ABSTRACT
Silver-Russell syndrome (SRS) is characterised by prenatal and postnatal growth retardation, dysmorphic facial features, and body asymmetry. In 35-60% of SRS cases the paternally methylated imprinting control region (ICR) upstream of the H19 gene (H19-ICR) is hypomethylated, leading to downregulation of IGF2 and bi-allelic expression of H19. H19 and IGF2 are reciprocally imprinted genes on chromosome 11p15. The expression is regulated by the imprinted methylation of the ICR, which modulates the transcription of H19 and IGF2 facilitated by enhancers downstream of H19. A promoter element of IGF2, IGF2P0, is differentially methylated equivalently to the H19-ICR, though in a small number of SRS cases this association is disrupted--that is, hypomethylation affects either H19-ICR or IGF2P0. Three pedigrees associated with hypomethylation of IGF2P0 in the probands are presented here, two with paternally derived deletions, and one with a balanced translocation of inferred paternal origin. They all have a breakpoint within the H19/IGF2 enhancer region. One proband has severe growth retardation, the others have SRS. This is the first report of paternally derived structural chromosomal mutations in 11p15 causing SRS. These cases define a novel aetiology of the growth retardation in SRS, namely, dissociation of IGF2 from its enhancers.
Subject(s)
Enhancer Elements, Genetic/genetics , Gene Deletion , Gene Rearrangement/genetics , Insulin-Like Growth Factor II/genetics , RNA, Untranslated/genetics , Silver-Russell Syndrome/genetics , Adult , Alleles , Child, Preschool , Chromosome Aberrations , Chromosomes, Human, Pair 11 , Female , Gene Order , Humans , Infant , Male , RNA, Long NoncodingABSTRACT
Fragile X syndrome is a common cause of inherited intellectual disability. It is caused by lack of the FMR1 gene product FMRP. The most frequent cause is the expansion of a CGG repeat located in the 5'UTR of FMR1. Alleles with 200 or more repeats become hypermethylated and transcriptionally silent. Only few patients with intragenic point mutations in FMR1 have been reported and, currently, routine analysis of patients referred for fragile X syndrome includes solely analysis for repeat expansion and methylation status. We identified a substitution in exon 2 of FMR1, c.80C>A, causing a nonsense mutation p.Ser27X, in a patient with classical clinical symptoms of fragile X syndrome. The mother who carried the mutation in heterozygous form presented with mild intellectual impairment. We conclude that further studies including western blot and DNA sequence analysis of the FMR1 gene should be performed in patients with typical symptoms of fragile X syndrome in whom no CGG repeat expansion is detected.
