normally occur between 20 and 30 years of age. The particularity of our case report is the manifestation of the disease at an earlier age (8 years). The progression is characterized by a centrifuge expansion of lesions.

We report the case of a newborn presenting an agenesis of corpus callosum (ACC) discovered in the prenatal period and initially related to cocaine exposure during the first trimester of gestation. The cytogenetic analysis revealed a trisomy 8 mosaicism. The putative role of prenatal cocaine exposure and mosaicism for chromosome 8 in ACC are discussed. This report emphasizes the specific analysis of chromosome 8 by using fluorescence in situ hybridization as a complement to routine cytogenetic analysis for prenatal diagnosis of ACC.

The identification of mutations leading to human genetic diseases has grown into an intensive research field during the last few years. Through novel DNA analysis progress, it is now possible to determine the mutational spectrum for a given genetic disease and international databases are now available online. Genetic diagnosis of hereditary diseases has become an essential tool in genetic counselling and prenatal diagnosis. The knowledge of the deleterious mutation type and the molecular associated mechanism is fundamental in order to devise the optimal molecular diagnosis strategy. This review aims to present the various mutation categories involved in genetic diseases (single base-pair substitutions, small deletions or insertions, dynamic mutations, gross DNA lesions...) and to summarize our current knowledge about the main molecular mechanisms responsible for these mutations. Their deleterious consequences on gene expression, including transcription and transcript maturation, and protein loss or gain of function are also discussed in this review.

Ito hypomelanosis-type pigmentary mosaicism is characterized by congenital pigmentation disorders along Blaschko's lines. We report a case of Ito-type pigmentary mosaicism associated with a congenital growth hormone deficiency having revealed trisomy 20 mosaicism.A 4 year-old boy presented with congenital pigmentation disorders. His history was marked by: inter-uterine delayed growth of unknown etiology, a dysmorphic syndrome, psychomotor retardation with speech problems, right cryptorchidia and an isolated, idiopathic, congenital growth hormone deficiency that had been treated with recombinant somatropine since the age of three. The clinical examination revealed alternating hypo and hyper-pigmented maculae with linear distribution on the limbs and in "twirls" on the trunk following Blaschko's lines. The blood karyotype was normal, the karyotype on fibroblasts of hypopigmented skin revealed trisomy 20 mosaicism.The occurrence of pigmentary mosaicism related to trisomy 20 mosaicism is exceptional. The combination of Ito hypomelanosis-type pigmentary mosaicism and delayed growth due to growth hormone deficiency has never been reported before. Our observation, unusual because of such an association, raises the question of the eventual existence of associated genes located on the chromosome 20 implied in the secretion of growth hormone and/or melanogenesis. It also underlines the interest of conducting cytogenic explorations on fibroblasts of damaged skin in the case of Ito-type pigmentary mosaicism, even if the blood karyotype is normal or in the absence of a patent phenotype abnormality.

Darier's disease or keratosis follicularis is an autosomal dominant acantholytic disorder that frequently arises as a result of spontaneous mutation. It is either a generalized or localized condition due to a mutation in the SERCA2 12q23-q24,1 resulting in a faulty organization of the tonofilaments. We present two siblings affected with the linear form of this disorder and discuss these cases as an example of the genetic mechanism of loss of heterozygosity.A 7 year-old girl was referred for evaluation of linear lesions present since the first year of age. Examination disclosed red, 1 to 2 mm papules that coalesced to form linear plaques on the left side of the vulvar and perianal areas, and on the left hand and foot. Her older brother had similar lesions in a linear arrangement on the left side of the face neck and homolateral foot. No lesions were found in their parents. Biopsies of both affected children revealed an intraepidermal suprabasal cleft. Dyskeratotic cells were present in the spinous layer, and corps ronds and grains near the granular layer.The linear form of Darier's disease could result from genetic mosaicism for this autosomal dominant disorder. As these children have a more pronounced involvement than the usual Darier's disease lesions, disposed in a linear arrangement, they probably represent a type 2 segmental manifestation of the disorder. Likewise, the presence of the same linear disorder in two siblings could be explained by loss of heterozygosity for the Darier's disease gene.

Hypomelanosis of Ito was first described by Ito in 1952 as incontinentia pigmenti achromians. The consistent feature of the disease is a characteristic cutaneous hypopigmentation area following the lines of Blaschko, but the clinical manifestations are varied and hypomelanosis of Ito is regarded as a neurocutaneous syndrome. Hypomelanosis of Ito is sporadic but is probably a non-specific expression of chromosomal mosaicism; we report a case with four clones.We report the case of a 26 year-old woman with neurocutaneous hypomelanosis of Ito and Trisomy 13 mosaicism. She also exhibited skeletal and ophthalmologic disorders. Immunohistology revealed a PS100 and Melan A decrease in hypopigmented skin. Cytogenetic study of normal and hypopigmented skin fibroblasts showed mosaicism with four clones.This is the third case of Trisomy 13 mosaicism associated with hypomelanosis of Ito, although other anomalies on chromosome 13 have been described. Happle published "phylloid" pigmented cases, which are mainly associated with Trisomy 13. This is the first observation of four-clone mosaicism and can be explained by successive mutations during embryogenesis. Anomalies on chromosomes 5,6 and 21 have never been described. The definition of hypomelanosis of Ito is not well established and the disease is presently included in a group of "pigmentary dysplasia" with genetic mosaicism.

Turner's syndrome is a gonadosomatic dysgenesis of female phenotype due to a more or less complete monosomy of one of the X chromosomes leading to a haploinsufficiency of the development genes situated at the level of the pseudoautosomal region of the gonosomes. Further experience of the karyotype showed a preponderance of mosaics and considerable variability of Turner's phenotype in proportion to the number of 45, X cells. The rare cases of monozygotism discordant with variable tissular distribution mosaics show that the phenotypic expression is a genic dosage effect. In patients with TS it would thus be of interest to study a second tissue such skin fibroblasts when a discordance is observed between the phenotype and the karyotype.

In 1968, Rapp and Hodgkin described a family (a mother and her son and daughter) with a combination of dysplasic hair, nails, hypodontia and hypohidrosis associated with oral clefting and hypoplasia of the maxillary. Forty other cases of this congenital disorder have since been reported, and provide better knowledge of this predominantly hereditary disease.We examined a 4 year-old boy for who had suffered since birth from a crusting and inflammatory scalp dermatitis which evolved periodically. He also had a coarse, wiry, light colored and slow growing hair, onychodysplasia, skin atrophy areas on the trunk, and distinctive craniofacial anomalies including a bifid uvula. He had never been seen to sweat. His father presented similar features, but less pronounced, and with a segmentary distribution.We report two new cases of Rapp-Hodgkin's ectodermal dysplasia. We suspect a mosaicism in the father who may be the first and sporadic case of the family. These discret forms are frequently misdiagnosed and we hope we have provided some new elements which will help in the description of this rare disease. Early diagnosis permits an early follow-up and avoids numerous complications.

A case of osteogenesis imperfecta type II diagnosed in the first trimester of pregnancy.Lethal osteogenesis imperfecta is a disorder characterized by collagen abnormalities resulting in dwarfism, bone fragility and deformity leading to death in utero or in the perinatal period. Molecular and biochemical studies demonstrate that OI type II results from mutations in either COL1 A1 or COL1 A2 which encode for the chains of type I procollagen. Early diagnosis by US examination in first trimester relies on shortening and bowing of long bones, multiple fractures and hypoechogenicity of the skeleton. When ultrasound examination suspects OI type II, the diagnosis can be accomplished in the first trimester by biochemical analysis if the collagen defect is characterized.OI type II results mainly from private mutations and parental mosaicism is an important cause of recurrence making genetic counselling difficult.

Several studies have shown an association between the presence of systemic sclerosis in females and the presence of fetal cells in peripheral blood. These results have led to the hypothesis that systemic sclerosis (SSc) may indeed be the consequence of an allogeneic fetomaternal reaction. However, certain normal female controls also exhibit microchimerism. In addition, there are several clinical and histological differences between SSc and sclerodermoid graft versus host reaction. If microchimerism plays a role in SSc, it would therefore comprise one step in a multistep process. This review focuses on recent papers targeting microchimerism and its evaluation in SSc.

We report a case of Rothmund-Thomson syndrome associated with a trisomy 8 mosaicism, and RECQ4 gene mutation.An 18-year-old man presented with a poikiloderma affecting photoexposed areas and the buttocks. This lesions appeared during the first year of life and was secondly associated with alopecia, sparse body hair, keratosis, and warts. He also had proportional short stature, thumb and patella aplasia, particular facies, and plantar malformations. Cytogenetic studies evidenced chromosomal instability and trisomy 8 mosaicism. The DNA repair capacity was normal. A mutation in RECQ4 helicase gene was found.Rothmund-Thomson syndrome is a rare hereditary syndrome characterized by early onset of poikiloderma. Patients exhibit variable features including skeletal abnormalities, juvenile cataracts, photosensitivity, and a higher than expected incidence of cutaneous or extracutaneous malignancies. Genetic patterns found in Rothmund-Thomson syndrome are heterogeneous. Normal karyotypes have been demonstrated in many patients. Various karyotypic abnormalities or reduced DNA repair was seen in others. Recently, five patients with Rothmund-Thomson syndrome were shown to segregate for mutations in RECQ4 helicase gene. Thus, clinical and genetic features in Rothmund-Thomson syndrome are polymorphous. Therefore, it could be interesting to correlate genotype and phenotype.

Confined placental mosaicism (CPM) occurs in approximately 2 per cent of viable pregnancies studies by chorionic villus sampling. The great majority of pregnancies with confined placental mosaicism proceed uneventfully, resulting in normal liveborn infants. However, a few specific chromosomal abnormalities have been associated with fetal growth retardation. Fetal karyotype should be checked by amniocentesis and such pregnancies should have careful ultrasound follow-up.

A central part of Pediatric Neurology is currently dominated by the search for genetic factors involved in developmental disorders of the nervous system, including cases where the cytogenetic examination remains uncontributive. The prerequisite for a good definition of the malformative phenotypes leads to distinguish: 1 cerebral malformations that can be identified at the macroscopic scale, by imaging. 2 polymalformative syndromes including mental retardation where cerebral imaging is not contributive, thus the syndromatic definition is based on associated somatic anomalies. 3 Non-syndromatic mental retardation, where a genetic origin is clear only in the familial forms. Various methodological approaches have included genetic linkage studies, search for inframicroscopic chromosomal rearrangements in the critical region and investigation of candidate genes. A great number of syndromes have been connected with a great diversity of genetic mechanisms, whose many examples are presented: genopathies with regular or variable expression, unstable mutations, contiguous gene syndromes or other complex infracytogenetic rearrangements, chromosomal or genic mosaicisms, mutations submitted to parental imprinting. New methods of genomic screening will be necessary to progress in this field, given the great number of genes involved in cerebral development. As for the early developmental disorders of the PNS and muscle, their diagnosis becomes frequent during the intrauterine life, raising the problem of a better definition of the fetopathological phenotypes.

The XX males represent a proportion of 1/25 of all patients suffering of the Klinefelter syndrome. From a clinical and endocrinological point of view, they exhibit a hypogonadotropic hypogonadism. Isolated cases are rare and familial forms are exceptional. The XX males may be divided in 3 subgroups: 46, XX males with the SRY gene; 46, XX males without the SRY gene and XX/XY mosaics.

Germline mosaicism is now well known to account for recurrence of hereditary human disorders. Facioscapulohumeral muscular dystrophy is an autosomal dominant disorder; its locus has been identified in the telomeric region of chromosome 4 at the q35 band. It appears to have a high rate of mutation.A young girl had presented from childhood signs of a severe form of facioscapulohumeral muscular dystrophy, but with no familial history. The diagnosis was ultimately confirmed at the age of 23 years by molecular studies evidencing the deletion. The same abnormality was sparsely found in the child's father who appeared to harbor the mutation as a germline mosaicism with no clinical expression.This case illustrates the possibility of severe facioscapulohumeral muscular dystrophy and the dominant transmission of the disorder which may be clinically occult. It underlines the importance of molecular biology and the difficulties of genetic counselling.

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