Background:

Recent analysis of CALGB 80405 showed that left sided colon tumors (LT) respond differently to biologics compared with right-sided tumors, likely due to molecular differences. Molecular variations between LT and rectal tumors remain undefined. Herein, we report our exploration of these variations.

Methods:

Tumors with origins clearly defined as splenic flexure to descending colon (SFT), sigmoid colon (SgT), or rectum (RT) were included. Protein expression, gene amplification and NextGen sequencing was tested. Microsatellite instability (MSI) was measured by PCR. Tumor mutational load (TML) was calculated using only somatic nonsynonymous missense mutations. Chi-square tests were used for comparative analyses.

Results:

In total, 1,457 primary tumors (SFT 125; SgT 460, and RT 872) were examined. When compared with SFT, RT had a higher frequency of TP53 (71% vs. 57%, p = 0.03) and APC (66% vs. 49%, p = 0.01); a lower frequency of PIK3CA (11% vs. 22%, p = 0.02), BRAF (3% vs. 15% p = 0.0001), GNAS (0.9% vs. 4%, p = 0.04), HNF1A(0.7% vs. 5%, p = 0.01), and CTNNB1(0.3% vs. 4%, p = 0.003); and a higher expression of TOPO1 (52% vs. 31%, p = 0.0001), ERCC1 (29% vs. 15%, p = 0.03), and MGMT (64% vs. 53%, p = 0.048). When compared with SgT, RT had higher expression of TLE3 (33% vs. 23%, p = 0.007), TOPO1 (52% vs. 35%, p < 0.001), TUBB3 (41% vs. 28%, p = 0.003), and MGMT (64% vs. 54%, p = 0.003). There were no differences between SFT, SgT, and RT in the frequency of PD-L1 expression (5%, 2%, and 2%) on tumor cells, PD-1 expression on tumor-infiltrating lymphocytes (54%, 42%, and 42%), or Her-2 expression (1%, 2%, and 3%) and amplification (3%, 3%, and 5%). MSI was seen in 7% of SFT, 4% of SgT, and 0.7% of RT (total LT vs. RT, p = 0.01). Mean TML was 23, 6.5, and 7 mutations (mut)/MB (332 tumors), and the portion of tumors carrying a TML of > 17mut/MB was 9%, 1.6%, and 4% for SFT, SgT, and RT, respectively. In all 3 cohorts, a TML > 17 mut/MB was highly concordant with MSI. There was a correlation between PD-1 and TML in RT (p = 0.04) but not in SFT or SgT. There were no correlations between PD-L1 and TML.

Conclusions:

Tumors arising in the rectum may carry genetic alterations that are distinct from LT. A better understanding of disease biology may help to identify therapeutic targets and advance precision medicine