‘Indirect’ Mutation Analysis in Cancer Families

1. ‘Indirect’ Mutation Analysis in Cancer Families SCOBEC & Birmingham Cancer Training Day 6th June 2007 Nicole Motton

2. Origin of this Approach to Mutation Analysis • Short Report by D G Cruger etal. • Clinical Genetics 2005: 68: 228-233. • ‘Indirect’ BRCA1/2 testing. A useful approach in hereditary breast and ovarian cancer families without a living affected relative.

3. Introduction • Currently genetic testing is offered based on the chance of identifying a disease-causing mutation • Several models can be used and a threshold of 10% for mutation analysis suggested for each gene • Practise is to test cancer families where an affected member will provide a blood sample • Testing a single unaffected member from a high risk family is limited if no mutation is found • Could be an undetectable mutation or a mutation in an unknown gene • Therefore the risk will still be thought to be increased

4. ‘Indirect’ Mutation Testing • For a large number of families no sample is available from the affected family member so no mutation screening will be offered • Even if affected relative DNA is available they could be a phenocopy • Many of these families have a history of cancer and are at very high risk of harbouring a mutation • May consider prophylatic surgery and in theory 50% may not carry a mutation • A method of genetic testing for high risk breast & ovarian cancer families where no DNA is available from affected relatives

5. ‘Indirect’ Mutation Testing Strategy • An alternative approach is by offering mutation screening to unaffected relatives • Test ≥2 unaffected relatives at 50% risk of being heterozygous for a potential mutation • Gives a chance of up to 99% of finding a disease-causing mutation that would have been found in an affected family member • By testing more relatives the probability of detecting a mutation increases

6. Probability of identifying a mutation present in an affected relative by number of unaffected individuals tested. Unaffected individuals must be first degree relatives at 50% heterozygous risk (1-0.5n)

7. Case Studies • Apply to Breast/Ovarian cancer families and HNPCC families (esp. with MSI & IHC studies available) • 3 Breast/Ovarian cancer families and 3 HNPCC families in which this approach has been successful • All families fulfilled local guidelines for mutation screening but no affected relatives were available to give DNA

8. BROV Family 1 p.Leu2092ProfsX7 exon 11 BRCA2 No Mutation 75% chance of detecting a mutation

9. BROV Family 2 p.Leu1098SerfsX4 exon 11 BRCA1 p.Leu1098SerfsX4 exon 11 BRCA1 75% chance of detecting a mutation

10. BROV Family 3 Missense c.1206C>A; p.Ser326Arg exon 10 BRCA2 Declined testing Missense c.1206C>A; p.Ser326Arg exon 10 BRCA2 No Mutation 87.5% chance of detecting a mutation

11. Loss MSH2 MSI +ve HNPCC Family 1 87.5% chance of detecting a mutation p.Lys675X Exon 13 hMSH2 No Mutation p.Lys675X Exon 13 hMSH2

12. Loss MSH2 MSI +ve HNPCC Family 2 Deletion exons 11-16 in hMSH2 No Mutation 75% chance of detecting a mutation

13. Loss MSH2 MSI +ve No sample No sample HNPCC Family 3 Duplication exons 1-6 in hMSH2 50% chance of detecting a mutation

14. Summary • In all but one of these families deleterious mutations were identified • Missense mutations are difficult to interpret especially if no DNA is available from an affected member • Proven to be a very useful technique • Allows family members to make decisions about risk-reducing surgery and surveillance based on their own mutation status • Allows presymptomatic testing to become available • Essentially performing presymptomatic testing but in an 8 week turnaround time • Thorough genetic counselling before consenting