Loss of carbonic anhydrase XII function in individuals with elevated sweat chloride concentration and pulmonary airway disease

Abstract

Elevated sweat chloride levels, failure to thrive (FTT), and lung disease are characteristic features of cystic fibrosis (CF, OMIM #219700). Here we describe variants in CA12 encoding carbonic anhydrase XII in two pedigrees exhibiting CF-like phenotypes. Exome sequencing of a white American adult diagnosed with CF due to elevated sweat chloride, recurrent hyponatremia, infantile FTT and lung disease identified deleterious variants in each CA12 gene: c.908-1 G>A in a splice acceptor and a novel frameshift insertion c.859_860insACCT. In an unrelated consanguineous Omani family, two children with elevated sweat chloride, infantile FTT, and recurrent hyponatremia were homozygous for a novel missense variant (p.His121Gln). Deleterious CFTR variants were absent in both pedigrees. CA XII protein was localized apically in human bronchiolar epithelia and basolaterally in the reabsorptive duct of human sweat glands. Respiratory epithelial cell RNA from the adult proband revealed only aberrant CA12 transcripts and in vitro analysis showed greatly reduced CA XII protein. Studies of ion transport across respiratory epithelial cells in vivo and in culture revealed intact CFTR-mediated chloride transport in the adult proband. CA XII protein bearing either p.His121Gln or a previously identified p.Glu143Lys missense variant localized to the basolateral membranes of polarized Madin-Darby canine kidney (MDCK) cells, but enzyme activity was severely diminished when assayed at physiologic concentrations of extracellular chloride. Our findings indicate that loss of CA XII function should be considered in individuals without CFTR mutations who exhibit CF-like features in the sweat gland and lung.

Carbonic anhydrase XII functions in health and disease

“…CAXII is also expressed in sweat glands and the enzyme might be responsible for salt homeostasis in sweat (29–32). …”

“…In luminal membranes of the kidney, membrane associated CAs catalyze the dehydration of carbonic acid (H 2 CO 3 ) to CO 2 and water for removal of bicarbonate (57). CAXII is localized at the basolateral surface of the nephrons where efflux of bicarbonate is enhanced by CAXII (31; 32). CAXII also plays an important role for fluid and bicarbonate homeostasis in epithelial cells (31; 34), suggesting an important role of CAXII in kidney physiology (31; 34).…”

“…CAXII also plays an important role for fluid and bicarbonate homeostasis in epithelial cells (31; 34), suggesting an important role of CAXII in kidney physiology (31; 34). CAXII may also function in the kidney as a metabolan of the membrane-associated CAXII and the chloride/bicarbonate exchanger (AE1) (31; 32) or sodium/bicarbonate co-transporter (NBCe) (32), and proton anti-porter (34; 58). …”

“…An alternatively spliced form of CAXII is expressed in brain tumors, where its expression is associated with poorer prognosis (28). Mutation in CAXII has also been associated with the cystic fibrosis-like syndrome, including hyponatremia (29–32). …”

“…Recombinant CAXII has been used to study the structure and function of the enzyme by crystallography (9) and enzyme kinetics (8). Affinity purified enzymes have been used to produce rabbit specific CAXII antibodies and the antibodies were characterized by immunological procedures and used for immunohistochemistry of CAXII (8; 9; 21–23; 28; 32; 35; 36). …”