Rett syndrome is a progressive neurologic disorder observed exclusively in females, and characterized by severe mental retardation, autism, ataxia, seizures and somatic growth retardation. The disorder is usually sporadic, but a few familial cases with inheritance through maternal lines have been identified. The absolute sex preference and the pattern of inheritance suggest that Rett syndrome could be due to an X-linked mutation. The pattern of X chromosome inactivation in putative carrier females and the finding of X;autosome translocations in two instances support this hypothesis. In order to gain insight into the molecular basis of Rett syndrome, we plan to make maximal use of the available cytogenetic abnormalities, and to develop strategies that will allow us to detect the molecular genetic mechanisms of this disease. The first phase of the work will focus on cloning the X chromosomal breakpoint from a patient with a de novo X;3 translocation. We recently sublocalized this breakpoint to an 80kb region in Xp21.3 and efforts to isolate this region in overlapping phage/cosmid clones are in progress. The other translocation which involves X;22 and maps by cytogenetic analysis to Xp11.2 will be molecularly characterized using Xp11.2 markers. Both breakpoints will be cloned and characterized; we will search for complex rearrangements in order to explain two different map positions on Xp. Genomic sequences at and/or adjacent to these breakpoints will be used to identify expressed sequences and candidate genes. The second phase of this project will involve characterization of candidate transcripts, searching for intragenic mutations in sporadic and familial Rett patients. Sequence and expression studies of such transcripts may also provide us with data about other related genes that could be candidates for Rett syndrome or other X-linked mental retardation syndromes. We will also investigate other possible mechanisms for Rett syndrome making use of recent advances in molecular technology. A PCR-mediated approach to scan for submicroscopic deletions along the length of the X chromosome will be used. We will also use a fluorescence in situ hybridization-based approach to investigate the possibility that amplification of triplet repeats could be the underlying mechanism for Rett syndrome. We will look for intragenic deletions/rearrangements, and single base mutations using RNA single-strand conformational polymorphism and sequencing for cDNAs which are good candidates for the Rett syndrome gene. Among such candidates, we will investigate growth factors given the phenotype of acquired microcephaly, decreased dendritic growth, and somatic growth retardation in this syndrome. In summary, our research efforts to understand the molecular genetic mechanisms for Rett syndrome will utilize the cytogenetic abnormalities, familial cases and candidate genes. Throughout the course of this work, the possibilities of unusual genetic mechanisms and/or the involvement of more than one genetic locus will be considered.
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