Molecular Vision 2006; 12:324-30 <http://www.molvis.org/molvis/v12/a35/> Received 12 January 2006 | Accepted 3 April 2006 | Published 10 April 2006 Maintenance of blood pH within a narrow range (7.35- 7.45) is crucial for essential biochemical and metabolic func- tions. The kidneys have a key role in homeostasis because of their ability to reabsorb HCO 3 - and excrete acid. The basolateral membrane, electrogenic Na + /HCO 3 - cotransporter (NBCe1) plays a major role in renal bicarbonate absorption via the proxi- mal tubules [1]. The human gene for NBCe1 protein (SLC4A4) is located on chromosome 4q21 and produces two major tran- scripts with 5'-end variants resulting from alternative promoter usage [2]. The pNBCe1 transcript is generated from the pri- mary promoter and is expressed in most tissues, including the pancreas [1]. The kNBCe1 transcript uses an alternative pro- moter and transcription initiation site within intron 3 of SLC4A4 and was originally cloned from kidney [2]. kNBCe1 and pNBCe1 are composed of 1,035 and 1,079 amino acid residues, respectively. kNBCe1 differs from pNBCe1 with its unique NH 2 -terminal that harbors 41 amino acids (in lieu of the first 85 amino acids of pNBCe1) [2]. A third isoform, re- ported in rat but not yet in human, is found in brain and uses the pNBCe1 promoter but a novel C-terminus [3]. NBCe1 expression has also been detected in other tissues such as heart, stomach, intestine, lung, thyroid, salivary glands, and pros- tate [1,4]. We have adopted the previously used nomenclature for SLC4A4 mutations based upon their locations in kNBCe1. The kidney seems to express all NBCe1 variants and their function accounts for 80-90% of renal HCO 3 - absorption [1]. Therefore, the renal findings constitute the major phenotype for SLC4A4 mutations. Renal tubular acidosis (RTA) is a clini- cal syndrome characterized by hyperchloremic, metabolic aci- dosis resulting from defective renal acidification. Proximal RTA (pRTA) is caused by impairment of bicarbonate absorp- tion in proximal tubules with decreased renal HCO 3 - thresh- old, while the distal tubule acidification remains intact. A rare syndrome was described in patients with SLC4A4 mutations that was characterized by autosomal recessive (AR) perma- nent isolated pRTA and hypokalemia (blood pH <7.2, blood HCO 3 - =5-11 mM/L, serum K + =2.6-3.3 mEq/l), short stature, and ocular pathology (glaucoma, band keratopathy, cataract) [5-8]. Additional features may include enamel defects of per- manent teeth and mental retardation [5-7]. Calcification of basal ganglia, hyperamylasemia, and hypothyroidism were also noted in some patients [4]. Eight homozygous SLC4A4 muta- tions (6 missense-R298S, S427L, T485S, R510H, A799V, R881C; one nonsense-Q29X; and one frameshift-2311delA) have been published to date [5-9]. One additional mutation (a ©2006 Molecular Vision Proximal renal tubular acidosis and ocular pathology: a novel missense mutation in the gene (SLC4A4) for sodium bicarbonate cotransporter protein (NBCe1) F. Yesim K. Demirci, 1,2 Min-Hwang Chang, 3 Tammy S. Mah, 1 Michael F. Romero, 3,4 Michael B. Gorin 1,2 1 Department of Ophthalmology, UPMC Eye Center, Ophthalmology and Visual Science Research Center, School of Medicine and 2 Department of Human Genetics, Graduate School of Public Health, University of Pittsburgh, Pittsburgh, PA; Departments of 3 Physiology and Biophysics and 4 Pharmacology, Case Western Reserve University, Cleveland, OH Purpose: The electrogenic Na + /HCO 3 - cotransporter (NBCe1) plays a major role in renal bicarbonate absorption via proximal tubules and therefore is crucial for maintaining normal blood pH. The human gene for NBCe1 (SLC4A4) pro- duces two major transcripts by alternative promoter usage (kNBCe1, originally cloned from kidney and pNBCe1, pancre- atic/general form). Though rare, recessive SLC4A4 mutations have been reported in patients with proximal renal tubular acidosis, short stature, and ocular pathology. A 27-year-old male presented with these findings. The purpose of this study was to investigate the molecular pathology responsible for this patient’s clinical findings. Methods: A comprehensive ophthalmic examination was performed, detailed ocular and systemic medical histories were taken and past medical records were obtained. Mutation screening was performed by using direct PCR sequencing of SLC4A4 exons and flanking intronic regions. Functional characterization of the mutation was made by expressing the wild-type and mutant NBCe1 proteins in Xenopus oocytes. Results: We identified a novel, homozygous, missense SLC4A4 mutation (Leu522Pro in kNBCe1) in our patient who had pRTA, short stature, enamel hypoplasia, and bilateral ocular disease (cataract, glaucoma, and band keratopathy). The patient also had a medical history of ataxia, migraine with transient hemiparesis attacks, and slight hypothyroidism. The mutant RNA failed to induce electrogenic transport activity. The L522P-protein was not effectively transported to the oocyte membrane and thus was unable to act as a transmembrane transporter. Conclusions: This novel mutation increases our understanding of the structural/functional aspects of the NBCe1 protein and the molecular basis of the multiorgan pathologies associated with its defects. Correspondence to: Michael B. Gorin, MD, PhD, UPMC Eye Cen- ter, University of Pittsburgh, 203 Lothrop Street, EEINS Building, Rm 1025, Pittsburgh, PA, 15213; Phone: (412) 647-7726; FAX: (412) 647-5880; email: gorinmb@upmc.edu 324