# Integrated Molecular Profiling of Colorectal Cancer by Tumor Location: Evidence from a Real-World Cohort with Primary and Metastatic Samples

## TL;DR

This study shows that colorectal cancer tumors on the right and left sides of the colon have different genetic and molecular features, which can affect diagnosis and treatment.

## Contribution

The study provides new insights into how tumor location correlates with specific molecular profiles in primary and metastatic colorectal cancer samples.

## Key findings

- Right-sided tumors are more likely to have DNA repair deficiencies, high mutation rates, and specific gene alterations compared to left-sided tumors.
- Left-sided tumors show more ERBB2 alterations and class 5 APC mutations, while right-sided tumors are associated with dMMR, MSI-H, and higher TMB.
- Metastatic samples were predominantly MSS/pMMR, highlighting differences in molecular profiles between primary and metastatic lesions.

## Abstract

Colorectal cancer is a biologically diverse disease, and growing evidence suggests that tumors arising on the right or left side of the colon are biologically and genetically distinct. These differences can influence both prognosis and treatment decisions. In this study, we examined tumor samples from 78 patients, including both primary tumors and metastatic lesions, to investigate the relationship between tumor location and key molecular features, such as microsatellite instability, mutation profiles, and expression of therapeutic targets. We found that right-sided tumors were more likely to have DNA repair deficiencies, a high number of specific mutations, and gene alterations that differ significantly from left-sided tumors. Understanding these patterns helps refine how we classify and treat colorectal cancer in clinical practice. Our findings highlight the need for location-informed molecular testing to support more personalized and effective cancer care.

Background/Objectives: Colorectal cancer (CRC) shows significant molecular diversity influenced by tumor location. Right- and left-sided CRCs differ in terms of microsatellite instability (MSI), mutational burden, and actionable biomarkers. This study aimed to characterize the clinicopathological and molecular features of CRC stratified upon tumor location. Methods: A consecutive series of CRC cases was retrospectively analyzed. Tissue samples were obtained from primary tumors (71%) or metastatic lesions (29%). All cases were evaluated by histopathology, immunohistochemistry (IHC), and targeted next-generation sequencing (NGS). Tumor location was assigned based on the primary tumor (43 right-sided and 35 left-sided cases). Results: Right-sided CRCs were more frequent in older patients and females and showed higher rates of deficient MMR (42% vs. 17%, p = 0.02), MSI-H (39% vs. 14%, p = 0.02), and high tumor mutational burden (TMB-high, ≥10 Mutations/Mb, 56% vs. 28%, p = 0.02). The most frequent pathogenic class 5 mutations were TP53 (65%), APC (49%), and KRAS (44%). APC was the most frequently mutated gene in both pathogenic (class 5) and likely pathogenic (class 4) categories, with class 5 variants more common in left-sided tumors and class 4 variants predominating in right-sided tumors. BRAF mutations showed a statistically significant trend toward higher frequency in right-sided tumors (p = 0.05). HER2/neu overexpression (3+) was seen in 15% of patients, exclusively in MSS left-sided tumors. PD-L1 expression (CPS ≥ 1) was detected in 20% of patients, irrespective of location, and pan-TRK IHC was negative in all cases. The 29% of samples derived from metastatic lesions were predominantly MSS/pMMR (87%). Conclusions: Tumor location in CRC correlates with distinct molecular patterns. Right-sided tumors are associated with dMMR, MSI-H, and higher TMB, while left-sided CRCs display more ERBB2 alterations and class 5 APC mutations. The results highlight the importance of integrating tumor location into personalized molecular diagnostics and therapeutic planning for CRC patients.