Table 1.

Summary of human and mouse disorders of calcification linked to ANK polymorphisms

ANK gene activity	Human	Mouse
Increased (hence increased ePPi)	1. CCAL2 Deposition of calcium pyrophosphate dihydrate crystals causing acute attacks of pseudo-gout in large joints. Mild, late onset. Autosomal dominant CCAL2: Gain of function mutations in M48T, E490del and −11CT [17]. Sporadic CCAL2: −4 bp G to A SNP (5'UTR) [18]. Demonstrated increased ANK expression and extracellular PPi in vitro. 2. Seizures in autosomal dominant CCAL Members of one such family experienced early childhood seizures; +4 N-terminal amino acids added due to premature initiation codon, causing likely gain of function [19].	There is no CCAL2 phenotype in mice. ANKM48T/null mice showed restored joint function over ANKnull/null mice but no increased PPi deposition [12].
Decreased (hence decreased ePPi)	1. CMD Increased mineralisation causes overgrowth and sclerosis of the craniofacial bones and abnormal modelling of long bone metaphyses, but normal joints. No renal calcification. Autosomal dominant CMD: Mutations: F376del (loss), S375del (loss), A380ins (loss) [20]; W292R, C331R, S375del, F377del, P380insA, G389R [21]. Sporadic CMD: Complex heterozygous mutation in exon 7 [22]. 2. Autosomal recessive syndrome Mental retardation, deafness, ankylosis, dental abnormalities. No renal calcification. Homozygous L244S mutation [6].	1. Progressive ankylosis Two mouse models featuring severe ankylosis, phenotypically indistinguishable from each other. ANKE440X/E440X is a naturally occurring nonsense mutation causing a 53-amino acid C-terminal truncation [3]. ANKnull/null had a very similar phenotype to ANKE440X/E440X [23], except that soft tissue calcification was absent. 2. CMD C331R and G389R mutations [21] abolish PPi influx in Xenopus oocytes and transgenic mice [12], suggesting a dominant negative effect on ANK. F377del homozygous knock-in is a mouse model of CMD [24]. 3. Nephrocalcinosis Increased calcification in the kidneys of adult ANKE440X/E440X mice [3].

Polymorphisms or mutations in the ANK gene causing either an increase or decrease in gene function exist in both humans and mice and may lead to increased extracellular PPi (ePPi), which reduces calcification, or vice versa. CCAL2 = Chondrocalcinosis type 2; SNP = single-nucleotide polymorphism; UTR = untranslated region; CMD = craniometaphyseal dysplasia.