SMAD4 loss in colorectal cancer: Correlation with recurrence, chemoresistance, and immune infiltrate

587 Background: Few markers reliably identify colorectal cancer (CRC) patients at risk of recurrence and death. SMAD4 loss occurs in 10-20% of cases and has shown promise in identifying high-risk stage II/III patients. We examined SMAD4 status and association with clinical/pathologic features in 446 stage I-IV CRC patients at Memorial Sloan Kettering (MSK). Methods: Patients undergoing curative resection were included (1981-2010). Familial polyposis syndrome patients and those with inadequate tissue were excluded. Tissue microarrays were constructed (n=364). Immunohistochemistry for SMAD4 and mismatch repair (MMR) proteins was completed. SMAD4 nuclear stain intensity was scored (scale=0-3; 0=loss). On whole sections, MMR proteins (present or absent), tumor-infiltrating lymphocytes (TILs) and peritumoral lymphocyte aggregates (PLAs) were scored (scale=0-3). Associations between clinical/pathologic features and SMAD4 loss vs. retention were analyzed. Kaplan-Meier estimates and log-rank test were used for recurrence-free and overall survival analyses (RFS and OS). Results: SMAD4 loss was noted in 13%. Median age at diagnosis was 53 years, and 51% were male. The cohort consisted of 61% hindgut tumors and 62% stage II/III patients. With up to 33 years of follow-up, the mean was 6 years. SMAD4 loss correlated with higher tumor and nodal stage, adjuvant therapy use, and lower TIL and PLA scores (pmedian) were noted to have worse OS with SMAD4 loss (p<0.01). SMAD4 loss did correlate with worse RFS (p=0.02), persisting even when excluding MMR-deficient patients. Additionally, SMAD4 loss was associated with worse RFS in both the adjuvant chemotherapy group (median RFS=3.8 vs. 13 years; p=0.06) and the resection-only group (median RFS=4.2 years vs. not yet reached; p< 0.01). Conclusions: SMAD4 loss correlates with worse RFS and resistance to adjuvant therapy. SMAD4 loss also correlates with lower TIL and PLA scores. Future work will address chemoresistance mechanisms, relevance to adjuvant therapy use, and loss of immune infiltrate in SMAD4-null tumors