FRA2A is a CGG repeat expansion associated with silencing of AFF3

Folate-sensitive fragile sites (FSFS) are a rare cytogenetically visible subset of dynamic mutations. Of the eight molecularly characterized FSFS, four are associated with intellectual disability (ID). Cytogenetic expression results from CGG tri-nucleotide-repeat expansion mutation associated with local CpG hypermethylation and transcriptional silencing. The best studied is the FRAXA site in the FMR1 gene, where large expansions cause fragile X syndrome, the most common inherited ID syndrome. Here we studied three families with FRA2A expression at 2q11 associated with a wide spectrum of neurodevelopmental phenotypes. We identified a polymorphic CGG repeat in a conserved, brain-active alternative promoter of the AFF3 gene, an autosomal homolog of the X-linked AFF2/FMR2 gene: Expansion of the AFF2 CGG repeat causes FRAXE ID. We found that FRA2A-expressing individuals have mosaic expansions of the AFF3 CGG repeat in the range of several hundred repeat units. Moreover, bisulfite sequencing and pyrosequencing both suggest AFF3 promoter hypermethylation. cSNP-analysis demonstrates monoallelic expression of the AFF3 gene in FRA2A carriers thus predicting that FRA2A expression results in functional haploinsufficiency for AFF3 at least in a subset of tissues. By whole-mount in situ hybridization the mouse AFF3 ortholog shows strong regional expression in the developing brain, somites and limb buds in 9.5-12.5dpc mouse embryos. Our data suggest that there may be an association between FRA2A and a delay in the acquisition of motor and language skills in the families studied here. However, additional cases are required to firmly establish a causal relationship

Loss-of-function mutations in the X-linked biglycan gene cause a severe syndromic form of thoracic aortic aneurysms and dissections.

Thoracic aortic aneurysm and dissection (TAAD) is typically inherited in an autosomal dominant manner, but rare X-linked families have been described. So far, the only known X-linked gene is FLNA, which is associated with the periventricular nodular heterotopia type of Ehlers-Danlos syndrome. However, mutations in this gene explain only a small number of X-linked TAAD families. We performed targeted resequencing of 368 candidate genes in a cohort of 11 molecularly unexplained Marfan probands. Subsequently, Sanger sequencing of BGN in 360 male and 155 female molecularly unexplained TAAD probands was performed. We found five individuals with loss-of-function mutations in BGN encoding the small leucine-rich proteoglycan biglycan. The clinical phenotype is characterized by early-onset aortic aneurysm and dissection. Other recurrent findings include hypertelorism, pectus deformity, joint hypermobility, contractures, and mild skeletal dysplasia. Fluorescent staining revealed an increase in TGF-β signaling, evidenced by an increase in nuclear pSMAD2 in the aortic wall. Our results are in line with those of prior reports demonstrating that Bgn-deficient male BALB/cA mice die from aortic rupture. In conclusion, BGN gene defects in humans cause an X-linked syndromic form of severe TAAD that is associated with preservation of elastic fibers and increased TGF-β signaling.Genet Med 19 4, 386-395

Segregation of the fragile X mutation from an affected male to his normal daughter

We report here a family in which the fragile X mutation segregates from an affected grandfather through his normal daughter to an affected grandson. The grandson shows clinical and cytogenetic expression of fragile X syndrome due to a full mutation (large methylated insertion) in the fragile X gene (FMR-1). The mother shows a premutation (small unmethylated insertion) in her FMR-1 gene as the sole manifestation of the fragile X syndrome. The grandfather expresses the fragile X syndrome at the clinical and cytogenetic level, whereas he is mosaic for a methylated full mutation and an unmethylated premutation. The absence of expression of the fragile X mutation when transmitted through an expressing male might present further evidence for genomic imprinting of the FMR-1 gene. Alternatively, it is possible that the grandfather transmitted his premutation to his daughter due to germline mosaicism with both the premutation and the full mutation present in his sperm.</p

Segregation of the fragile X mutation from an affected male to his normal daughter

We report here a family in which the fragile X mutation segregates from an affected grandfather through his normal daughter to an affected grandson. The grandson shows clinical and cytogenetic expression of fragile X syndrome due to a full mutation (large methylated insertion) in the fragile X gene (FMR-1). The mother shows a premutation (small unmethylated insertion) in her FMR-1 gene as the sole manifestation of the fragile X syndrome. The grandfather expresses the fragile X syndrome at the clinical and cytogenetic level, whereas he is mosaic for a methylated full mutation and an unmethylated premutation. The absence of expression of the fragile X mutation when transmitted through an expressing male might present further evidence for genomic imprinting of the FMR-1 gene. Alternatively, it is possible that the grandfather transmitted his premutation to his daughter due to germline mosaicism with both the premutation and the full mutation present in his sperm.</p

The Reduced Expression of the HADH2 Protein Causes X-Linked Mental Retardation, Choreoathetosis, and Abnormal Behavior

Recently, we defined a new syndromic form of X-linked mental retardation in a 4-generation family with a unique clinical phenotype characterized by mild mental retardation, choreoathetosis, and abnormal behavior (MRXS10). Linkage analysis in this family revealed a candidate region of 13.4 Mb between markers DXS1201 and DXS991 on Xp11; therefore, mutation analysis was performed by direct sequencing in most of the 135 annotated genes located in the region. The gene (HADH2) encoding l-3-hydroxyacyl-CoA dehydrogenase II displayed a sequence alteration (c.574 C→A; p.R192R) in all patients and carrier females that was absent in unaffected male family members and could not be found in 2,500 control X chromosomes, including in those of 500 healthy males. The silent C→A substitution is located in exon 5 and was shown by western blot to reduce the amount of HADH2 protein by 60%–70% in the patient. Quantitative in vivo and in vitro expression studies revealed a ratio of splicing transcript amounts different from those normally seen in controls. Apparently, the reduced expression of the wild-type fragment, which results in the decreased protein expression, rather than the increased amount of aberrant splicing fragments of the HADH2 gene, is pathogenic. Our data therefore strongly suggest that reduced expression of the HADH2 protein causes MRXS10, a phenotype different from that caused by 2-methyl-3-hydroxybutyryl-CoA dehydrogenase deficiency, which is a neurodegenerative disorder caused by missense mutations in this multifunctional protein