At least 10% of both breast cancers and colorectal cancers are estimated to develop because of heredity. The BRCA1 and BRCA2 genes are the major causes of hereditary breast and ovarian cancer, but the underlying genetic defect remains unresolved in the majority of the families with familial or hereditary breast cancer 12. In colorectal cancer, familial adenomatous polyposis due to mutations in APC and the hereditary nonpolyposis colorectal cancer (HNPCC) syndrome caused by mutations in the mismatch-repair genes MLH1, MSH2, MSH6 and PMS2 are the underlying causes of many families with hereditary colorectal, whereas the genetic defects remains unknown in most of the families with hereditary colorectal cancer with higher age at onset and lack of other associated tumor types 34. Searches for additional high-penetrant disease-causing genes in these cancer types have so far not been successful and it is likely that low-penetrant genetic variants may contribute a substantial fraction of hereditary breast cancer and colorectal cancer 35.

Double-strand DNA breaks lead to activation of the cell cycle checkpoint kinase 2, CHEK2 [GenBank:AL121825], through ATM phosphorylation 6. CHEK2 plays an important role in cell cycle regulation and DNA damage repair, processes that are central in prevention of tumor development. The CHEK2 1100delC mutation leads to premature termination and abrogates kinase activity. This variant allele occurs in about 1% of normal populations, but has been identified at increased frequencies (1.2–1.9%) in individuals with breast cancer 78. An approximately 2-fold increased risk of breast cancer has been associated with the CHEK2 1100delC mutation 8. Higher frequencies, 5.1–11.4%, of the CHEK2 1100delC allele have by some investigators been reported in non-BRCA1/2 families with hereditary breast cancer 891011, although other studies from e.g. Australia and Spain have not found an increased frequency in such families 1213. These differences may be ascribed to geographical variations as well as to different sample sets, particularly regarding the number of family members affected, age at onset, and selection of e.g. families with male breast cancer. Since the CHEK2 1100delC variant does not show clear co-segregation with disease it is considered to be a modifier, and may act in cooperation with yet unidentified high-penetrance genes or together with multiple low-penetrant genes as part of polygenic inheritance.

In studies from Finland and the Netherlands the CHEK2 1100delC has been reported at frequencies of 1.6–2.6% among individuals with colorectal cancer, which is not significantly higher than in unaffected individuals from the population, but these studies may be compatible with a risk of 1.5–2.0 for development of colorectal cancer among carriers 141516. A particularly high frequency of the CHEK2 1100delC allele has been reported in families with hereditary breast and colorectal cancer where 10/55 Dutch families (18%) carried the mutation 16.

Since development of multiple primary tumors in an individual is a sign of hereditary cancer we utilized a population-based cancer registry to identify women who had developed breast cancer and colorectal cancer in order to assess the contribution of the CHEK2 1100delC mutation to the development of metachronous cancers of these types.

Of the 84 patients, 75 (89%) were successfully analyzed. In total, 36 breast cancers and 67 colorectal cancers were analyzed and from 34 patients normal tissue was also analyzed. The majority of the patients developed breast cancer as their first tumor (table 1) and 68 of the patients developed other malignant tumors, most commonly malignant melanoma (n = 18), endometrial cancer (n = 12) and urinary bladder cancer (n = 6). The CHEK2 1100delC mutation was detected in 2 patients (2.5%) (table 2). Both the breast cancer and the colorectal cancer was analyzed and found to carry the mutation in these cases and, in case 17 the mutation was verified also in normal tissue. Chromatograms over the results are shown in figure 1.

In order to determine the expected frequency of the CHEK2 1100delC in the Southern swedish population dHPLC analysis and direct sequencing was performed from 300 healthy individuals with identification of the variant allele in 3/300 individuals, thus at a population frequency of 1% 18.

A subtype of familial breast cancer that includes colorectal cancer was recognized by Lynch et al. already in 1972 19 and recently the CHEK2 1100delC mutation was proposed to represent a low-penetrant breast cancer susceptibility allele 1011. This variant has been identified at a particularly high frequency (18%) in families with a hereditary breast- and colorectal cancer phenotype 16. Since development of multiple primary tumors is a hallmark of hereditary cancer and a high frequency of the CHEK2 1100delC had been described in hereditary breast and colorectal families, we assessed the contribution of this variant to the development of double primary breast and colorectal cancer in a population-based patient material. Among the 75 patients with metachronous tumors of the breast and the colorectum successfully studied, 2 (2.5%) carried the CHEK2 1100delC mutation compared to 1% in the control group 18. These frequencies were not significantly different (p = 0.26), but the small size of the population-based patient material limits the strength of this comparison. However, the low frequency of this alteration in our material implies that development of metachronous breast cancer and colorectal cancer in women is per se not alone to identify individuals with a high likelihood of being carriers of the CHEK2 1100delC mutation. Although development of double primary tumors may be a sign of heredity it is not enough to recommend genetic analysis, though the family history of cancer should be carefully reviewed. Our findings are in line with previous studies that exclude CHEK2 1100delC as a major contributor to the breast and colorectal cancer phenotype 16. Huang et al. 20 did not identify any CHEK2 1100delC mutation among 24 patients from the US with breast cancer and colorectal cancer. Also, in the study by Meijers-Heijboer et al. that primarily identified the link between CHEK2 1100delC and familial breast and colorectal cancer in Dutch families, the vast majority, 45/55 families, who fulfilled these criteria did indeed not carry this variant 16. However geographical differences may influence the importance of the CHEK 1100delC mutation.

Among the two women with the CHEK2 1100delC allele in our study, one presented with two separate, synchronous, breast cancers. An increased risk of multiple primary breast cancers (OR 5.7–6.5) has been reported in individuals carrying this CHEK2 1100delC allele 112122. However, the CHEK2 1100delC mutation has not only been suggested to act as a low-penetrant susceptibility gene in breast cancer families, but the CHEK2 I157T alteration has been proposed to act as a multiorgan cancer susceptibility allele based on observations of an increased risk for development of breast cancer, colon cancer, prostate cancer, thyroid cancer, and renal cancer in Polish families 23. Both patients in our study, did in addition to breast cancer and colorectal cancer, develop a third tumor, two renal cancers at ages 49 and 73 (table 2). In the Swedish cancer registry 8–10% of the patients develop 2 or more malignancies (The Board of Social Health and Welfare, Cancer Incidence in Sweden 2003). The development of renal cancer in two of our patients was intriguing in relation to Cybulski et al. who identified an increased risk of renal cancer associated with CHEK2, albeit in carriers of the CHEK2 I157T variant 23. However, since our study was registry-based no data on additional cancer cases in these families are available.

In summary, our findings demonstrate that the CHEK2 1100delC occurs at a low frequency in Swedish women with double primary breast cancer and colorectal cancer, and thus suggests that development of these two tumor types is not sufficient to recommend mutation analysis of CHEK2.

The author(s) declare that they have no competing interests.

AI conceived of the study, performed the sequencing analysis, interpret data and drafted the manuscript. MB also carried out the sequencing. AB participated in its design and helped to draft the manuscript. MN conceived of the study, participated in its design and coordination and helped to draft the manuscript. All authors read and approved the manuscript.