MECP2 mutations in sporadic cases of Rett syndrome are almost exclusively of paternal origin

Abstract

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder that apparently is lethal in male embryos. RTT almost exclusively affects female offspring and, in 99.5% of all cases, is sporadic and due to de novo mutations in the MECP2 gene. Familial cases of RTT are rare and are due to X-chromosomal inheritance from a carrier mother. We analyzed the parental origin of MECP2 mutations in sporadic cases of RTT, by analysis of linkage between the mutation in the MECP2 gene and intronic polymorphisms in 27 families with 15 different mutations, and we found a high predominance of mutations of paternal origin in 26 of 27 cases (P<.001). The paternal origin was independent of type of mutation and was found for single-base exchanges as well as for deletions. Parents were not of especially advanced age. We conclude that de novo mutations in RTT occur almost exclusively on the paternally derived X chromosome and that this is most probably the cause for the high female:male ratio observed in patients with RTT. Affected males recently have been described in a few cases of familial inheritance. Identification of the parental origin may be useful to distinguish between the sporadic form of RTT and a potentially familial form. This distinction will allow geneticists to offer more-specific counseling and discriminate between higher (maternal origin) and lower (paternal origin) recurrence risk.

Figure 1

A, Sequence analysis of patient 18, who is heterozygous for mutation R270X (c.808C→T). The wild-type allele shows the recognition site (GCCG) of the NlaIV restriction enzyme; in the mutant allele, the NlaIV recognition site (GTCG) is abolished. B, Linkage analysis of mutation R270X in patient 18. The status, at the polymorphic region c.377+266C→T of patient 18 and her parents was determined by amplification of the respective region from genomic DNA and subsequent AluI digestion (lanes 2–4). The patient showed a DNA restriction pattern with fragments 195 and 169 bp in length (upper bands), indicating heterozygosity for the c.377+266C→T polymorphism. The patient's father showed a restriction pattern with a 195-bp fragment (missing the 169-bp fragment) indicating that he carries the cytosine variant of the c.377+266C→T polymorphism on his single X chromosome. The patient's mother showed the same restriction pattern as was seen in the patient and is also heterozygous for the c.377+266C→T polymorphism. Conclusively, in patient 18, the allele containing cytosine must be of paternal origin, and the allele containing thymine must be of maternal origin. Lanes 6–15 show polymorphism genotyping and mutation detection in five different clones of patient 18. Clones 1–3 show an AluI restriction pattern indicated by cytosine variant alleles of the c.377+266C→T polymorphism (lanes 6–8), and clones 4 and 5 show a pattern indicated by thymine variant alleles (lanes 9 and 10). Mutation detection in these five clones was done by second-round PCR and subsequent digestion with NlaIV. Mutant alleles miss an NlaIV cleavage site, at position 490 of the generated 590-bp PCR product, resulting in a DNA fragment that is 191 bp in length (upper band), instead of two smaller fragments. Clones 1–3 (lanes 11–13) show this 191-bp fragment, and clones 4 and 5 (lanes 14 and 15) do not. Mutation R270X is linked to the cytosine variant allele of the c.377+266C→T polymorphism and therefore is of paternal origin.