N-acetyltransferase 2 genotype in colorectal cancer and selective gene retention in cancers with chromosome 8p deletions

Abstract

Background: Genetic polymorphisms in N-acetyltransferase (NAT2) can change the normally fast acetylation of substrates to slow acetylation, and have been associated with the development of some cancers. The NAT2 locus may also suffer dysregulation during cancer progression, as the gene resides on chromosome 8p22, a region which is frequently deleted in colorectal cancer.

Subjects and methods: A polymerase chain reaction based method was used to determine NAT2 genotype in 275 patients with colon cancer and 343 normal control DNAs. Within the cancer group, 65 cases known to contain deletions in chromosome 8p were examined for loss of heterozygosity at the NAT2 locus.

Results: Overall, there was no statistical difference in frequency or distribution of NAT2 alleles and genotype between colon cancer and control groups. There was a significant association between the slow acetylation genotype and early age of onset. NAT2 genotype did not vary with other clinical features of colon cancer, which included Dukes's stage, site of tumour, and sex. Of 48 informative cases, only three (6%) showed loss of heterozygosity, indicating that the NAT2 locus is not commonly deleted in colorectal cancer. This suggests that NAT2 is retained during the process of allele loss possibly because of its proximity to a gene necessary for cell viability.

Conclusions: NAT2 does not play a major role in colorectal cancer risk, but may influence risk in some age groups. The nature of the loss of heterozygosity at the chromosome 8p site is complex and is worthy of further study.

Figure 1

: NAT2 PCR products from constitutive DNA of patients with colorectal cancer, following digestion with restriction enzymes: (A) Kpn I; (B) DdeI; (C) TaqI; and (D) BamHI, illustrating four NAT2 polymorphic sites. Band sizes (in base pairs) are given for polymorphic sites (+ and -) and constant bands (c). Restriction patterns for each enzyme are illustrated: homozygous wild type; heterozygous; and homozygous mutant. MkV denotes molecular weight marker.

Figure 2

: Heterozygosity of NAT2 was retained in 94% of cancers which had demonstrated LOH on chromosome 8p in a previous study of colon cancer. Gene markers used in that study are listed according to approximate chromosomal location from telomere to centromere and heterozygosity status was examined using constitutive and tumour DNA in PCR and Southern blotting studies.²⁵ The approximate region to which NAT2 maps is indicated on the left of the figure. The pattern of allele loss in 13 patients with colorectal cancer is shown. In patients 1-10 NAT2 heterozygosity was retained, and in patients 11-13 NAT2 heterozygosity was lost.

Figure 3

: NAT2 alleles of two patients with colorectal cancer which had previously shown LOH on chromosome 8p. Patient A demonstrates allele retention and patient B allele loss. Restriction digests for each enzyme, KpnI, DdeI, TaqI, and BamHI are shown for matched pairs of constitutional DNA (N) and tumour DNA (T). Arrows indicate alleles showing loss.