Journal of Pathology J Pathol 2005; 206: 198–204 Published online 8 April 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/path.1764 Original Paper Homozygosity for a CHEK2 * 1100delC mutation identified in familial colorectal cancer does not lead to a severe clinical phenotype Marjo van Puijenbroek, 1 Christi J van Asperen, 2 Anneke van Mil, 2 Peter Devilee, 1,3 Tom van Wezel 1 and Hans Morreau 1 * 1 Department of Pathology, Leiden University Medical Centre, The Netherlands 2 Department of Human and Clinical Genetics, Leiden University Medical Centre, The Netherlands 3 Department of Human Genetics, Leiden University Medical Centre, The Netherlands *Correspondence to: Dr Hans Morreau, Leiden University Medical Centre, Department of Pathology, Building L1Q, PO Box 9600, 2300 RC Leiden, The Netherlands. E-mail: J.Morreau@lumc.nl Received: 2 December 2004 Revised: 7 January 2005 Accepted: 1 February 2005 Abstract It has recently been suggested that the frequency of the germline CHEK2 * 1100delC mutation is higher among breast cancer families with colorectal cancer, although the mutation does not seem to be significantly associated with familial colorectal cancer. Five hundred and sixty-four familial colorectal tumours were studied for expression of CHEK2 using tissue microarrays and an antibody against the NH 2 -terminal SQ regulatory domain of the CHEK2 protein. Normal colonic tissue from patients whose tumours showed loss of CHEK2 expression was investigated further using fragment and sequence analysis for the presence of a CHEK2 * 1100delC mutation and five other (R117G, R137Q, R145W, I157T, and R180H) known germline variants in CHEK2. Twenty-nine tumours demonstrated loss of expression for CHEK2. Analysis of matched normal colonic tissue from these patients revealed germline CHEK2 * 1100delC mutation in three cases. In two of these, the mutation was heterozygous but, interestingly, the third patient proved to be homozygous for the deletion, using six different primer pair combinations. None of the other tested germline variants were identified. No CHEK2 * 1100delC mutations were found in patients whose tumours stained positive. Homozygosity for the CHEK2 * 1100delC mutation appears not to be lethal in humans. No severe clinical phenotype was apparent, although the patient died from colonic carcinoma at age 52 years. This observation is in line with recent knockout mouse models, although in the latter, cellular defects in apoptosis and increased resistance to irradiation seem to exist. It is also concluded that CHEK2 protein abrogation is not caused by the CHEK2 germline variants R117G, R137Q, R145W, I157T, and R180H in familial colorectal cancer. Copyright  2005 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd. Keywords: TMA; familial colorectal cancer; HNPCC; CHEK2 Introduction CHEK2 on chromosome 22q is the human homologue of the yeast Cds1 and Rad53 G2 checkpoint kinases. The Chek2/Rad53/Cds1 family of proteins identifies DNA damage in eukaryotic cells [1]. Pseudo-genes of exons 10–14 of CHEK2 are found on chromo- somes 15 and 16 and, with lower homology, on chromosomes 2, 10, 13, X, and Y [2,3]. The pro- tein truncating mutation CHEK2 ∗ 1100delC, present in exon 10 of the functional gene on chromosome 22q, abolishes the kinase function of CHEK2 [4,5]. The role of the CHEK2 ∗ 1100delC and other germline variants has been well studied in breast cancer. The 1100delC allele has been claimed to be a low pene- trance susceptibility allele for breast cancer and carri- ers appear to have a two-fold increase in breast can- cer risk [6]. CHEK2 protein is abrogated or reduced to a large extent in breast tumours of heterozygous CHEK2 ∗ 1100delC mutation carriers [7–9]. The inci- dence of the 1100delC mutation has been suggested to be higher among breast cancer families with col- orectal cancer than in those without colorectal cancer, identifying a hereditary breast and colorectal cancer (HBCC) phenotype [10]. Recently, the incidence of the CHEK2 ∗ 1100delC mutation in familial and non- familial colorectal cancer (CRC) patients was deter- mined to be 1.3% and 2.9%, respectively, which is not significantly higher than the 1.1–1.4% frequency with which this allele is found in the healthy Euro- pean population studied so far. With an estimated range of 1.3 – 1.6%, this frequency seems similar in the Dutch population [11]. These results suggest that the CHEK2 ∗ 1100delC mutation may not be significantly associated with familial colorectal cancer or with col- orectal cancer risk in the population, although a very Copyright  2005 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.