Hyperchlorhidrosis caused by homozygous mutation in CA12, encoding carbonic anhydrase XII

Abstract

Excessive chloride secretion in sweat (hyperchlorhidrosis), leading to a positive sweat test, is most commonly indicative of cystic fibrosis yet is found also in conjunction with various metabolic, endocrine, and dermatological disorders. There is conflicting evidence regarding the existence of autosomal-recessive hyperchlorhidrosis. We now describe a consanguineous Israeli Bedouin kindred with autosomal-recessive hyperchlohidrosis whose sole symptoms are visible salt precipitates after sweating, a preponderance to hyponatremic dehydration, and poor feeding and slow weight gain at infancy. Through genome-wide linkage analysis, we demonstrate that the phenotype is due to a homozygous mutation in CA12, encoding carbonic anhydrase XII. The mutant (c.427G>A [p.Glu143Lys]) protein showed 71% activity of the wild-type enzyme for catalyzing the CO₂ hydration to bicarbonate and H(+), and it bound the clinically used sulfonamide inhibitor acetazolamide with high affinity (K(I) of 10 nM). Unlike the wild-type enzyme, which is not inhibited by chloride, bromide, or iodide (K(I)s of 73-215 mM), the mutant is inhibited in the submicromolar range by these anions (K(I)s of 0.37-0.73 mM).

Figure 1

Pedigree of the Affected Israeli-Bedouin Kindred The pedigree is compatible with autosomal-recessive heredity.

Figure 2

Fine Mapping of the Chromosome 15 Locus Partial pedigree of the large Israeli-Bedouin kindered and disease-haplotype segregation. The two lines indicate a crossing over event. Disease associated haplotype shown in boxes. Markers chr15:61090417-61090468 and rs16949924 define the minimal homozygosity locus associated with the disease (Individual IV:15). The affected haplotype is shaded. Numbers below symbols correspond to the position of individuals within the pedigree (see Figure 1). Blank spaces indicate non-genotyped markers.

Figure 3

The c.427G>A (p.Glu143Lys) Mutation in Exon 4 of CA12, and CA12 Expression Sequence analysis is shown for an unaffected individual (A), an obligatory carrier (B), and an affected individual (C). (D) ConSeq analysis for the p.Glu143Lys residue (marked with an arrow), demonstrating conservation grade 9 (highly conserved). (E) RT-PCR demonstrating that CA12 (as well as actin control) are transcribed in normal human foreskin (1,2) and arm skin (3,4). PCR primer sequences available upon request. Lane 5 = size marker. Lanes 6,7 represent negative controls for CA12 and actin PCR reactions, with no cDNA template.

Figure 4

Inhibition Constants for Anionic Inhibitors of Human Wild-Type and Mutant hCA XII, for the CO₂ Hydration Reaction, at 20°C