Lissencephalies (smooth brain) are human disorders affecting the establishment of normal neuronal migration and position in developing cerebral cortex. Two phenotypically related forms of LIS have been identified: an autosomal form 17LIS (LIS1), associate with Miller-Dieker syndrome on 17p13 and another with an X-linked inheritance pattern, called X-linked lissencephaly and subcortical band heterotopia, a less severe phenotype, in carrier females. Linkage analysis in several XLIS kinships places the XLIS locus in Xq21-24. An unusual case of classical lissencephaly is associated with a de novo X;2 balanced translocation with a breakpoint in Xq22.3 which very likely disrupts or alters the XLIS gene. Physical mapping of this breakpoint using somatic cell hybrids which contain the der2 or derX chromosomes from this patient and YACs containing linked DNA markers has narrowed the breakpoint to 1 -2 Mb between DSX1105 and DSX1072. This project aims to identify the gene involved in XLIS and begin to examine its function in cortical development and the pathogenesis of lissencephaly. This will entail 1) further definition of the XLIS locus with new markers and additional families, identification of additional YACs, fine mapping of the Xq22 breakpoint, and construction of cosmid contigs from the YACs spanning the breakpoint; 2) identification and isolation of candidate XLIS genes through mapping of CpG islands in YACs, isolation of genomic subclones, genomic sequence comparison to identify highly conserved sequences, exon trapping, and recombination based recovery of cDNAs; 3) confirmation of XLIS candidate identity by Southern analysis of XLIS families, FISH screen using candidate gene probes of LIS males without detectable mutation at 17p, heteroduplex, and SSCP analysis of XLIS families; 4) characterization of XLIS gene by sequence analysis of full length cDNA, and mapping of intron-exon boundaries; and 5) examination of XLIS function by temporal and anatomic localization of gene expression during cortical development, and production of transgenic mouse models by homologous recombinant knockout or over-expression of the mutated gene.
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