DNA Replication and Cancer

Proper control and execution of DNA replication is critical for faithful transmission of the genome.  Replication stress, whether exogenous or genetically induced, can cause genomic instability (GIN), cancer susceptibility, and compromised cell proliferation.  In a phenotype-based screen for mutations causing elevated genomic instability in mice, we previosuly isolated a hypomorphic mutation of the essential and highly conserved DNA replication gene Mcm4.  This mutation, named Chaos3 (Chromosome aberrations occurring spontaneously 3), caused exclusively mammary adenocarcinomas in homozygous nulliparous females in the C3H strain background, and other tumor types in different strain backgrounds.  Mcm4 encodes a subunit of the MCM2-7 DNA replication licensing complex and replicative helicase.  The Mcm4Chaos3 mouse demonstrated for the first time that a defective endogenous allele in the replication licensing machinery can cause cancer.  aCGH analysis of yeast engineered to contain the identical amino acid change in Mcm4 and Chaos3 mouse tumors reveal recurrent breakpoints of chromosome rearrangments, indicating that the mutant replisome has difficulties in particular region of the genome.  In vitro studies have shown that depletion of the MCM2-7 DNA replication licensing factors, which form the replicative helicase, can cause GIN and cell proliferation defects.  To explore the effects of incremental attenuation of MCM licensing in whole animals, we generated and analyzed the phenotypes of mice that were hemizygous for Mcm2, 3, 4, 6, and 7 null alleles, combinations thereof, and also in conjunction with Mcm4Chaos3Mcm4Chaos3/Chaos3  embryonic fibroblasts have ~40% reduction in all MCM proteins coincident with reduced Mcm2-7 mRNA.  These mRNA decreases occurred posttranscriptionally, and we are currently investigating the regulatory mechanism.  Further reductions in Mcm2, 6, or 7, achieved in Mcm4Chaos3/Chaos3 background, caused synthetic lethality and growth defects.  Surviving Mcm4Chaos3/Chaos3  Mcm2+/- animals die of highly penetrant, early onset cancer.  These and other data indicate that mutations or polymorphisms causing relatively minor dysregulation or destabilization of MCM levels can be an important contributor to cancer, birth defects, and aging. Most of these studies have been published in Chuang et al.

Ongoing projects seek to answer the following questions, amoung others: 

1) What genomic alterations occur in Chaos3 cells that drive cancer?  What are the responsible “driver” genes?  

2) How are MCM levels and replication licensing controlled in mammalian cells?

3) Chaos3 mutant mice always get cancer, but the cancer type is determined by genetic background.  What are the modifier genes that underlie this phenomenon?

Questions?  Contact John at jcs92@cornell.edu