SMAD4 has shown promise in identifying patients with colorectal cancer (CRC) at high risk of recurrence or death.

A discovery cohort and independent validation cohort were classified by SMAD4 status. SMAD4 status and immune infiltrate measurements were tested for association with recurrence-free survival (RFS). Patient-derived xenografts from SMAD4-deficient and SMAD4-retained tumors were used to examine chemoresistance.

The discovery cohort consisted of 364 patients with stage I-IV CRC. Median age at diagnosis was 53 years. The cohort consisted of 61% left-sided tumors and 62% stage II/III patients. Median follow-up was 5.4 years (interquartile range, 2.3 - 8.2). SMAD4 loss, noted in 13% of tumors, was associated with higher tumor and nodal stage, adjuvant therapy use, fewer tumor-infiltrating lymphocytes (TIL) and lower peritumoral lymphocyte aggregates (PLA) scores (all p<0.04). SMAD4 loss was associated with worse RFS (p=0.02). When stratified by SMAD4 and immune infiltrate status, patients with SMAD4 loss and low TIL or PLA had worse RFS (p=0.002 and p=0.006, respectively). Among patients receiving 5-fluorouracil-based systemic chemotherapy, those with SMAD4 loss had a median RFS of 3.8 years compared with 13 years for those patients with SMAD4 retained. In xenografted mice, the SMAD4-lost tumors displayed resistance to 5- fluorouracil. An independent cohort replicated our findings, in particular the association of SMAD4 loss with decreased immune infiltrate, as well as worse disease-specific survival.

Our data show SMAD4 loss correlates with worse clinical outcome, resistance to chemotherapy, and decreased immune infiltrate-supporting its use as a prognostic marker in CRC patients.