Mutations in the amino terminus of ANKH in two US families with calcium pyrophosphate dihydrate crystal deposition disease

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Abstract
Objective To analyze ANKH in families with calcium pyrophosphate dihydrate crystal deposition disease (CPPD) for disease‐causing mutations. Methods Two US families (one of British ancestry and the other of German/Swiss ancestry) with autosomal‐dominant CPPD, whose disease phenotypes were found to be linked to chromosome 5p15.1 (locus symbol CCAL2), were screened by direct sequencing for mutations in ANKH, a gene in the CCAL2 candidate interval that has been shown to harbor mutations in other families with CPPD. Observed sequence variants were confirmed by antisense sequencing, and expression of the mutant allele was verified by reverse transcriptase–polymerase chain reaction amplification of messenger RNA followed by direct sequencing. Results The two US families displayed the same mutation at position 5 of the ANKH gene product (P5T). All affected members were heterozygous for the P‐to‐T variant, and the mutation was not seen in 204 control alleles. The two families displayed distinct disease haplotypes, suggesting that they were unrelated to each other. Conclusion These observations represent the fourth and fifth families with heritable CPPD whose disease phenotypes are linked to the CCAL2 locus and who have missense mutations in the amino terminus of ANKH. This same position (P5) was the site of a missense mutation in an Argentinean family of northern Italian ancestry; however, the sequence variant in that family generated a P5L mutation. The distinct disease haplotypes among the 3 families with P5 mutations suggest that the mutations arose independently and that the evolutionarily conserved P5 position of ANKH may represent a hot spot for mutation in families with autosomal‐dominant CPPD.

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