[Analysis of the parental origin of de novo MECP2 mutations and X chromosome inactivation in fifteen sporadic cases with Rett syndrome].
OBJECTIVE
Rett syndrome (RTT) is a neurodevelopmental disorder occurring almost exclusively in females as sporadic cases due to de novo mutations in the methyl-CpG-binding protein 2 gene (MECP2). Familial cases of RTT are rare and are due to X-chromosomal inheritance from a carrier mother. Recently, DNA mutations in the MECP2 have been detected in approximately 84.7% of patients with RTT in China. To explain the sex-limited expression of RTT, it has been suggested that de novo X-linked mutations occur exclusively in male germ cells resulting therefore only in affected daughters. To test this hypothesis, we have analyzed the parental origin of mutations and the XCI status in 15 sporadic cases with RTT due to MECP2 molecular defects.


METHODS
Allele-specific PCR was performed to amplify a fragment including the position of the mutation. The allele-specific PCR products were sequenced to determine which haplotype contained the mutation. It was then possible to determine the parent of origin by genotyping the single nucleotide polymorphism (SNP) in the parents. The degree of XCI and its direction relative to the X chromosome parent of origin were measured in DNA prepared from peripheral blood leucocytes by analyzing CAG repeat polymorphism in the androgen receptor gene (AR).


RESULTS
Except for 2 cases who had a frameshift mutation; all the remaining 13 cases had a C-->T transition mutation. Paternal origin has been determined in all cases with the C-->T transition mutation. For the two frameshift mutations, paternal origin has been determined in one case and maternal origin in the other. The frequency of male germ-line transmission in mutations is 93.3%. Except for 2 cases who were homozygotic at the AR locus, of the remaining 13 cases, 8 cases had a random XCI pattern; the other five cases had a skewed XCI pattern and they favor expression of the maternal origin allele.


CONCLUSIONS
De novo mutations in sporadic 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 sporadic cases with RTT. Random XCI was the main XCI pattern in sporadic RTT patients. The priority inactive X chromosome was mainly of paternal origin.
Zhu
Xing-wang
XW
Department of Pediatrics, Peking University First Hospital, Beijing 100034, China.
Pan
Hong
H
Li
Mei-rong
MR
Bao
Xin-hua
XH
Zhang
Jing-jing
JJ
Wu
Xi-ru
XR
D002869:Chromosome Aberrations; D041321:Chromosomes, Human, X; D005260:Female; D006801:Humans; D008297:Male; D051783:Methyl-CpG-Binding Protein 2 / Q000235:genetics*; D009154:Mutation; D020641:Polymorphism, Single Nucleotide; D015518:Rett Syndrome / Q000235:genetics*; D049951:X Chromosome Inactivation
D051783:Methyl-CpG-Binding Protein 2