Analysis of full fragile X mutations in fetal tissues and monozygotic twins indicate that abnormal methylation and somatic heterogeneity are established early in development

Abstract

The fragile X syndrome, the most common cause of inherited mental retardation, is characterized by unique genetic mechanisms, which include amplification of a CGG repeat and abnormal DNA methylation. We have proposed that 2 main types of mutations exist. Premutations do not cause mental retardation, and are characterized by an elongation of 70 to 500 bp, with little or no somatic heterogeneity and without abnormal methylation. Full mutations are associated with high risk of mental retardation, and consist of an amplification of 600 bp or more, with often extensive somatic heterogeneity, and with abnormal DNA methylation. To analyze whether the latter pattern is already established during fetal life, we have studied chorionic villi from 10 fetuses with a full mutation. In some cases we have compared them to corresponding fetal tissues. Our results indicate that somatic heterogeneity of the full mutation is established during (and possibly limited to) the very early stages of embryogenesis. This is supported by the extraordinary concordance in mutation patterns found in 2 sets of monozygotic twins (9 and 30 years old). While the methylation pattern specific of the inactive X chromosome appears rarely present on chorionic villi of normal females, the abnormal methylation characteristic of the full mutation was present in 8 of 9 male or female chorionic villi analyzed. This suggests that the methylation mechanisms responsible for establishing the inactive X chromosome pattern and the full mutation pattern are, at least in part, distinct. Our results validate the analysis of chorionic villi for direct prenatal diagnosis of the fragile X syndrome.

Comparative analysis of DNA methylation in transgenic mice with unstable CGG repeats from FMR1 gene

“…24 During early embryogenesis extensive methylation and demethylation occurs, corresponding approximately to the stages at which FRAXA instability is postulated to occur in the foetus. [25][26][27] Our analysis shows that repeat instability and DNA methylation are independent events in mice. The founder mice in the three lines FVB120, CD38 and CD41 show instability of CGG repeats in the transgene but completely lack methylation.…”

Hotspot for deletions in the CGG repeat region of FMR1 in fragile X patients

“…Two different types of mosaicism have been observed. Firstly, there is the wide range of repeat lengths of the full mutation, visible as a smear on Southern blot analysis (7,29,30). Secondly, 20% of the fragile X patients are mosaic for a premutation allele in addition to the full mutation (7,11,15).…”

DNA Methylation and Trinucleotide Repeat Expansion Diseases

“…The CGG deletions occur during replication and are dependent on replication fork dynamics, size of repeat and CpG methylation status (Nichol Edamura et al, 2005). The period of somatic CGG instability is restricted to early stages of embryonic and foetal growth and ends when expanded CGG sequences become abnormally methylated (Devys et al, 1992;Taylor et al, 1999). Subsequent CpG methylation of FRAXA 'full mutation' expanded CGG repeats causes somatic stability (Wohrle et al, 1995).…”