Genetic studies of neurological disorders : Rett syndrome and HD-like familial prion disease

Abstract

Rett syndrome (RTT) is a severe childhood neurodevelopmental disorder, which almost exclusively affects females with a prevalence of 1 in 10 000-15 000, >99% of the cases are sporadic. It is commonly thought of as an X-linked dominant disorder lethal to males. Concordance among monozygotic twins and disease transmission in a few familial cases suggest genetic aetiology of the disease.

Based on the hypothesis by Thomas that a high male to female ratio of germline mutations can explain the lack of affected males in RTT and other X-linked dominant diseases, genomic screening of the whole X chromosome was performed in order to identify the grand-paternal alleles in one RTT family with three generations. The results indicate that the RTT gene is likely to be located on the telomere of X. Further investigations using eight additional families narrowed down the region to Xq28, which was further confirmed by two other groups. Xq28 was then intensively screened for RTT gene until Amir et al. identified mutations in the MECP2 gene.

Mutations of the MECP2 gene in our RTT familial and sporadic cases, classical and atypical RTT, were detected in 35%. Mutations included missense, nonsense, deletion or insertion. Due to the low mutation frequency, suggesting that other genes may be involved in the pathogenesis of RTT, mutation screening of six additional candidate genes on the X or autosomal chromosomes (UBE1, UBE21 GdX, SOX3, GABRA3 and CDR2) was performed. These genes were selected based on clinical, pathological, and genetic grounds. No mutation was found in these genes by direct sequencing. Furthermore, gene expression profiles of MECP2, GdX, GABRA3 and LICAM were investigated using in situ hybridization. No gross differences were observed in brain neurons from Rett patients comparing to normal controls.

To determine if X-chromosome inactivation (XCI) plays an important role in the pathogenesis of RTT, we analysed XCI pattern in five RTT families. The result showed that all mothers and six out of eight affected girls exhibited a totally skewed pattern of XCI and the paternally inherited X chromosome was active in the patients with a skewed pattern of XCI. Genotyping of the whole X chromosome indicate that a potential locus responsible for the skewed XCI in these families could be located on the short arm of the X chromosome. Mutation screening of MECP2 gene revealed a mutation in only one of the five families. These data led us to propose a model for familial RTT transmission in which two phenomena would be involved: an X-linked locus abnormally escaping XCI and the presence of a skewed XCI in carrier women.

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder. The HD gene on chromosome 4p, containing a CAG repeat, was identified in 1993. A family with three generations was referred to us for HD predictive testing. Seven affected family members were diagnosed as HD because of clinical and neuropathological similarities. No CAG repeat expansions were detected and the region containing the HD gene was completely excluded by linkage analysis.

In order to map the disease trait in this HD-like family, whole genome screening was per performed using highly polymorphic DNA microsatellite markers. A Lod score of 3.01 was obtained in this family for the marker D20S482 on chromosome 20p, which was the highest Lod score predicted by a simulation test. Haplotype analysis indicated that the gene responsible for this disease was located in a region of 2.7 cM between markers D20S193 and D20S895. Candidate genes from the mapped region were screened for mutations. A 194 bp octapeptide repeat expansion in the N-terminus of the PrP gene was identified in all affected members and not in healthy individuals. This indicates that the HD-like neurodegenerative disorder is in fact a familial prion disease and suggests that clinicians should consider screening for prion gene mutation in individuals with HD-like diseases in which the HD CAG repeat expansions are absent.