Microsatellite instability and the clinicopathological features of sporadic colorectal cancer

Abstract

Background and aims: In this study, we prospectively examined the clinical significance of the microsatellite instability (MSI) phenotype in sporadic colorectal cancer, and investigated methods for effective identification of these tumours in routine pathology practice.

Methods: DNA was extracted from 310 tumours collected from 302 consecutive individuals undergoing curative surgery for sporadic colorectal cancer. Microsatellite status was determined by polymerase chain reaction amplification using standard markers, while immunostaining was used to examine expression of MLH1, MSH2, and p53.

Results: Eleven per cent of tumours showed high level instability (MSI-H), 6.8% had low level instability (MSI-L), and the remainder were stable. MSI-H tumours were significantly more likely to be of high histopathological grade, have a mucinous phenotype, and to harbour increased numbers of intraepithelial lymphocytes. They were also more likely to be right sided, occur in women, and be associated with improved overall survival. In total, 25 (8%) tumours showed loss of staining for MLH1 and a further three tumours showed absence of staining for MSH2. The positive and negative predictive value of immunohistochemistry in the detection of MSI-H tumours was greater than 95%.

Conclusions: We conclude that the MSI-H phenotype constitutes a pathologically and clinically distinct subtype of sporadic colorectal cancer. Immunohistochemical staining for MLH1 and MSH2 represents an inexpensive and accurate means of identifying such tumours.

Figure 1

Microsatellite analysis of paired normal (N) and tumour (T) DNA using Bat 26 (A, B), and D5S346 (C, D). The tumours in (A), (C), and (D) are unstable. The tumour in (B) (stable) has a germline polymorphism that is only identifiable as such by the presence of the same pattern in normal tissue. (C) (laddering) and (D) (new allele) are representative of the patterns of changes observed at the dinucleotide markers. Size in base pairs is indicated, and the scale on the right shows signal amplitude.

Figure 2

Kaplan-Meier survival curves for individuals with colorectal carcinoma. (A, B) Survival according to microsatellite instability status in stage II and stage III tumours, respectively. (C) Overall survival as a function of sex. MSI-H, high level of microsatellite instability; MSS, microsatellite stable.

Figure 3

Typical appearance of tumours showing loss of mismatch repair enzymes, as demonstrated by immunohistochemistry for MLH1. (A) Poorly differentiated carcinoma cells (upper) show no nuclear staining for MLH1 while normal crypt epithelium (lower left) and stromal lymphocytes show nuclear positivity (brown stain); bar=500 µm. (B) Details of negative tumour cell staining in the presence of dark staining positive lymphocytes, both within tumour epithelium and in surrounding stroma. Normal epithelium is seen in the lower left; bar=100 µm; immunoperoxidase with haematoxylin counterstain.