# Effect of chemotherapy on passenger mutations in metastatic colorectal cancer

## TL;DR

Chemotherapy in metastatic colorectal cancer reduces passenger mutations, which may affect immunotherapy response and treatment outcomes.

## Contribution

The study reveals chemotherapy's impact on passenger mutations and their potential role in influencing immunotherapy response in metastatic colorectal cancer.

## Key findings

- Passenger mutations in large genes like TTN are enriched in chemonaïve mCRC tumors and linked to higher tumor mutational burden.
- Chemotherapy-treated samples show a significant reduction in passenger mutations, indicating depletion of hypermutated subclones.
- Passenger mutation patterns may serve as biomarkers for treatment response and immunotherapy considerations in MSS mCRC.

## Abstract

Metastatic colorectal cancer (mCRC), particularly microsatellite stable (MSS) cases, often exhibits limited responsiveness to immunotherapy, leaving chemotherapy as the primary treatment option. While chemotherapy effectively targets tumor cells, its impact on the broader mutational landscape, including passenger mutations in large genes such as Titin (TTN), remains poorly understood. Passenger mutations, traditionally deemed biologically inert, may reflect tumor mutational burden (TMB) and influence treatment outcomes. In our study involving whole exome sequencing of paired primary and metastatic tumor samples from 22 mCRC patients, recurrent driver mutations in APC, KRAS, and TP53 were consistently observed. However, passenger mutations in large genes, particularly TTN, were notably enriched in chemonaïve specimens and associated with higher TMB. Chemotherapy‐treated samples exhibited a significant reduction in these mutations, suggesting selective depletion of hypermutated subclones. Our findings demonstrate that chemotherapy may selectively reduce passenger mutations in mCRC, potentially influencing the persistence of hypermutated subclones. This highlights the potential role of passenger mutation patterns and TMB as biomarkers for treatment response and raises the hypothesis that they could help guide immunotherapy considerations for patients with MSS mCRC.

Changes in passenger mutation load and predicted immunotherapy response after chemotherapy treatment. Tumor cells rich with passenger mutations have increased sensitivity to chemotherapy. Correlation of passenger mutations with neoantigen load suggests highly mutated clones promote a more effective response to immunotherapy, and therefore, first‐line chemotherapy may limit response to subsequent immunotherapy due to selective loss of highly mutated clones.