Genome Instability in Cancer Development

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Transmitting genetic information without creating deleterious genetic alterations is one of the most important tasks. Cells have evolved systems that check for and repair potentially lethal DNA damage. However, when these systems do not work properly, DNA damage accumulates and causes genetic changes or cell death. Accumulation of genetic changes, which is defined as a genomic instability is frequently observed in various types of genetic disorders including cancers. Genomic instability has been documented as a preceding step for multiple inactivations of tumor suppressor genes and activations of proto-oncogenes. One type of genomic instability observed frequently in many cancers is gross chromosomal rearrangement (GCR). GCR includes translocations, deletions of chromosome arm, interstitial deletions, inversions, amplifications, chromosome end-to-end fusion and aneuploidy. Although little is known about the origin and mechanisms of GCRs observed in cancer cells, recent studies on genes mutated in inherited cancer predisposition syndromes have started to demonstrate that proteins that function in DNA damage responses, DNA repair, and DNA recombination, play crucial roles in the suppression of spontaneous and/or DNA damage-induced GCRs. To understand mechanisms how GCRs are generated and how such GCR formation can lead tumorigenesis, we screened the entire yeast genome for mutations or overexpression that increase the rate of GCR formation. RAD5 and ELG1 from mutation screening were selected for further studies of molecular mechanisms of these proteins to protect genome from deleterious GCR formation. 1. Determine the role of RAD5 orthologs, SHPRH. Previously, we identified two RAD5 orthologs in mammals and demonstrated that RAD5 orthologs, SHPRH and HLTF function to prevent collapse of persistent stalled replication forks by assisting template switching DNA damage bypass mechanism that uses the nascent strand of the sister chromatid for recombination mechanism for damage bypass. Among different modifications of Proliferating Cell Nuclear Antigen (PCNA) that determine the bypass mechanisms, we demonstrated that PCNA is poly-ubiquitinated by SHPRH and HLTF. Although we tested for association of SHPRH and HLTF with tumorigenesis in vivo, the inactivation of these genes did not result in an increase in tumorigenesis. SHPRH has a unique histone interaction domain called PHD domain. We recently found that this domain is important for SHPRH localization in the nucleolus. In our characterization of the general DNA damage response regulating PCNA ubiquitination, we unexpectedly found that phosphorylation of Serine 4 and 8 of RPA32, which we used as a DNA damage response control, depends on DNA dependent protein kinase. We found a novel function of SHPRH in the nucleolus that drove the project into a novel molecular mechanism of SHPRH in rRNA transcription in the nucleolus. We found the function of SHPRH for rRNA transcription is mTOR-dependent. 2. ATAD5 (mammalian ELG1 homolog): determine whether alternative Replication Factor C (RFC) complex protein directs DNA repair pathways and replication. We reported that mice haploinsufficient in Atad5 showed a high incidence of tumorigenesis. We recently confirmed that embryonic day 7.5 to 8.5 as embryonic lethality caused by homozygous null mutation of ATAD5. In addition, in collaboration with Dr. Daphne Bells group in NHGRI, we found human somatic mutations of ATAD5 gene in many endometrial tumors. We also found several rare polymorphisms as well as cancer mutations in other tumor types. We recovered a zebrafish atad5 null allele from the TILLING project in the NHGRI zebrafish core facility. Unlike the atad5 null mouse, which dies at embryonic day 8.5, an atad5 null zebrafish survives until seven days post fertilization (dpf). The lethal phenotype of the atad5 null zebrafish became obvious right after hatching when fertilized eggs were treated with MMS. Therefore, we believe that in vivo functional activity of ATAD5 variants identified can be tested with the atad5 null zebrafish model. We have generated various ectopic expression constructs with a single sequence variant found in somatic cancer mutations as well as database search in the International Cancer Genome Consortium for the Cancer Genome Atlas using site-directed mutagenesis. We are currently testing whether these rare mutations found in affect ATAD5s molecular function and cause phenotypes observed in mice and zebrafish as well as in tissue culture system.