ATRX mutation in two adult brothers with non-specific moderate intellectual disability identified by exome sequencing

In this report, we describe two adult brothers affected by moderate non-specific intellectual disability (ID). They showed minor facial anomalies, not clearly ascribable to any specific syndromic patterns, microcephaly, brachydactyly and broad toes. Both brothers presented seizures. Karyotype, subtelomeric and FMR1 analysis were normal in both cases.

We performed array-CGH analysis that revealed no copy-number variations potentially associated with ID. Subsequent exome sequence analysis allowed the identification of the ATRX c.109C>T (p.R37X) mutation in both the affected brothers. Sanger sequencing confirmed the presence of the mutation in the brothers and showed that the mother is a healthy carrier.

Mutations in the ATRX gene cause the X-linked alpha thalassemia/mental retardation (ATR-X) syndrome (MIM #301040), a severe clinical condition usually associated with profound ID, facial dysmorphism and alpha thalassemia. However, the syndrome is clinically heterogeneous and some mutations, including the c.109C>T, are associated with a broad phenotypic spectrum, with patients displaying a less severe phenotype with only mild-moderate ID. In the case presented here, exome sequencing provided an effective strategy to achieve the molecular diagnosis of ATR-X syndrome, which otherwise would have been difficult to consider due to the mild non-specific phenotype and the absence of a family history with typical severe cases.

Evidence by expression analysis of candidate genes for congenital heart defects in the NF1 microdeletion interval

It was recently reported that congenital heart disease is significantly more frequent in patients with NF1 microdeletion syndrome than in those with classical NF1. The outcome of congenital heart disease in this subset of patients is likely caused by the haploinsufficiency of gene/s in the deletion interval. Following in silico analysis of the deleted region, we found two genes known to be expressed in adult heart, the Joined to JAZF1 (SUZ12) and the Centaurin-alpha 2 (CENTA2) genes, and seven other genes with poorly defined patterns of expression and function. With the aim of defining their expression profiles in human fetal tissues (15th-21st weeks of gestation), expression analysis by RT-PCR and Northern blotting was performed. C17orf40, SUZ12 and CENTA2 were found to be mainly expressed in fetal heart, and following RT-PCR on mouse embryos and embryonic heart and brain at different stages of development, we found that the orthologous genes C17orf40, Suz12 and Centa2 are also expressed in early stages of development, before and during the formation of the four heart chambers. The presence of binding sites for Nkx2-5, a transcription factor expressed early in heart development, in all three mouse orthologous genes was predicted by bioinformatics, thus reinforcing the hypothesis that these genes might be involved in heart development and may be plausible candidates for congenital heart disease.

Identification of duplicated genes in 17q11.2 using FISH on stretched chromosomes and DNA fibers

Recent evidence has been provided that links duplicons (REP-P and REP-M) in 17q11.2 flanking the neurofibromatosis type 1 (NF1) gene to the breakpoints of the NF1 microdeletion syndrome. The physical mapping and structural definition of duplicated regions is often impossible by conventional approaches, and so we have used high resolution fluorescence in situ hybridization (FISH) with locus-specific probes of limited size on chromosomes stretched to different degrees to identify novel duplicated genes and expressed sequence tags (ESTs) mapping to 17q11.2. This approach has allowed us to detect and map duplications of the BLMH and GOS28 genes, with one copy lying centromeric and one telomeric to the NF1 gene, and an SHGC30113 transcript with one copy being adjacent and the other distal to the NF1 3' untranslated region. Double-color FISH with a BLMH-specific probe on stretched chromosomes showed that the telomeric BLMH repeat lacked the 5' end of the gene and was 0.8 Mb from its centromeric copy. The distance between the SHGC30113 repeats was estimated as being 500 kb by double-color FISH on highly stretched chromosomes and DNA fibers. The latter approach revealed adjacent SHGC30113-BLMH-specific signals relating to the incomplete BLMH copy. The use of FISH on stretched chromosomes and, where applicable DNA fibers, is a powerful tool for identifying and finely characterizing duplicated regions whose mapping by the classical physical mapping approach is often precluded.