Generalized Arterial Calcification of Infancy and Pseudoxanthoma Elasticum (PXE) via the ABCC6 Gene

Generalized arterial calcification of infancy (also referred as Idiopathic infantile arterial calcification) is a rare infantile onset calcification of the internal elastic lamina of medium and large arteries and arterial stenosis due to myointimal proliferation (Rutsch et al. 2003. PubMedID: 12881724; Chong and Hutchins. 2008. PubMed ID: 17990935). The major early clinical features include fetal distress, heart failure, hydramnios, respiratory distress, hydrops fetalis, fetal edema, cyanosis, effusions in the pleural, peritoneal, pericardial spaces and cardiomegaly. The major late clinical features are respiratory distress, cyanosis, feeding difficulties, and heart failure (Chong and Hutchins. 2008. PubMed ID: 17990935). Without proper medical management, ~60% of the patients die from cardiovascular complications or renal artery stenosis within the first 6 months of life (Rutsch et al. 2001. PubMed ID: 11159191; Rutsch et al. 2008. PubMed ID: 20016754; Ferreira et al. 2014. PubMed ID: 25392903).

Pseudoxanthoma elasticum (PEX) is a progressive condition characterized by ectopic mineralization in elastic fibers of connective tissues mainly in skin, retina, and blood vessels. Patients may present yellowish papules in skin, progressive visual loss and cardiovascular symptoms. Cardiovascular symptoms include rupture of blood vessels particularly within the gastrointestinal tract, and early myocardial infarction (Bergen et al. 2000. PubMed ID: 10835643; Ringpfeil et al. 2000. PubMed ID: 10811882). The estimated disease prevalence is between 1/25,000 and 1/50,000 in general populations and the estimated carrier frequency is one in 111 to one in 80 individuals (Legrand et al. 2017. PubMed ID: 28102862).

Generalized arterial calcification of infancy is inherited in an autosomal recessive manner caused by pathogenic variants in the ENPP1 and ABCC6 genes.

The ABCC6 protein (ATP-binding cassette subfamily C member 6) coded by ABCC6 is a member of the ATP-binding cassette (ABC) protein family and serves serves as an efflux transporter and regulates plasmatic inorganic pyrophosphate (Jansen et al. 2014. PubMed ID: 24969777).

ABCC6 is the major gene involved in autosomal recessive pseudoxanthoma elasticum (Legrand et al. 2017. PubMed ID: 28102862). To date, ~350 unique pathogenic variants have been reported. They include miseense (~53%), truncating (~28%), splicing (~7%), large deletion and gross rearrangements (~11%) (Human Gene Mutation Database). A nonsense variant c.3421C>T, p.Arg1141* in exon 24 was found in 25% of cases from all ethnic backgrounds, and a large deletion involving exons 23 to 29 was found in 28% of American patients of European descent (Legrand et al. 2017. PubMed ID: 28102862).

Pathogenic ABCC6 variants have also been reported in patients with autosomal recessive generalized arterial calcification of infancy (Nitschke et al. 2012. PubMed ID: 22209248) and found in a few families with debatable autosomal dominant inheritance pattern of Pseudoxanthoma elasticum, forme fruste (Bergen et al. 2000. PubMed ID: 10835643; Bergen. 2006. PubMed ID: 16541094). No genotype-phenotype correlations have been established.

In one study, pathogenic variants in the ABCC6 gene were reported in 88% of patients with clinically diagnosed Pseudoxanthoma elasticum (Legrand et al. 2017. PubMed ID: 28102862). A nonsense variant c.3421C>T, p.Arg1141Ter in exon 24 was found in 25% cases from all ethnic backgrounds, and a large deletion involving exons 23 to 29 was found in 28% of American patients of European descent (Legrand et al. 2017. PubMed ID: 28102862).

This test provides full coverage of all coding exons of the ABCC6 gene, plus ~10 bases of flanking noncoding DNA. We define full coverage as >20X NGS reads or Sanger sequencing. This test also detects the ABCC6 c.1780-29T>A and c.2248-12_2248-11del variants.

Due to the homologous sequence at the region of exons 1 to exon 9 of ABCC6, we utilize a long-range PCR strategy to specifically amplify and sequence these exons, which is able to overcome the limitation of current short-read NGS technology in dealing with this type of homologous sequence (Mandelker et al. 2016. PubMed ID: 27228465).