A mitotic recombination map proximal to the APC locus on chromosome 5q and assessment of influences on colorectal cancer risk.
BACKGROUND
Mitotic recombination is important for inactivating tumour suppressor genes by copy-neutral loss of heterozygosity (LOH). Although meiotic recombination maps are plentiful, little is known about mitotic recombination. The APC gene (chr5q21) is mutated in most colorectal tumours and its usual mode of LOH is mitotic recombination.


METHODS
We mapped mitotic recombination boundaries ("breakpoints") between the centromere (~50 Mb) and APC (~112 Mb) in early colorectal tumours.


RESULTS
Breakpoints were non-random, with the highest frequency between 65 Mb and 75 Mb, close to a low copy number repeat region (68-71 Mb). There were, surprisingly, few breakpoints close to APC, contrary to expectations were there constraints on tumorigenesis caused by uncovering recessive lethal alleles or if mitotic recombination were mechanistically favoured by a longer residual chromosome arm. The locations of mitotic and meiotic recombination breakpoints were correlated, suggesting that the two types of recombination are influenced by similar processes, whether mutational or selective in origin. Breakpoints were also associated with higher local G+C content. The recombination and gain/deletion breakpoint maps on 5q were not, however, associated, perhaps owing to selective constraints on APC dosage in early colorectal tumours. Since polymorphisms within the region of frequent mitotic recombination on 5q might influence the frequency of LOH, we tested the 68-71 Mb low copy number repeat and nearby tagSNPs, but no associations with colorectal cancer risk were found.


CONCLUSIONS
LOH on 5q is non-random, but local factors do not greatly influence the rate of LOH at APC or explain inter differential susceptibility to colorectal tumours.
Howarth
Kimberley
K
Molecular and Population Genetics Laboratory, Wellcome Trust Centre for Human Genetics, University of Oxford, Oxford, UK. kimberley.howarth@well.ox.ac.uk
Ranta
Susanna
S
Winter
Eitan
E
Teixeira
Ana
A
Schaschl
Helmut
H
Harvey
John J
JJ
Rowan
Andrew
A
Jones
Angela
A
Spain
Sarah
S
Clark
Susan
S
Guenther
Thomas
T
Stewart
Aengus
A
Silver
Andrew
A
Tomlinson
Ian
I
D045744:Cell Line, Tumor; D002874:Chromosome Mapping; D002895:Chromosomes, Human, Pair 5; D015179:Colorectal Neoplasms / Q000235:genetics*; D017491:Genes, APC; D020022:Genetic Predisposition to Disease; D006801:Humans; D019656:Loss of Heterozygosity; D018895:Microsatellite Repeats; D008938:Mitosis; D020411:Oligonucleotide Array Sequence Analysis; D020641:Polymorphism, Single Nucleotide; D011995:Recombination, Genetic; D017422:Sequence Analysis, DNA