Straightening out the renal tubule: advances in the molecular basis of the inherited tubulopathies.

Advances in the molecular genetics of inherited renal tubulopathies have allowed some insight into the normal mechanisms of tubular cation and anion reabsorption. It is now possible to view Bartter's syndrome, Gitelman's syndrome and pseudohypoaldosteronism type 1 as having genetic abnormalities which produce tubular defects that are similar to those induced by the pharmacological actions of loop diuretics, thiazide diuretics or potassium-sparing diuretics, respectively. Although these rare monogenic disorders with dramatic phenotypes seem to have little relevance to everyday clinical practice, it is possible that subtle abnormalities of the regulation of the ENaCs may play a role in low-renin forms of 'essential' hypertension. Similarly, subtle abnormalities in the function of the electroneutral sodium-(potassium)-chloride cotransporters (NKCC2 and NCCT) and the renal CLC-type chloride channels (CLC5) may be major determinants of urinary calcium excretion with roles in the pathogenesis of 'idiopathic' hypercalciuria and osteoporosis. Because of the intricate and diverse molecular mechanisms by which tubular reabsorption of water and solutes takes place in each different nephron segment, it is likely that other renal channels and transporters will be implicated in the pathogenesis of further monogenic disorders, and that these will allow additional insights into tubular functioning. Recent studies have demonstrated that in addition to abnormalities in the NKCC2 and ROMK1 genes, mutations at a third genetic locus can also cause Bartter's syndrome. Linkage studies, followed by mutational analyses have found deletions and point mutations in the gene encoding one of the TAL-specific chloride channels, CLCKB, in 17 Bartter's families. This chloride channel is similar in structure to CLC5, and is located on the long arm of chromosome 1. Importantly, there appears to be a phenotypic difference between subjects with Bartter's syndrome due to CLCKB abnormalities and those with NKCC2 or ROMK1 mutations. Despite the fact that all of these Bartter's patients had significant hypercalciuria, nephrocalcinosis was not found in any of the 17 subjects with CLCKB mutations, compared to 19 of 20 patients with NKCC2 or ROMK1 mutations. These findings have also demonstrated a key role for CLCKB as a major basolateral chloride channel involved in mTAL sodium and chloride reabsorption (Figure 2).