Hampton, Garret Malcolm; (1992) Molecular genetic analysis of the adenomatous polyposis coli gene region. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Familial Adenomatous Polyposis (FAP) is a rare, autosomal dominant predisposition to colorectal cancer, affecting about one in ten thousand individuals in all populations studied. The gene responsible for this syndrome, designated APC (for Adenomatous Polyposis Coli) was mapped to 5q21-q22 by linkage analysis following a cytogenetic report of a male patient with polyposis and an interstitial deletion on 5q. The high incidence of allele loss at 5q21-q22 in carcinomas of sporadic patients suggests that mutation of the APC gene is a very frequent step in the tumorigenic pathway to nonfamilial colorectal carcinomas and emphasises the importance of isolating the gene and identifying its function. Attempts were made to identify this gene using a positional cloning strategy, on the basis of its genomic location rather than by a knowledge of its function. As a first step toward this goal, two approaches were taken to identify a large number of DNA probes mapping within the breakpoints of two, and later three independent deletions encompassing the APC gene. In the first approach, a novel method, termed 'alu-PCR' was developed. By comparing the PCR patterns generated from normal and deleted chromosomes 5, a probe was identified mapping close to the APC gene. In the second approach, genomic libraries constructed from physically dissected DNA around chromosomal region 5q21-q22 were used to derive a large number of DNA probes. These probes facilitated the definition of a new minimally deleted region harbouring the gene and assisted with the construction of a physical map of this region. For the second phase of the cloning project, these DNA probes, in addition to others sub-localised to this region, were used to isolate a collection of yeast artificial chromosomes (YACs) which in total cover some 4-megabase-pairs (Mb) of DNA. Two of the YACs identified in this study, which cover a total distance of 1.1- Mb, were used indirectly to identify potential alterations, particularly small deletions, in homologs of chromosome 5 from FAP patients. One such deletion of 260-kb was identified and shown to be entirely overlapped by one of these YACs. This deletion, and the YAC used to identify it, contain the entire coding sequence of the APC gene.

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