Association of Mutations in SLC12A1 Encoding the NKCC2 Cotransporter With Neonatal Primary Hyperparathyroidism

Context: Primary hyperparathyroidism with hypercalciuria has not been described in the newborn period. Objective: Our objectives are to identify the genetic basis for neonatal primary hyperparathyroidism in a family with 2 affected children. Subjects: An African American boy presenting with mild neonatal primary hyperparathyroidism and hypercalciuria was evaluated at The Children's Hospital of Philadelphia. His older brother with neonatal primary hyperparathyroidism had died in infancy of multiple organ failure. Methods: We collected clinical and biochemical data and performed exome sequencing analysis on DNA from the patient and his unaffected mother after negative genetic testing for known causes of primary hyperparathyroidism. Results: Exome sequencing followed by Sanger sequencing disclosed 2 heterozygous mutations, c.1883C>A, p.(A628D) and c.2786_2787insC, p.(T931fsX10), in the SLC12A1 gene, which was previously implicated in antenatal type 1 Bartter syndrome. Sanger sequencing confirmed the 2 mutations in the proband and his deceased brother; both parents were heterozygous for different mutations and an unaffected sister was homozygous for wild-type alleles. Conclusions: These results demonstrate a previously unrecognized association between neonatal primary hyperparathyroidism and mutation of SLC12A1, the cause of antenatal Bartter syndrome type 1, and suggest that the loss of sodium-potassium-chloride cotransporter-2 cotransporter activity influences parathyroid gland function.

Alternatively spliced isoform of apical Na+-K+-Cl− cotransporter gene encodes a furosemide-sensitive Na+-Cl−cotransporter

In the absence of vasopressin, medullary thick ascending limb cells express a K+-independent, furosemide-sensitive Na+-Cl− cotransporter that is inhibited by hypertonicity. The murine renal specific Na+-K+-2 Cl− cotransporter gene ( SLC12A1) gives rise to six alternatively spliced isoforms. Three feature a long COOH-terminal domain that encodes the butmetanide-sensitive Na+-K+-2 Cl−cotransporter (BSC1–9/NKCC2), and three with a short COOH-terminal domain, known as mBSC1-A4, B4, or F4 (19). Here we have determined the functional characteristics of mBSC1-A4, as expressed in Xenopus laevis oocytes. When incubated at normal oocyte osmolarity (∼200 mosmol/kgH2O), mBSC1–4-injected oocytes do not express significant Na+ uptake over H2O-injected controls, and immunohistochemical analysis shows that the majority of mBSC1–4 protein is in the oocyte cytoplasm and not at the plasma membrane. In contrast, when mBSC1–4 oocytes are exposed to hypotonicity (∼100 mosmol/kgH2O), a significant increase in Na+uptake but not in 86Rb+ uptake is observed. The increased Na+ uptake is Cl− dependent, furosemide sensitive, and cAMP sensitive but K+independent. Sodium uptake increases with decreasing osmolarity between 120 and 70 mosmol/kgH2O ( r = 0.95, P < 0.01). Immunohistochemical analysis shows that in hypotonic conditions mBSC1-A4 protein is expressed in the plasma membrane. These studies indicate that the mBSC1-A4 isoform of the SLC12A1 gene encodes a hypotonically activated, cAMP- and furosemide-sensitive Na+-Cl− cotransporter. Thus it is possible that alternative splicing of the BSC1 gene could provide the molecular mechanism enabling the Na+-Cl−-to-Na+-K+-2Cl−switching in thick ascending limb cells.