SUMMARY: Parathyroid hormone (PTH) is a central regulator of calcium and phosphate homeostasis and thereby essential for bone mineralization. In primary hyperparathyroidism parathyroid glands (PTGs) produce increased amounts of PTH despite normal or high serum calcium levels. The molecular mechanisms that normally control transcriptional regulation of PTH in parathyroid cells are poorly defined. Moreover, PTH production is also influenced by mechanisms that control its protein stability or secretion from cells. Identifying transcriptional or posttranscriptional regulators of PTH production is important for our understanding of the basic mechanisms that control PTH production. Here, we have identified the BTB-domain-containing nuclear protein KCTD1 as a novel key regulator of PTH production and we propose that lack of KCTD1 in PTGs leads to primary hyperparathyroidism through loss of KCTD1-mediated transcriptional repression of PTH. We generated KCTD1lacZ reporter mice and KCTD1-/- mice and show that KCTD1 is strongly expressed in PTGs and that mice lacking KCTD1 have highly increased active PTH serum levels and hypercalcemia. Moreover, KCTD1 is expressed in human parathyroid tissue as well. KCTD1-/- mice develop bone mineralization abnormalities, as they are seen in patients with hyperparathyroidism. Notably, some of the bone abnormalities in KCTD1-/- mice phenocopy bone defects seen in patients with Scalp-Ear-Nipple syndrome in which we identified KCTD1 missense mutations. To address the question if the abnormalities observed in KCTD1-/- mice are a consequence of loss of KCTD1 specifically in the PTGs or whether functions of KCTD1 in other organs contribute to the observed phenotype as well, we have generated mice that lack KCTD1 only in PTGs and express a fluorescent reporter allele, allowing us to dissect PTGs from these mice (PTH-Cre+KCTD1fl/flEYFP+ mice). We will compare the phenotypes in these mice with those observed in KCTD1-/- mice and thereby determine if the observed hyperparathyroidism and bone defects are a consequence of KCTD1 deficiency exclusively in the PTGs. KCTD1 can function as a transcriptional repressor and as an inhibitor of canonical Wnt/β-catenin signaling. To determine how KCTD1 regulates PTH production we will test in dissected PTGs of PTH- Cre+KCTD1fl/flEYFP+ mice whether KCTD1 regulates PTH production through transcriptional repression or through effects on protein stability or cellular secretion of PTH and whether it affects sensitivity of PTH production in response to changes in calcium. We will investigate whether canonical Wnt/β-catenin signaling is increased in PTGs that lack KCTD1 and whether β-catenin inhibitors can rescue the increased PTH production. RNA-Seq will identify downstrea...