Taylor, Catherine Louise; (1997) The isolation and testing of candidate genes for DiGeorge syndrome. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

DiGeorge Syndrome (DGS) is a developmental field defect associated with hemizygosity for human chromosome 22q11. Deletion mapping of patients has previously identified the DiGeorge critical region (DGCR), of approximately 500-600kb. One DGS individual (ADU) has a balanced translocation with a breakpoint that maps within the DGCR. It was suggested that a gene of major effect is disrupted in this rearrangement. As part of the strategy to isolate genes from the DGCR, a conserved genomic clone known to span the ADU breakpoint, was used to screen a human foetal brain cDNA library. The gene IDD was identified and shown to map distal to the breakpoint. Although hemizygous patients with a deletion of 22q11, no gross re-arrangements or mutations of this gene were found in any affected individual of normal karyotype. The murine homologue was identified and sequenced. It was found to share 92% similarity with its human counterpart at the protein level. Expression analysis of murine Idd showed high transcript levels in the branchial arches and limb buds at 9.5 dpc. and it was seen to be expressed early in embryogenesis. Other genes that map within the DGCR, including splice variants of the gene HIRA, were extensively tested for mutations. No mutations were detected, but a number of polymorphisms were observed in the genes ES2 and DGCR5. A second locus on human chromosome 10p13 has been associated with DGS. A candidate gene BMI-1 had been previously mapped to this region. This gene was tested both by quantitative Southern blotting and fluorescence in situ hybridisation, in affected individuals, and it was found to map proximal to the DGCRII on chromosome 10. The Japanese pufferfish, Fugu rubripes, has a haploid genome 7.5x smaller than the human, due to fewer repetitive DNA sequences. Conservation of synteny between Fugu and man, has been observed in several loci. A contig of genomic Fugu cosmid clones was isolated by hybridisation with human genes that map within the DGCR. Sequencing and mapping data show that there is homology of synteny between human 22q11 and Fugu. Both the genes IDD and ES2 are within this syntenic region.

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