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PWE-067 Recessively inherited non-polyposis colorectal cancer: genotype and phenotype
  1. K J Monahan1,
  2. K Pack2,
  3. C Cummings1,
  4. H J W Thomas1,
  5. I P M Tomlinson2
  1. 1Family Cancer Clinic, Imperial College and St Mark's Hospital, London, UK
  2. 2Department of Molecular and Population Genetics, Cancer Research UK, London, UK

Abstract

Introduction Patients diagnosed before 50 years of age have a likely strong genetic or environmental aetiological factor. There is good evidence from population studies1 that recessive inheritance is common in young colorectal cancer patients.

Methods A cohort of 133 colorectal cancer patients without multiple polyps or a family history of dominant inheritance were diagnosed under the age of 50 years. They were identified and recruited from the Bobby Moore database in the Family Cancer Clinic, St Mark's Hospital, Harrow. MYH was screened for germline mutations. As these patients fulfilled Bethesda criteria they were tested for hereditary non-polyposis colorectal cancer (HNPCC) by microsatellite instability analysis and immunohistochemistry of mismatch repair proteins. Immunohistochemistry was also performed on β-catenin and P53. Loss of heterozygosity of the APC locus at 5q21–22 was tested using a set of microsatellite markers. Sequencing was used to identify somatic mutations in KRAS and BRAF.

Results Forty-four patients (33%) had cancers proximal to the splenic flexure, 79 (59%) distal and had 11 (8%) synchronous colorectal cancers. Thirty-seven patients (28%) had an affected sibling and 33 (25%) patients had a second-degree relative with cancer at any site. The median age of diagnosis of colorectal cancer was 39 years (range 14–49 years of age). Twenty-six patients (20%) were found to harbour sequence variation in the MYH gene but none of these variants were likely to be pathogenic, and there was no difference in the frequency of these compared to a control group of 50 patients. Eighty percent of tumours were found to be microsatellite stable. 20/30 cancers had nuclear localisation of β-catenin and 21/30 had nuclear localisation of P53 antibodies on immunohistochemistry. Loss of heterozygosity of the APC locus at 5q21–22 was present in 14/30 cases. Thus Wnt pathway activation is likely by over half of this group of cancers. Four cancers had BRAF V600E mutations and five had KRAS codon 12 or 13 mutations.

Conclusion In a cohort of 133 young colorectal cancer patients without multiple polyps, most tumours demonstrated Wnt pathway activation and other somatic changes consistent with the classical adenoma-to-carcinoma sequence. Germline mutations in the colorectal neoplasia predisposition gene MYH appear to be rare events in such patients. The majority of recessive inheritance in young patients is probably caused by mutations in unknown predisposition genes.

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