Search

Chromosomal imbalances are a major cause of intellectual disability (ID) and multiple congenital anomalies. We have clinically and molecularly characterized two patients with chromosome translocations and ID. Using whole genome array CGH analysis, we identified a microdeletion involving 4q21.3, unrelated to the translocations in both patients. We confirmed the 4q21.3 microdeletions using fluorescence in situ hybridization and quantitative genomic PCR. The corresponding deletion boundaries in the patients were further mapped and compared to previously reported 4q21 deletions and the associated clinical features. We determined a common region of deletion overlap that appears unique to ID, short stature, hypotonia, and dysmorphic facial features.

We have identified disruptions in the dedicator of cytokinesis 8 gene, DOCK8, in two unrelated patients with mental retardation (MR). In one patient, a male with MR and no speech, we mapped a genomic deletion of approximately 230 kb in subtelomeric 9p. In the second patient, a female with mental retardation and ectodermal dysplasia and a balanced translocation, t(X;9) (q13.1;p24), we mapped the 9p24 breakpoint to a region overlapping with the centromeric end of the 230-kb subtelomeric deletion. We characterized the DOCK8 gene from the critical 9p deletion region and determined that the longest isoform of the DOCK8 gene is truncated in both patients. Furthermore, the DOCK8 gene is expressed in several human tissues, including adult and fetal brain. Recently, a role for DOCK8 in processes that affect the organization of filamentous actin has been suggested. Several genes influencing the actin cytoskeleton have been implicated in human cognitive function and thus a possibility exists that the rare mutations in the DOCK8 gene may contribute to some cases of autosomal dominant mental retardation.

We have studied a male patient with significant developmental delay, growth failure, hypotonia, girdle weakness, microcephaly, and multiple congenital anomalies including atrial (ASD) and ventricular (VSD) septal defects. Detailed cytogenetic and molecular analyses revealed three de novo X chromosome aberrations and a karyotype 46,Y,der(X)inv(X) (p11.4q11.2)inv(X)(q11.2q21.32∼q22.2)del(X)(q22.3q22.3) was determined. The three X chromosome aberrations in the patient include: a pericentric inversion (inv 1) that disrupted the Duchenne muscular dystrophy (DMD) gene, dystrophin, at Xp11.4; an Xq11.2q21.32∼q22.2 paracentric inversion (inv 2) putatively affecting no genes; and an interstitial deletion at Xq22.3 that results in functional nullisomy of several known genes, including a gene previously associated with X-linked nonsyndromic mental retardation, acyl-CoA synthetase long chain family member 4 (ACSL4). These findings suggest that the disruption of DMD and the absence of ACSL4 in the patient are responsible for neuromuscular disease and cognitive impairment.

Two angiotensin II (Ang II)–specific receptors, AGTR1 and AGTR2, are expressed in the mammalian brain. Ang II actions on blood pressure regulation, water electrolyte balance, and hormone secretion are primarily mediated by AGTR1. The function of AGTR2 remains unclear. Here, we show that expression of the AGTR2 gene was absent in a female patient with mental retardation (MR) who had a balanced X;7 chromosomal translocation. Additionally, 8 of 590 unrelated male patients with MR were found to have sequence changes in the AGTR2 gene, including one frameshift and three missense mutations. These findings indicate a role for AGTR2 in brain development and cognitive function.

Alport syndrome with intellectual disability (ID) is a contiguous gene deletion syndrome involving several genes on Xq22.3 including COL4A5 and ACSL4. We report on a family with two males with this disorder and a Xq22.3 deletion. Fluorescent in situ hybridization and genomic analyses mapped the deletion region to between exon 1 of COL4A5 and exon 12 of ACSL4. The patients' mother has microscopic hematuria and was found to be heterozygous for the Xq22.3 deletion. Analysis using reverse transcription polymerase chain reaction of lymphoblastoid cell line RNA from an affected male in the family revealed a stable chimeric transcript with the ACSL4 exons 13-17 replaced by a cryptic exon from intron 1 of the COL4A5 gene. A truncated 54 kDa protein was predicted from this transcript but Western blot analysis and ACSL4 enzyme assay both showed functional nullisomy of ACSL4. We also compared the clinical features of the family with three previously reported families with the ACSL4 gene deletion and found that ID with absent or severely delayed speech, midface hypoplasia, and facial hypotonia are consistent features observed in the absence of ACSL4 gene.

Mental retardation (MR) is not a common feature observed in patients with classical ectodermal dysplasias (EDs). Several genes responsible for EDs and MR have been identified. However, the causation has yet to be identified in a significant number of patients with either ED or MR. Here, we have molecularly characterized a de novo balanced translocation t(1;6)(p22.1;p22.1) in a female patient who had mild features of ED with hypodontia, microcephaly, and cognitive impairment. Mapping of the translocation breakpoints in the patient revealed no obvious causative gene for either ED or MR. Whole genome array CGH analysis unveiled two novel submicroscopic deletions at 2q12.2 and 6q22.3, unrelated to the translocation in the patient. The 2q12.2 deletion contains a known ED gene, ectodysplasin-A receptor (EDAR), and the loss of one copy of this gene is considered to be responsible for the ectodermal phenotype in the patient. It is plausible that a potential autosomal MR gene deleted at 2q12.2 or 6q22.3 is likely the cause of the neurodevelopmental defects in the patient.

Cell-adhesion molecules play critical roles in brain development, as well as maintaining synaptic structure, function, and plasticity. Here we have found the disruption of two genes encoding putative cell-adhesion molecules, CDH15 (cadherin superfamily) and KIRREL3 (immunoglobulin superfamily), by a chromosomal translocation t(11;16) in a female patient with intellectual disability (ID). We screened coding regions of these two genes in a cohort of patients with ID and controls and identified four nonsynonymous CDH15 variants and three nonsynonymous KIRREL3 variants that appear rare and unique to ID. These variations altered highly conserved residues and were absent in more than 600 unrelated patients with ID and 800 control individuals. Furthermore, in vivo expression studies showed that three of the CDH15 variations adversely altered its ability to mediate cell-cell adhesion. We also show that in neuronal cells, human KIRREL3 colocalizes and interacts with the synaptic scaffolding protein, CASK, recently implicated in X-linked brain malformation and ID. Taken together, our data suggest that alterations in CDH15 and KIRREL3, either alone or in combination with other factors, could play a role in phenotypic expression of ID in some patients.