Familial pure proximal renal tubular acidosis - a clinical and genetic study

Background. Inherited proximal renal tubular acidosis (pRTA) is commonly associated with more generalized proximal tubular dysfunctions and occasionally with other organ system defects. Inherited combined pRTA and distal RTA with osteopetrosis and pure pRTA associated with ocular abnormalities, a rare disease which has been recently described. Only one family with pure isolated pRTA has been reported so far and the genetic cause for this disease is unknown. Objectives. We report a unique family with isolated pRTA. The aim of the project was to define the phenotype and to try to find the gene defect causing the disease. Methods. Clinical and metabolic evaluation of all family members was performed and a family pedigree was constructed. DNA was extracted from blood samples of affected and unaffected family members. We amplified by PCR and sequenced the coding areas and splice-sites of the genes that contribute to HCO3- reclamation in the proximal tubule. The genes studied were as follows: CA II, CA IV, CA XIV, NCB1, Na+/H+ exchanger (NHE)-3, NHE-8, the regulatory proteins of NHE3, NHRF1 and NHRF2 and the Cl-/HCO3- exchanger, SLC26A6. Results. The father and all four children had RTA with blood HCO3- levels of 11-14 meq/1 and urine pH of 5.3-5.4. Increased HCO3- fractional excretion after bicarbonate loading to 40-60% confirmed the diagnosis pRTA. No other tubular dysfunction was found, and no organ system dysfunction was detected, besides short stature. No mutation was found in all candidate genes studied. Conclusions. We presented a second family in the literature with familial isolated pure pRTA. The mode of inheritance is compatible with an autosomal dominant disease. Because of the small size of the family, wide genome search was not applicable and the gene candidate approach was chosen. Nine important candidate genes were extensively studied but the molecular basis of the disease was not yet found and genotyping nine important gene candidates were negative.