Higher risk of mismatch repair-deficient colorectal cancer in α₁-antitrypsin deficiency carriers and cigarette smokers

Microsatellite instability (MSI) is a genomic alteration observed in 15-30% of colorectal cancer (CRC). Two MSI phenotypes have been defined for CRC: MSI-H is characterized by MSI at ≥30% of the examined loci and MSI-L by MSI at 1-30% of the loci. An absence of MSI at any examined loci has been defined as a microsatellite stable (MSS) phenotype. Current data suggest the majority of MSI tumors are the result of defective DNA mismatch repair (MMR). In this study, we have determined the α₁-antitrypsin deficiency carrier (α₁ATD-ht) status of 161 CRC patients whose MSI phenotype and protein expression states had previously been determined. Cases were selected to enrich a larger number of MSI-H cases. Among 51 CRC patients with MSI-H tumors, the α₁ATD-ht rate was 21.6%; among 110 patients with MSI-L/MSS tumors, the rate was 9.1% (MSI-H vs MSI-L/MSS. P = 0.02): and among the 191 population-based controls the α₁ATD-ht rate was 9.4.% (MSI-H vs controls, P = 0.02). The estimated relative risk of having MSI-H CRC among α₁ATD-ht was 3.1 after adjusting for age, gender, and smoking history. The risk of having MSI-H CRC among current and past smokers was 6.6 and 2,7, respectively. Patients who were α₁ATD-ht and smoked had a 20-fold increased risk of developing an MSI-H CRC compared to nonsmokers who were homozygous normal at the α₁ATD locus. Our findings suggest an etiologic link between α₁ATD alleles and development of CRC with defective MMR, and a synergistic effect between smoking and α₁ATD allele in the development of MSI-H CRC.