BRCA1 and BRCA2 founder mutations and the risk of colorectal cancer

BRCA1 and BRCA2: the genetic testing and the current management options for mutation carriers

Approximately 5-10% of breast carcinomas and 10% of ovarian carcinomas are ascribable to a genetic susceptibility. Of these, about 40% are related to genetic mutations in the genes BRCA1 and BRCA2. Despite the increasing demand for genetic testing arising from the patients and their relatives, the genetic testing can be offered yet only to individuals belonging to high-risk families in which the probability that there is a germline mutation in a BRCA gene is high and thus cancer occurrence is likely the expression of a highly penetrant genetic predisposition. In this article, we review how the current knowledge on the biological mechanisms underlying BRCA1 and BRCA2 dysfunction may contribute to the understanding of breast and ovarian cancer predisposition. The most currently employed methods for genetic testing are critically overviewed, together with some indications for the interpretation of the test outcome and the clinical management of mutation carriers.

CDX2 polymorphisms, RNA expression, and risk of colorectal cancer

In adult mammals, CDX2 acts as a transcription factor and is expressed in intestinal epithelial cells. Down-regulation of CDX2 is frequently observed in colorectal cancer, suggesting its loss may cause dedifferentiation of gastrointestinal epithelial cells. However, it is not clear whether inherited variants of CDX2 are associated with risk of colorectal cancer. Using epidemiologic data and tumors from a population-based case-control study in Israel, we identified novel single nucleotide polymorphisms (SNPs) by resequencing 35 cases, compared genotype and haplotype frequencies in 455 matched pairs, and characterized the tumor characteristics of all 455 cases by microsatellite instability analysis, in addition to a partially overlapping set of 201 frozen tumors with expression profiling data (82/201) from the same study. Nine polymorphisms were identified in the 35 cases, and none of the SNPs or haplotypes were associated with risk of colorectal cancer in the 455 matched pairs. These variants were not associated with CDX2 expression in the 83 subjects with expression data. We evaluated subject and tumor characteristics in the 201 subjects with CDX2 tumor expression data. Reduced CDX2 expression was associated with tumor location (right sided), poor differentiation, high microsatellite instability status, and a positive first-degree family history. We conclude that it is unlikely that common CDX2 variants account for a measurable fraction of susceptibility to colorectal cancer in this population. However, CDX2 expression levels were strongly associated with microsatellite instability and tumor location in the gastrointestinal tract, consistent with a possible role in the specification of gastrointestinal epithelial cell fate in humans.

Low allele frequency of MLH1 D132H in American colorectal and endometrial cancer patients

PURPOSE

Hereditary nonpolyposis colon cancer is caused by mutations in DNA mismatch repair genes, predominantly MLH1 and MSH2. Classic MLH1 mutations cause an approximately 20-fold increase in colorectal cancer susceptibility. Recently, we identified a hypomorphic allele, MLH1 D132H , which impairs, but does not completely eliminate the function of MLH1 in tumor suppression. MLH1 D132H confers an approximately fivefold increase in colorectal cancer susceptibility and was first described in a cohort of Israeli colorectal cancer patients, with an estimated allele frequency of 1.3 percent. Because MLH1 D132H has only recently been described, the ethnic distribution of this risk allele is not well understood. This study was undertaken to determine both the frequencies of this risk allele in ethnic groups outside of Israel and whether families harboring this mutation have susceptibility to extracolonic cancers in the hereditary nonpolyposis colon cancer spectrum.

METHODS

We genotyped two independent cohorts: 629 population-based colorectal cancer patients ascertained from clinics in Orange, Imperial, and San Diego Counties, and 515 endometrial cancer patients ascertained from gynecologic oncology clinics in the Midwestern United States.

RESULTS

MLH1 D132H was not detected in either study cohort, which together totaled more than 1,100 American colorectal cancer and endometrial cancer patients.

CONCLUSIONS

The MLH1 D132H risk variant has significantly lower allele frequency in American compared with Israeli cancer patients and, alone, is unlikely to explain significant amounts of American sporadic colorectal cancer or uterine cancer susceptibility. Genetic testing for the MLH1 D132H allele exclusively is therefore unlikely to be cost effective for genetic risk assessment in American population-based and clinic-based colorectal cancer and endometrial cancer patients.

Genetics of hereditary colorectal cancer

Genetic factors can dramatically influence the risk of colorectal cancer, and the molecular bases of many hereditary colorectal cancer syndromes, including familial adenomatous polyposis (FAP), attenuated FAP (AFAP), and hereditary nonpolyposis colorectal cancer (HNPCC) have been elucidated. Additional syndromes continue to be defined as new genes, including MYH , are linked to the development of colonic polyps and cancer. The risks of colorectal cancer are variable and depend on the specific germline alterations. Some mutations are associated with a 100% lifetime risk of developing cancer, while others are associated with only a mild increase in risk. Although there are overlapping clinical features in many of these syndromes, they can be distinguished by the age at cancer diagnosis, inheritance pattern, number and distribution of polyps, specific histologic features of the cancers, and the presence of distinctive extracolonic features. The introduction and refinement of genetic testing has provided a new and invaluable tool for the diagnosis and assessment of cancer risk for suspected cases of hereditary colon cancer.

Characterization of a novel large deletion and single point mutations in the BRCA1 gene in a Greek cohort of families with suspected hereditary breast cancer

BACKGROUND

Germline mutations in BRCA1 and BRCA2 predispose to breast and ovarian cancer. A multitude of mutations have been described and are found to be scattered throughout these two large genes. We describe analysis of BRCA1 in 25 individuals from 18 families from a Greek cohort.

METHODS

The approach used is based on dHPLC mutation screening of the BRCA1 gene, followed by sequencing of fragments suspected to carry a mutation including intron--exon boundaries. In patients with a strong family history but for whom no mutations were detected, analysis was extended to exons 10 and 11 of the BRCA2 gene, followed by MLPA analysis for screening for large genomic rearrangements.

RESULTS

A pathogenic mutation in BRCA1 was identified in 5/18 (27.7 %) families, where four distinct mutations have been observed. Single base putative pathogenic mutations were identified by dHPLC and confirmed by sequence analysis in 4 families: 5382insC (in two families), G1738R, and 5586G > A (in one family each). In addition, 18 unclassified variants and silent polymorphisms were detected including a novel silent polymorphism in exon 11 of the BRCA1 gene. Finally, MLPA revealed deletion of exon 20 of the BRCA1 gene in one family, a deletion that encompasses 3.2 kb of the gene starting 21 bases into exon 20 and extending 3.2 kb into intron 20 and leads to skipping of the entire exon 20. The 3' breakpoint lies within an AluSp repeat but there are no recognizable repeat motifs at the 5' breakpoint implicating a mechanism different to Alu-mediated recombination, responsible for the majority of rearrangements in the BRCA1 gene.

CONCLUSIONS

We conclude that a combination of techniques capable of detecting both single base mutations and small insertions/deletions and large genomic rearrangements is necessary in order to accurately analyze the BRCA1 gene in patients at high risk of carrying a germline mutation as determined by their family history. Furthermore, our results suggest that in those families with strong evidence of linkage to the BRCA1 locus in whom no point mutation has been identified re-examination should be carried out searching specifically for genomic rearrangements.

Genetic factors and colorectal cancer in Ashkenazi Jews

The observed increased incidence of colorectal cancer in Ashkenazi Jews compared to other populations is unexplained but likely has a genetic component. The I1307K APC polymorphism/mutation is carried by 6-8% of Ashkenazim and increases the risk of colorectal cancer 1.5-2 fold. There are few differences between the phenotype of colorectal cancer in I1307K carriers and sporadic cases. It is estimated that the mutation accounts for 6% of cases of colorectal cancer in Jews of Eastern European heritage. It should not be the subject of mass screening in Ashkenazi Jews, although it may be important in cases of familial colorectal cancer. Even rarer is the 1906G-->C MSH2 mutation carried by less than 1% of Ashkenazim, but as with other HNPCC mutations likely associated with a high risk of malignancy. Mutations at 15q13-14 are associated with the colorectal adenoma and carcinoma syndrome (CRAC) described in Ashkenazi families. The prevalence of the mutation is not known, nor its significance as a cause of colorectal cancer. Despite the paucity of genetic explanations for the high risk of colorectal cancer in Ashkenazim, that risk warrants aggressive colorectal cancer screening and particular attention to family history of malignancy in all Jews of Ashkenazi descent.

Frequency of BRCA1 and BRCA2 mutations in unselected Ashkenazi Jewish patients with colorectal cancer

Mutations in BRCA1 and BRCA2 that predispose to breast and ovarian cancer are detected in approximately 2.5% of the Ashkenazi Jewish population. To explore whether carriers of Ashkenazi founder mutations in BRCA1 or BRCA2 have an increased risk for colorectal cancer, we screened 586 unselected Ashkenazi Jewish case patients with colorectal cancer for the three common founder mutations in BRCA1 and BRCA2. We identified six carriers (1.02%) among these case patients. After adjusting for age at diagnosis and sex by use of logistic regression analysis, we compared the incidence of carriers in this group of 586 case patients with that of 5012 Ashkenazi Jewish control subjects without a known history of colorectal cancer. The presence of a founder BRCA mutation was not associated with the risk of colorectal cancer (relative risk = 0.50, 95% confidence interval = 0.22 to 1.14). We thus recommend that counseling for colorectal cancer screening and prevention in individuals with BRCA mutations be based on the personal and family history of colorectal cancer or associated syndromic malignancies.