Polycystic ovary syndrome (PCOS), characterized by increased ovarian androgen biosynthesis, anovulation, and infertility, affects 5-7% of reproductive-age women. Genome-wide association studies identified PCOS candidate loci that were replicated in subsequent reports, including DENND1A, which encodes a protein associated with clathrin-coated pits where cell-surface receptors reside. However, these studies provided no information about functional roles for DENND1A in the pathogenesis of PCOS. DENND1A protein was located in the cytoplasm as well as nuclei of theca cells, suggesting a possible role in gene regulation. DENND1A immunostaining was more intense in the theca of PCOS ovaries. Using theca cells isolated and propagated from normal cycling and PCOS women, we found that DENND1A variant 2 (DENND1A.V2) protein and mRNA levels are increased in PCOS theca cells. Exosomal DENND1A.V2 RNA was significantly elevated in urine from PCOS women compared with normal cycling women. Forced overexpression of DENND1A.V2 in normal theca cells resulted in a PCOS phenotype of augmented CYP17A1 and CYP11A1 gene transcription, mRNA abundance, and androgen biosynthesis. Knock-down of DENND1A.V2 in PCOS theca cells reduced androgen biosynthesis and CYP17A1 and CYP11A1 gene transcription. An IgG specific to DENND1A.V2 also reduced androgen biosynthesis and CYP17 and CYP11A1 mRNA when added to the medium of cultured PCOS theca cells. We conclude that the PCOS candidate gene, DENND1A, plays a key role in the hyperandrogenemia associated with PCOS. These observations have both diagnostic and therapeutic implications for this common disorder. P olycystic ovary syndrome (PCOS) is one of the most common endocrinopathies affecting 5-7% of reproductive age women world-wide (1). PCOS is associated with hyperandrogenemia/ hyperandrogenism, anovulation, infertility, and a characteristic ovarian morphology consisting of multiple small subcortical follicular "cysts" embedded in bilaterally enlarged ovaries (2-5). The presence of elevated circulating concentrations of testosterone results primarily from increased production of androgens by the ovaries, and is a classic endocrine phenotype of women with PCOS. Although there has been debate about the diagnostic criteria for PCOS, hyperandrogenemia/hyperandrogenism and anovulation, not explained by other causes, is a hallmark of the disorder, and is included as a key element in all "consensus" diagnosis schemes (6-10).There is consensus that the ovarian theca cells are the primary source of excess androgen biosynthesis in women with PCOS (11-13). Studies on freshly isolated thecal tissue or cultures of theca cells derived from normal and PCOS women have demonstrated that PCOS theca secretes greater amounts of androgen than theca tissue or cells from regularly ovulating women (12,(14)(15)(16)(17)(18)(19). Increased androgen biosynthesis in PCOS theca cells results from increased expression of the key enzymes involved in androgen biosynthesis, steroid-17-α-hydroxylase/17,20 lyase (encoded by the CYP17...