Search

The objective of this study is to provide a detailed description of clinical characteristics of disorders associated with FMR1 gene, which is located on the long arm of chromosome X. The most frequent FMR1 mutations are related to CGG-repeat expansion in the promoter region: premutation (from 55 to 199), full mutation (more than 200 repeats). The first section of the article is devoted to the fragile X mental retardation syndrome (FX syndrome) caused by FMR1 full mutation. The clinical characteristics of FX syndrome are presented. The second section provides information about specific phenotypes associated with FMR1 premutation that can be observed in maternal relatives (grandmother, mother's siblings, grandfather) of the child. The most frequent symptoms that observed in permutation carriers are mild cognitive impairment, ASD, ADHD in children, fragile X-associated tremor/ataxia syndrome (FXTAS) in older carries, fragile X-associated primary ovarian insufficiency (FXPOI) in women. The last section provides information about screening diagnostic instruments that help to identify the risk of fragile X syndrome. It also presents the key questions to be asked to family members in order to identify the risk of the permutation.

Fragile X syndrome is one of the most common reasons for human hereditary mental retardation. It is associated with the expansion of CGG repeats in the 5'-untranslated region of the FMR1 gene, which results in the suppression of its expression and the development of the disease. At present, methods based on PCR and Southern blot analysis are used for diagnostics of the fragile X syndrome. The presence of a fragile site FRAXA on the X chromosome is typical for patients with this pathology. We developed a method of visualizing this site in cell cultures obtained from patients using the fluorescent in situ hybridization (FISH) and the combination of two probes. The method allows one to detect five types of signals on the X chromosome, three of which are normal, while two are associated with the emergence of fragile site FRAXA. An analysis of the distribution of all signal types in cell lines from healthy individuals and patients with fragile X syndrome demonstrated that the method allows one to determine differences between lines with a high statistical significance and that it is applicable to detecting cells that are carriers of the syndrome.

The purpose of this article is to provide the overview of visual cognitive development in subjects with FMRP deficit. Description of fragile X mental retardation syndrome is presented in the article, that is the most common cause of inherited intellectual disability and one of the most prevalent genetic causes of autism spectrum disorder. The syndrome is associated with deficit of fragile X mental retardation protein following FMR1-gene mutation. Researches of static and dynamic object perception, face perception and oculomotor control are discussed in the article. The results obtained by subjects with FX syndrome are compared with ASD data, syndrome with closed behavioral phenotype. Several factors that underlie visual cognitive deficit are discussed in the article.

Fragile X syndrome is inheritable neurodegenerative disease with frequency 1/4000-1/6000. It is the main cause of inheritable mental retardation. Progression of the disorder is caused by CGG repeat expansion in 5' UTR of fmr1 (fragile X mental retardation 1) gene. Normal allele contains ≤ 54 repeats. Allele containing 55-200 repeats induce fragile X-associated disorders: fragile X-associated tremor/ataxia syndrome and fragile-X associated primary ovarian insufficiency. Allele containing ≥ 200 repeats induce fragile X syndrome. Absence of FMRP protein is the main reason for the syndrome progression. FMRP is RNA-binding protein responsible for neuronal differentiation. In case of increasing CGG triplets number the expression of fmr1 gene is repressed. Results of CGG expansion are DNA methylation, histone methylation and deacetylation. Repression transcription factors bind such chromatin and lead to disorder progression. In this review we discuss the mechanisms of heterochromatinization induced by CGG repeat expansion in the promoter region of fmr1 gene.

Three groups of children with learning difficulties (a total of 62 children) are dealt with in this study according to their cognitive and psychological special features. In the first group the difficulties are a symptom of the FMR1-gene dysfunction. In the second group the poor school performance is associated with social interaction problems and high intelligence level. In the third group of gifted children with low verbal intelligence parameters the poor school performance occurred because of unevenly developed verbal and non-verbal abili-ties. We demonstrate the importance of complex neurophysiological, neuropsychological and clinical psychological investigation for establishing the reasons for school difficulties.

The human FMR1 gene encodes an RNA-binding protein taking part in translation regulation. The 5'-untranslated region of FMR1 gene contains a large number of tandem repeats of GCC triplets (5-50) which increasing (more then 200) is responsible for X-fragile syndrome (human congenital anomaly). As it has been shown earlier, al least two transcription factors (ZF5 and CGGBP-20) are capable of interacting specifically with GCC-repeats in regulatory regions of some genes. In this work, their roles in FMR1 gene expression regulation were studied. It was demonstrated by electrophoretic mobility shift assay that ZF5 recombinant protein specifically bound with GCC-triplet repeats (GCC9). Tissue-specific distributions of ZF5 and FMR1 proteins are very overlapped in mammalian. Inhibition of ZF5 expression in HepG2 cells (by RNA interference) leads to at least 1.5 times stimulations of FMR1 gene expression in these cells. To estimate the contribution of GCC-triplet repeats in FMR1 gene expression regulation we used two alternative variants of genetic construction: containing luciferase reporter gene under 5'-regulatory region fragment devoid of GCC-triplet repeats or including the GCC9 nucleotide sequence. HepG2 cells were co-transfected by these constructions and expressions vectors of ZF5 or (and) CGGBP-20 respectively. It was found that ZF5 downregulated the activity of 5'-regulatory region of FMR1 gene in both cases (acting probably through canonic 5'-GCGCGC3' sites). The presence of GCC-triplet repeats in the construction weakens this ZF5 effect. CGGBP-20 downregulates the activity of 5'-region of FMR1 gene in the presence of GCC-triplets only. The data obtained evidently indicate differently directed ZF5 effects on FMR1 gene expression and suggest the mechanism to explain the earlier demonstrated phenomenon about increasing of mRNA level in permutation FMR1 allele carries.

A PCR-based survey of allelic polymorphism of three microsatellite markers, DXS998, DXS548, and FRAXAC1, mapped to chromosome region Xp27.3, and two microsatellite markers, DXS8091 and DXS1691 located on Xq28 was carried out using a series of DNA samples obtained from 98 unrelated individuals from Russia. The number of alleles detected on electrophregrams for each marker tested was 4, 6, 4, 5, and 3, respectively. The values of heterozygosity index for the markers examined were 0.65, 0.27, 0.38, 0.70, and 0.29, respectively. The observed distribution of the allelic frequencies for each microsatellite marker examined fitted Hardy--Weinberg expectations. The values of individualization potential determined for each marker were 0.24, 0.53, 0.43, 0.12, and 0.52, respectively. In the sample tested the genotype distribution with regard to above loci was determined. The perspectives of using the analyzed allelic polymorphisms for indirect DNA diagnostics of the monogenic diseases located in this chromosome region (X-linked mental retardations, FRAXA and FRAXE) as well as for human population genetics and personal identification is discussed.

The methylation status of the FMR1 gene promotor was studied in 8 patients with detected CGG-expansion or negative PCR analysis (CGG-expansion or deletion). In all these causes the methylation status was detected. The modified PCR protocol of Hin6.I-restricted DNA has been proposed for performing diagnosis, postnatal and prenatal diagnostics and for screening patients with mental retardation and newborn boys.

Polymorphism of CGG and GCC trinucleotide repeats, whose expansions at the FRAXA and FRAXE loci have been identified as causative mutations in two forms of mental retardation, was studied in Slavic population of Tomsk. At the FRAXA locus a total of 31 allelic variants ranging from 8 to 56 copies of CGG repeat with two modal classes of 28-29 and 18-20 repeat units (with the frequencies of 24.6 and 11.5% respectively) were revealed. Compared to other populations, this locus was characterized by unusually high frequency of intermediate alleles with the sizes of more than 40 CGG repeat units (12.4%). Since intermediate repeats of the FRAXA locus were more prone to instability than normal alleles, it was suggested that Slavic population of Siberia had higher risk of the development of FMR1 dynamic mutations, giving rise to the Martin-Bell syndrome. The FRAXE allele frequency distribution was demonstrated to be normal with 18 allelic variants ranging from 9 to 27 GCC repeat units. In the population of Tomsk this locus had higher than in other populations frequency (26.7%) of short (less than 15 repeat units in size) alleles. In addition, in the Tomsk population both loci were characterized by high level of heterozygosity and low frequencies of modal allele classes. These results can be explained by the high level of outbreeding typical of the population of Siberia.

A number of genetic syndromes and cancer are caused by chromosomal microstructural rearrangements. It is sometimes impossible to detect this pathology microscopically. A complex of molecular methods makes it possible to detect subchromosomal deletions, point mutations, and functional anomalies on the respective genes, which cause a disease. Achievements of comprehensive analysis have been demonstrated in cases of some well-known diseases, the Prader-Willi syndrome, Engelmann's syndrome, the Martin-Bell syndrome, hereditary multiple exostotic chondrodysplasia, and retinoblastoma. DNA-diagnosis protocols are the result of this study.

Results of phenotypical, patho-psychological and molecular-genetic analysis of the 53 probands with clinical features of the fragile X syndrome and 10 female carriers are presented. The clinical heterogeneity, diagnostic criteria, methods of genetic risk estimation, perspectives of prevention of this disease are discussed.

The spontaneous deletion in FMR1 gene in a patient with the typical Fragile X phenotype was described. It was assumed that this deletion is located around CGG region in Pst1 fragment of the gene. The alleles with normal range of CGG copies and corresponding to premutation and full mutation ranges were revealed in mother-mosaic of proband. The possible mechanisms of trinucleotide repeat sequence instability are discussed.

30 patients (4-22 years old) and their 12 mothers were examined by means of method of electroencephalography (EEG). Healthy individuals of corresponding age were included in the control group. EEG changes of the same type were observed in all the patients with syndrome of fragile X-chromosome: reduction of occipital alpha-rhythm, prevalence of theta-rhythm in central-parietal and central frontal regions as well as epileptoid activity in parietal and central-parietal cortical regions. These peculiarities could be considered as the markers in syndrome's diagnostics. EEG of mothers (heterozygous carriers of mutant gene) were characterised by considerable variations: increase of rolandic rhythm's presentation in central cortical zones, accumulation of hypersynchronous EEG, increase of rhythmical theta-activity, etc. EEG of mother with shift-like schizophrenia resembled one of proband in one case.

A cytogenetic examination of a cow and its offsprings, obtained by pair transplantations of the embryos was carried out. The fragile X and the heteromorphism of the chromosomes XX of this cow and some of its offsprings were revealed. The variability of the hematological chimerism of the pair transplanted embryos was observed. The immigrated cells were, as a rule, distinguished by the comparatively low frequencies of the metaphases with the nearly located chromosomes XX. The heterogeneity of the stem blood cells of the embryos for some traits, and for the ability to migrate in the blood of the twin is under discussion.

Overall 21 women, heterozygous carriers of fragile X-chromosome (7 cases of obligate carriership and 14 sporadic cases) were examined by electroencephalography and cytogenetically. In none of them the clinical examination revealed a typical somatic appearance characteristic of Martin-Bell syndrome. Certain somatic disorders, psychasthenic traits, tearfulness and uncommunicativeness were recorded in single cases. The level of intellectual development corresponded to the low norm. Analysis of the EEG has demonstrated that in 76% of the women examined, the character of alpha-rhythm differed from normal. According to its intensity, three variants of the EEG were distinguished. In the majority of women, the EEG manifested the signs of the dysfunction of the diencephalic structures and not gross focal alterations. The character of the EEG does not provide any data which may be of diagnostic importance in revealing heterozygous carriership of fragile X-chromosome.

A total of 368 patients aged 1 year 11 months to 35 years with oligophrenia were examined cytogenetically. Out of 58 boys with a typical disease picture, the fragile X chromosome (fra-X) was revealed in 57. In all the cases, the syndrome could be diagnosed clinically before chromosomal analysis was made. In a group of children with undifferentiated oligophrenia including those with the sex-linked inheritance type, fra-X was not discovered. Besides, the fragile X chromosome was revealed in 4 girls. Of these, 3 were probands' sisters and one case was sporadic. The girls manifested both symptomatology typical for Martin-Bell syndrome and nonspecific mental retardation. Another 55 probands' relatives were examined clinically and cytogenetically. The fra-X was discovered in 9 mothers of the patients. The mean IQ in the women with the revealed fragile X chromosome was much lower than the mean IQ of all the women, mothers of the sick children. It is concluded that in spite of a wide range of the variability of somatic and psychopathological disorders in patients with Martin-Bell syndrome, the possibility of the clinical diagnosis of the disease without making cytogenetic studies attains as much as almost 100%. Symptoms, specific for the clinical diagnosis of the disease were distinguished.

A case of fragile-X syndrome (the Martin-Bell syndrome) in two male half-sibs from different marriages of their mother was described. Both patients suffered from mental retardation and had some characters of the Martin-Bell syndrome. Somatically healthy mother was characterized by IQ-101. The EEG study showed similar changes both in mother and her sons. The diagnostic specificity of the method presented was discussed.

Based on somatic, psychopathological and electroencephalographic examination of 26 boys aged 3-11.5 years with Martin-Bell syndrome it was found that the disease is characterized by noticeable polymorphism. Three types of the encephalograms and 3 clinical types of diseased children were distinguished. The first two types of the EEG accounting for 73.1% of cases in whose clinical picture the teta-rhythm appeared dominant may be of diagnostic importance. The majority of that group of children included those with ambidexterity and left-handedness. In the control group children, the EEG materially differed in 92.3% of cases. No correlation was found between the clinical groups and the EEG patterns.

The X-chromosome was studied in blood lymphocytes of 68 males with aspecific mental retardation (MR), their 57 relatives and 15 intellectually normal males. The incidence of a fragile X-chromosome (fra(X)) was found to be 4.7% in an unselected group of 42 patients, 50% among 10 probands in which pedigree data were suggestive of X-linked MR diagnosis, and 75% in the group of 15 patients selected for phenotype characteristic of the fragile X syndrome. The fra(X) was present in 1-43% of metaphases in different individuals, no such marker being observed in cells of 15 normal individuals. No significant difference was found when the incidence of the fra(X) was compared in cells cultured in the medium 199 with low folic acid content and the Eagle's medium supplemented with 5-fluorodeoxyuridine (10.62 +/- 2.94 SEM and 13.53 +/- 2.85 SEM, respectively). The possibility of false-positive diagnosis of the fragile X syndrome was quantitatively appreciated. A half of the patients showing a fra(C) in conventionally stained chromosomes were found to have fragile 6 autosome as the only marker in these cells, and in patients with the evident fragile (X) syndrome the fra(6) constituted about one-third of the fra(X) frequency. Both culture media employed were similar in the fra(6) induction.

Psychopathological, neurological and somatic manifestations of oligophrenia-with fragile X chromosome were investigated in 13 mentally retarded boys in prepuberty and in 9 such post-puberty++ patients. Age-related features of several psychopathological and somatic signs of the disease were revealed. The authors discuss the possibilities of clinical diagnosis of the disease and stress the importance of its early cytogenetical diagnosis in mentally retarded males.

Spontaneous and degranol- and dimatiph-induced chromosomal instability in the lymphocyte culture of patients with fra-X syndrome was investigated. The cultures contained TC 199 and 5% FC serum. It was found that the frequency of spontaneous chromosomal aberrations (CA) was 7.3% in cells from patients with fra(X), 3.9% in patients with MR of unknown origin, and 1.3% in normal individuals. Spontaneous break-points in the patients with fra(X) were localized in 1p, 2q, 3p, 6q, 7q, 16 q more often than in normal individuals. No significant difference was found in SCEs. The cells of patients with fra(X) were not sensitive to the induction of CA by degranol. It was found that chromosomal telomeric changes (CTC) were mutagen-independent, remaining at the spontaneous level: in the patients with fra(X) CTC were 10.5% (9.5% fra-Xq27, and 1% autosomal telomeric changes); in normal individuals CTC were 0.1%.