Understanding the Mechanism of a Gut Microbial Genotoxin Involved in Colorectal Carcinogenesis

PROJECT SUMMARY
Colorectal cancer (CRC) is the third most prevalent form of cancer in the US and the second leading cause of
cancer deaths. Studies over the last several decades have revealed that the gut microbiota influences CRC, and
recent work has implicated gut bacterial genotoxins as key effectors in cancer development and progression.
One of the bacterial genotoxins most strongly connected to cancer is colibactin, a metabolite produced by human
gut commensal bacteria, including certain E. coli strains, that possess a biosynthetic gene cluster termed the
pks island. The increased prevalence of pks+ E. coli in CRC patients and the ability of pks+ strains to potentiate
tumorigenesis in mouse models of CRC suggest colibactin may play a causal role in cancer progression.
However, achieving a mechanistic understanding of colibactin's genotoxicity and contribution to CRC has been
impeded by an inability to isolate and structurally characterize the active genotoxic metabolite(s). During the
previous funding period of this grant, we gained critical information about colibactin's structure and mode of
action by studying the enzymes involved in its biosynthesis. Most notably, we established that colibactin is a
DNA alkylating agent and proposed a potential structure for the active genotoxin. The overall objective of this
renewal application is to elucidate additional molecular mechanisms underlying colibactin's activity and role in
CRC carcinogenesis. Building off of the central hypothesis that the genotoxic activity of colibactin derives from
the formation of DNA interstrand cross-links (ICLs), our three specific aims will: 1) characterize the specificity
and structure of colibactin-DNA ICLs; 2) develop small molecules that inhibit colibactin biosynthesis in microbial
communities; 3) elucidate the physiological location and timing of colibactin-mediated DNA damage in CRC
development. These advances will be enabled by our multidisciplinary approach, which merges knowledge and
techniques from chemical biology, structural biology, microbiology, toxicology, and cancer biology. Overall, this
effort will fill critical gaps in fundamental knowledge needed to elucidate the role of colibactin-producing gut
bacteria in CRC carcinogenesis and ultimately impact cancer prevention, diagnosis, and treatment. Additionally,
by successfully demonstrating that studying and manipulating individual disease-associated microbial pathways
can provide key mechanistic insights, this work will also support and validate future efforts to understand how
other gut microbial activities influence CRC initiation and development.

Grant Number: 5R01CA208834-10
NIH Institute/Center: NIH
Principal Investigator: Emily Balskus