RESEARCH

Double-strand breaks in the DNA duplex can lead to irreversible genetic loss and various forms of improper repair in a cell. Improper repair of DNA double-strand breaks can cause genomic instability such as gene mutations and deletions. Genomic instability is a hallmark of cancer, and it can serve as the first step in cancer development and contributes to cancer progression. I am interested in understanding the cellular mechanisms that govern DNA double-strand break repair and how environmental exposure to heavy metals disrupts these normal repair processes.

Cancer caused by environmental heavy metal pollution is one of the major challenges faced by modern society. Rapid industrialization over the past century has played a major role in elevating heavy metal exposure risk for the general population. Heavy metal pollution is increased by industrial activities such as mineral resources extraction, metal processing and smelting, chemical production, factory emissions, and sewage irrigation.

Targeting of the MRN Complex by Heavy Metals

We recently demonstrated that long-term, low-dose exposure to heavy metals (inorganic arsenic and cadmium) interferes with key DNA damage response pathways. However, these metals act at common and different points within the DNA damage response to promote genomic instability. Whereas skin keratinocytes exposed long-term to inorganic arsenic display inhibited ataxia telengectasia protein mutated (ATM) phosphorylation normally mediated by the MRE11/RAD50/NBN (MRN) complex, ATM phosphorylation is not higher in breast epithelial cells exposed chronically to cadmium. These results suggest that Cd may prevent further ATM activation in cells with Cd-induced DSB accumulation. The MRN complex is an evolutionarily conserved molecular machine responsible for processing DNA ends, bringing broken strands together, and signaling the presence of damage within the cell. A key question is whether MRN complex function is inhibited by exposure to either metal. To address this question, we are utilizing proteomics and single-molecule-based imaging techniques with recombinant MRN complex proteins to determine whether either metal can displace zinc from RAD50, and, whether the MRN complex can bind to DNA after inorganic arsenic or cadmium exposure.

Does cadmium exposure drive triple-negative breast cancer initiation and progression by mimicking homologous recombination deficiencies?

Individuals with BReast CAncer 1 (BRCA1) germline mutations (BRCA1+/-) have an increased risk of developing breast cancer, notably triple-negative breast cancers, compared to individuals without BRCA1 germline mutations. However, genetic predisposition is absent in the large majority (~80%) of breast cancers, suggesting that environmental factors may be a major driver of breast cancer susceptibility. Breast epithelial cells are malignantly transformed by cadmium after 40 weeks of exposure. Recently, we found that only 8 weeks of cadmium exposure inhibits BRCA1 phosphorylation mediated by ATM and Ataxia telengectasia-related (ATR) tyrosine kinases in breast epithelial cells with normal BRCA1. These data suggest that long-term cadmium exposure mimics BRCA1 deficiency normally seen in individuals with BRCA1 germline mutations. However, it is unclear how this inhibition is occuring. Does cadmium bind to BRCA1, displace zinc, and lock the protein in a confirmation that prevents its normal activation by ATM or ATR? We are beginning to address this question using proteomics and CRISPR-based DNA repair assays to determine whether BRCA1 function is compromised by cadmium exposure.

Figure 1. Summary of ongoing projects. Cadmium (Cd) and inorganic arsenic (iAs) are known to promote DNA double-strand break accumulation in human cells. iAs is known to inhibit Ataxia mutated telengectasia (ATM) autophosphorylation (pSer1981) and downstream Checkpoint Kinase 2 (CHEK2) phosphorylation (pThr68). Our recent work demonstrates that Cd may also inhibit the MRN complex by preventing further ATM-pSer1981 phosphorylation. Proteomics and single-molecule-based imaging techniques are being used to determine how heavy metal exposure may disrupt MRN function. Chronic Cd exposure inhibits BRCA1 phosphorylation (pSer1524) that is mediated by ATM and ATR Serine/Threonine Kinase (ATR). However, ATM and ATR do not rely on zinc for their function and are therefore not hypothesized to be directly targeted by Cd. BRCA1 is a protein that relies on zinc for its function. Proteomics and CRISPR-based DNA repair assays, and additional experiments, are being used to determine whether BRCA1 function is impaired in breast epithelial cells exposed long-term to Cd. Additional experiments are underway to determine how Cd may also directly inhibit Tumor Protein 53 (TP53) function.