Low level microsatellite instability may be associated with reduced cancer specific survival in sporadic stage C colorectal carcinoma.

Background: Colorectal cancers (CRCs) may be categorised according to the degree of microsatellite instability (MSI) exhibited, as MSI-high (MSI-H), MSI-low (MSI-L), or microsatellite stable (MSS). MSI-H status confers a survival advantage to patients with sporadic CRC.

Aims: To determine if low levels of MSI are related to the clinicopathological features and prognosis of sporadic stage C CRC.

Patients: A total of 255 patients who underwent resection for sporadic stage C CRC were studied. No patient received chemotherapy. Minimum follow up was five years.

Methods: DNA extracted from archival malignant and non-malignant tissue was amplified by polymerase chain reaction using a panel of 11 microsatellites. MSI-H was defined as instability at > or =40% of markers, MSS as no instability, and MSI-L as instability at >0% but <40% of markers. Patients with MSI-H CRC were excluded from analysis as they have previously been shown to have better survival.

Results: Thirty three MSI-L and 176 MSS CRCs were identified. There was no difference in biological characteristics or overall survival of MSI-L compared with MSS CRC but MSI-L was associated with poorer cancer specific survival (hazard ratio 2.0 (95% confidence interval 1.1-3.6)).

Conclusions: Sporadic MSI-L and MSS CRCs have comparable clinicopathological features. Further studies are required to assess the impact of MSI-L on prognosis.