aglogo.gif (1177 bytes)Search for genetic modifiers of cancer risks conferred by BRCA1 and BRCA2


Background of research project

Protein truncating mutations in the BRCA1 and BRCA2 genes strongly predispose to the development of breast and ovarian cancer. Despite the high risks of breast and ovarian cancer conferred by deleterious BRCA1 and BRCA2 mutations, a strong variability in phenotype has been observed among families segregating the same mutation. This can range from early-onset breast cancer and ovarian cancer, to late-onset breast cancer without ovarian cancer. Even within a single pedigree, ages of onset of cancer can vary substantially. These observations support the idea that disease outcome in carriers is co-determined by other factors, some of which may be environmental, others genetic.

Research questions

To identify genetic factors that may modify the risk conferred by BRCA1 and BRCA2 mutations.

Project description

Our working hypothesis is that genes which modify the penetrance in BRCA1/2 carriers can be identified through candidate gene analysis. We shall employ a case-case association study-design to test for association between polymorphisms in candidate genes and disease status. Many of the carriers are expected to have developed either breast or ovarian cancer at predominantly early, but nonetheless variable ages. Likewise, healthy carriers will generally be young, because of the high penetrance of BRCA1/2 mutations, and because many were identified through presymptomatic testing which mostly concerns young women. Yet a small proportion (~5%) are still cancer-free at ages 60 or over. Subgrouping the carriers in this way will allow the analysis of the frequency of modifier alleles according to age at onset or current age. A ‘protective’ allele is expected to be enriched among older healthy carriers and late-onset breast cancer cases, relative to early-onset cases. Alternatively, one can compute the cumulative cancer risks in the subgroups defined on the basis of modifier allele status and test any differences for statistical significance. The candidate genes are genes which are plausibly involved in or related to the cellular function of BRCA1 and BRCA2, with focus on DNA damage response. A number of proteins are known to bind or interact with Brca1 and Brca2, such as RAD50, RAD51, CtIP, BARD1, p53, ATM, and Chk2. Others can be inferred on the basis of their involvement in DNA damage repair (XRCC1, XRCC3), or are regulated by BRCA1 or BRCA2 (GADD45, cyclin B1 and cylin D1).

Experimental design

We currently have about 400 BRCA gene carriers available for analysis. These will be genotyped for DNA polymorphisms in candidate genes. We will also genotype a series of 300 healthy controls to establish the allele frequency of the DNA variant in the population. The results of genotyping will be analysed by several different statistical methods (in collaboration with Department of Medical Statistics). If no known polymorhisms can be found in candidate genes in the public databases, we shall perform small scale sequencing of their coding regions in a small number of controls to identify them.

Techniques to be applied

Automated single nucleotide polymorphism (SNP) genotyping (using TaqMan molecular beacon technology); PCR; database-mining (internet searches of genome databases); DNA sequencing (if necessary); software-assisted analysis of genotype- and DNA sequence-data.

Statistical methods

Fisher's exact test; maximum likelihood methods

Plan of work and time schedule

The length of the project will determine how many SNPs will be analysed. For any SNP to be analysed, the whole procedure must be completed: genotyping of relevant samples, data-processing, and statistical analysis. For a single SNP this can easily be completed within 3 months; longer projects may also include SNP development in candidate genes (by DNA sequencing 10-20 controls to search for low-frequent SNPs).

Equipment to be used

TaqMan Molecular Beacon technology; automated DNA sequencer.

Patients involved

Yes, 200

Laboratory animals involved

No

Clinical/Non-clinical

Non-Clinical

Approval required from the Committee on medical ethics

No

Approval required from the committee on the use of laboratory animals

No

Updated:  19-02-2003
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