SO33: Assessment of MUTYH Associated Polyposis (MAP) phenotype in order to refine testing guidelines

Kate Simon, Sam Loughlin, Lucy Jenkins, Hospital Lucy Side

Great Ormond Street Hospital

Aim

To review our genetic testing criteria for MAP and improve understanding of the MAP phenotype.

Method

We reviewed patient records in those whom we requested MAP testing from 01/01/2009 to 31/12/2014. We documented the number and type of polyps and age of polyp/bowel cancer diagnosis.

Results

Common Caucasian or Asian mutation testing or a full screen was completed in 122 families.7/122 (5.7%) patients were MUTYH compound heterozygotes for pathogenic mutations, 1/122 (0.8%) had a homozygous VUS, 2/122 (1.6%) were carriers and 105/122 (86%) had no mutations identified. 7/122 (5.7%) had another condition identified.

6/8 patients with two mutations/variants had at least one rare mutation. These 8 patients had 6 to ~30 polyps identified; mostly low grade adenomas. Average age of polyp diagnosis was 54; cancer diagnosis 55.

Those without a mutation had a broader range of polyp type. Average age of polyp/cancer diagnosis was 45. Mutations were not found in any patients with serrated polyps or with isolated bowel cancer <35 years without polyps.

Conclusion

We no longer test for MAP in isolated colorectal cancer cases <35 years or patients with <5 adenomas. We will offer a full screen if phenotype is classical. Serrated polyps appear to represent a phenotype distinct to MAP.

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