Environmental chromium and cadmium co-exposure induces GADD45A-mediated rDNA instability in human cohorts and cell models.

Exposure to heavy metals such as hexavalent chromium [Cr(VI)] and cadmium (Cd) poses a major environmental and occupational health concern due to their widespread presence and genotoxic potential. While the toxicological profiles of individual metals are well-characterized, their combined effects on genome stability remain poorly understood. Ribosomal DNA (rDNA), a repetitive and transcriptionally active genomic region essential for protein synthesis, is increasingly recognized as a sensitive target of environmental stress and genomic instability. Here, we investigated electroplating workers stratified by blood Cr and Cd concentrations and conducted complementary mechanistic studies in BEAS-2B human bronchial epithelial cells. Individuals with high co-exposure exhibited significantly elevated rDNA copy number (CN) across 5S, 5.8S, 18S, 28S, and 45S subunits, along with increased γH2AX-positive lymphocytes. Multivariable regression confirmed independent associations with both endpoints. Cr(VI) and Cd co-treatment triggered γH2AX accumulation, ATM activation, nucleolar disruption, p53 phosphorylation, and apoptosis in vitro. These changes coincided with rDNA promoter hypomethylation and strong induction of GADD45A, a stress-inducible gene involved in DNA demethylation. Knockdown of GADD45A attenuated rDNA CN expansion. Furthermore, plasma GADD45A levels were elevated in co-exposed individuals and positively correlated with rDNA CN. Together, these findings uncover a GADD45A-dependent mechanism by which metal mixtures induce rDNA instability and nucleolar stress. Our results support the utility of rDNA CN and DNA damage markers as sensitive indicators of environmental metal exposure and early genomic injury.