HPV-positive HNSCC cells are unresponsive to TGFβ. Abrogated TGFβ signaling compromises repair by HRR and increases reliance on alt-EJ, which provides a mechanistic basis for sensitivity to PARP inhibitors. The effect of HPV in HNSCC provides critical validation of TGFβ's role in DNA repair proficiency and further raises the translational potential of TGFβ inhibitors in cancer therapy.

Purpose: Following cytotoxic therapy, 70% of patients with human papillomavirus (HPV) positive oropharyngeal head and neck squamous cell carcinoma (HNSCC) are alive at 5 years compared to 30% of those with similar HPV-negative cancer, which is thought to be due to dysregulation of DNA repair. Loss of transforming growth factor β (TGFβ) signaling is a poorly studied consequence of HPV that could contribute to this phenotype.Experimental Design: Human HNSCC cell lines (n=9), patient-derived xenografts (n=9), tissue microarray (n=194), TCGA expression data and primary tumor specimens (n=10) were used to define the relationship between TGFβ competency, response to DNA damage, and type of DNA repair.Results: Analysis of HNSCC specimens in situ and in vitro showed that HPV associates with loss of TGFβ signaling that increases the response to radiation or cisplatin. TGFβ suppressed miR-182 that inhibited both BRCA1, necessary for homologous recombination repair, and FOXO3, which is required for ATM kinase activity. TGFβ signaling blockade by either HPV or inhibitors released this control, compromised HRR and increased response to PARP inhibition. Antagonizing miR-182 rescued the homologous recombination deficit in HPV+ cells. Loss of TGFβ signaling unexpectedly increased error-prone, alternative end-joining repair.Conclusions: HPV-positive HNSCC cells are unresponsive to TGFβ. Abrogated TGFβ signaling compromises homologous recombination and shifts reliance on alt-EJ repair that provides a mechanistic basis for sensitivity to PARP inhibitors. The effect of HPV in HNSCC provides critical validation of TGFβ’s role in DNA repair proficiency and further raises the translational potential of TGFβ inhibitors in cancer therapy.

Maintenance of mammary functional capacity during cycles of proliferation and regression depend on appropriate cell fate decisions of mammary progenitor cells to populate an epithelium consisting of secretory luminal cells and contractile myoepithelial cells. It is well established that transforming growth factor β (TGFβ) restricts mammary epithelial cell proliferation and that sensitivity to TGFβ is decreased in breast cancer. Here we show that TGFβ also exerts control of mammary progenitor self-renewal and lineage commitment decisions by stringent regulation of breast cancer early onset 1 (BRCA1), which controls stem cell self-renewal and lineage commitment. Either genetic depletion of Tgfb1or transient blockade of TGFβ increased self-renewal of mammary progenitor cells in mice and cultured primary mammary epithelial cells, and also skewed lineage commitment towards the myoepithelial fate. TGFβ stabilized the expression of BRCA1 by suppressing miR-182. Ectopic expression of BRCA1 or antagonism of miR-182 in cultured TGFβ-deficient mammary epithelial cells restored lineage commitment. These findings reveal that TGFβ modulation of BRCA1 directs mammary epithelial cell fate and, since stem/progenitor cells are thought to be the cell of origin for aggressive breast cancer subtypes, suggest that TGFβdysregulation during carcinogenesis may promote distinct breast cancer subtypes.

Transforming growth factor β1 (TGFβ) affects stroma and epithelial composition and interactions that mediate mammary development and determine the course of cancer. The reduction of TGFβ in Tgfβ1 heterozygote mice, which are healthy and long-lived, provides an important model to dissect the contribution of TGFβ in mammary gland biology and cancer. We used both intact mice and mammary chimeras in conjunction with Tgfβ1 genetic depletion and TGFβ neutralizing antibodies to evaluate how stromal or epithelial TGFβ depletion affect mammary development and response to physiological stimuli. Our studies of radiation carcinogenesis have revealed new aspects of TGFβ biology and suggest that the paradoxical TGFβ switch from tumor suppressor to tumor promoter can be resolved by assessing distinct stromal versus epithelial actions.

Germline pathogenic variants in BRCA1 confer a high risk of developing breast and ovarian cancer. The BRCA1 exon 11 (formally exon 10) is one of the largest exons and codes for the nuclear localization signals of the corresponding gene product. This exon can be partially or entirely skipped during pre‐mRNA splicing, leading to three major in‐frame isoforms that are detectable in most cell types and tissue, and in normal and cancer settings. However, it is unclear whether the splicing imbalance of this exon is associated with cancer risk. Here we identify a common genetic variant in intron 10, rs5820483 (NC_000017.11:g.43095106_43095108dup), which is associated with exon 11 isoform expression and alternative splicing, and with the risk of breast cancer, but not ovarian cancer, in BRCA1 pathogenic variant carriers. The identification of this genetic effect was confirmed by analogous observations in mouse cells and tissue in which a loxP sequence was inserted in the syntenic intronic region. The prediction that the rs5820483 minor allele variant would create a binding site for the splicing silencer hnRNP A1 was confirmed by pull‐down assays. Our data suggest that perturbation of BRCA1 exon 11 splicing modifies the breast cancer risk conferred by pathogenic variants of this gene.