Loughrey M B, Waring P M, Tan A, Trivett M, Kovalenko S, Beshay V, Young M-A, McArthur G, Boussioutas A, Dobrovic A
Molecular Pathology Research Laboratory, Peter MacCallum Cancer Centre, Melbourne, VIC, Australia.
Fam Cancer. 2007;6(3):301-10. doi: 10.1007/s10689-007-9124-1. Epub 2007 Apr 24.
Patients suspected on clinical grounds to have hereditary non-polyposis colorectal cancer (HNPCC) may be offered laboratory testing in order to confirm the diagnosis and to facilitate screening of pre-symptomatic family members. Tumours from an affected family member are usually pre-screened for microsatellite instability (MSI) and/or loss of immunohistochemical expression of mismatch repair (MMR) genes prior to germline MMR gene mutation testing. The efficiency of this triage process is compromised by the more frequent occurrence of sporadic colorectal cancer (CRC) showing high levels of MSI (MSI-H) due to epigenetic loss of MLH1 expression. Somatic BRAF mutations, most frequently V600E, have been described in a significant proportion of sporadic MSI-H CRC but not in HNPCC-associated cancers. BRAF mutation testing has therefore been proposed as a means to more definitively identify and exclude sporadic MSI-H CRC cases from germline MMR gene testing. However, the clinical validity and utility of this approach have not been previously evaluated in a familial cancer clinic setting. Testing for the V600E mutation was performed on MSI-H CRC samples from 68 individuals referred for laboratory investigation of suspected HNPCC. The V600E mutation was identified in 17 of 40 (42%) tumours showing loss of MLH1 protein expression by immunohistochemistry but in none of the 28 tumours that exhibited loss of MSH2 expression (P < 0.001). The assay was negative in all patients with an identified germline MMR gene mutation. Although biased by the fact that germline testing was not pursued beyond direct sequencing in many cases lacking a high clinical index of suspicion of HNPCC, BRAF V600E detection was therefore considered to be 100% specific and 48% sensitive in detecting sporadic MSI-H CRC amongst those cases showing loss of MLH1 protein expression, in a population of patients with MSI-H CRC and clinical features suggestive of HNPCC. Accordingly, we recommend the incorporation of BRAF V600E mutation testing into the laboratory algorithm for pre-screening patients with suspected HNPCC, whose CRCs show loss of expression of MLH1. In such tumours, the presence of a BRAF V600E mutation indicates the tumour is not related to HNPCC and that germline testing of MLH1 in that individual is not warranted. We also recommend that in families where the clinical suspicion of HNPCC is high, germline testing should not be performed on an individual whose CRC harbours a somatic BRAF mutation, as this may compromise identification of the familial mutation.
临床上怀疑患有遗传性非息肉病性结直肠癌（HNPCC）的患者可接受实验室检测，以确诊并便于对无症状的家庭成员进行筛查。在进行种系错配修复（MMR）基因突变检测之前，通常会对受影响家庭成员的肿瘤进行微卫星不稳定性（MSI）预筛查和/或MMR基因免疫组化表达缺失检测。由于MLH1表达的表观遗传缺失，散发性结直肠癌（CRC）中高MSI（MSI-H）的情况更为常见，这影响了这种分类过程的效率。在相当一部分散发性MSI-H CRC中发现了体细胞BRAF突变，最常见的是V600E，但在HNPCC相关癌症中未发现。因此，有人提出进行BRAF突变检测，以便更明确地从种系MMR基因检测中识别和排除散发性MSI-H CRC病例。然而，这种方法的临床有效性和实用性此前尚未在家族性癌症诊所环境中进行评估。对68例因疑似HNPCC而转诊进行实验室检查的MSI-H CRC样本进行了V600E突变检测。在40例（42%）通过免疫组化显示MLH1蛋白表达缺失的肿瘤中，有17例检测到V600E突变，但在28例显示MSH2表达缺失的肿瘤中均未检测到（P<0.001）。在所有已鉴定出种系MMR基因突变的患者中，该检测均为阴性。尽管在许多临床高度怀疑HNPCC的病例中，种系检测仅进行了直接测序，存在一定偏差，但在一组具有MSI-H CRC且临床特征提示HNPCC的患者中，BRAF V600E检测在检测显示MLH1蛋白表达缺失的病例中的散发性MSI-H CRC时，被认为特异性为100%，敏感性为48%。因此，我们建议将BRAF V600E突变检测纳入实验室算法，用于对疑似HNPCC且CRC显示MLH1表达缺失的患者进行预筛查。在这类肿瘤中，BRAF V600E突变的存在表明该肿瘤与HNPCC无关，该个体无需进行MLH1的种系检测。我们还建议，在临床高度怀疑HNPCC的家族中，对于CRC存在体细胞BRAF突变的个体，不应进行种系检测，因为这可能会影响家族性突变的识别。
