Genomic alterations in signet ring and mucinous patterned colorectal carcinoma.

Background: The genetic alterations (GAs) in two specific histological subtypes of colorectal cancer (CRC), signet ring cell colorectal carcinoma (SRC) and mucinous colorectal carcinoma (MC), are not well known. In the present study, we employed next-generation sequencing to perform genetic profiling of SRC and MC, and compared the spectrum of GAs with the alterations found in conventional type colorectal cancer (CON).

Materials And Methods: We selected 46 CRCs comprising 17 SRCs and mucinous carcinoma with signet ring cell component (SRCCs), 17 MCs, and 12 CONs with microsatellite stability or microsatellite instability-low. Deep sequencing was performed using a targeted cancer panel composed of 171 cancer-related genes. SMAD4 protein expression was evaluated by immunohistochemical staining.

Results: We detected 108 mutations in 18 different genes. Overall, 2.34 GAs were detected per tumor (range, 0-14). The overall frequency of GA and alteration in targetable genes was less prevalent in SRC/SRCC compared to the frequency of alteration in MC/CON (p = 0.040 and p < 0.001, respectively). The GA profile of SRC/SRCC included TP53 (8/17, 47.1%), SMAD4 (5/17, 29.4%), KRAS (4/17.23.5%), APC (4/17.23.5%), PIK3CA, ATM, BRAF, and PIK3R1 (1/17, 5.9%, each). KRAS mutation was significantly less prevalent in SRC/SRCC compared to the number of KRAS mutations in MC (12/17, 70.6%) and CON (9/12, 75.0%) (p = 0.015 and 0.01, respectively). Compared to the 152 non-hypermutated CONs from TCGA database, SMAD4 alteration was predominant in SRC/SRCC (p = 0.045) with aberrant loss of SMAD4 expression (13/17, 76.5%) compared to the SMAD4 alterations in CON (5/15, 33.3%) (p = 0.031). Accordingly, KRAS (12/17, 70.6%), APC (6/17, 35.3%), SMAD4, TP53 (4/17, 23.5%, each), PIK3CA (3/17, 17.6%), AKT1, ATM, BRAF, EGFR, and EZH2 (1/17, 5.9%, each) were altered in MC. APC and TP53 mutations were less frequent in MC compared to those in TCGA-CON (p < 0.001 and 0.003, respectively) whereas KRAS mutation was prevalent (p = 0.041).

Conclusion: Alterations of known cancer associated genes and targetable genes in SRC/SRCC are infrequent. The profile of GAs in SRC/SRCC and MC differs from the GA profile of CON. Specifically, SMAD4 mutation and loss of SMAD4 expression is frequently found in SRC/SRCC. The genetic profiles revealed in the present study may aid in developing precision medicine for CRC treatment based on histological subtype.