Generalized Arterial Calcification of Infancy and Pseudoxanthoma Elasticum Panel

Generalized arterial calcification of infancy (also referred to 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. PubMed ID: 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 the general population 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.

Pathogenic variants in the ENPP1 gene have been reported in 75% of patients with autosomal recessive generalized arterial calcification of infancy (Rutsch et al. 2008. PubMed ID: 20016754). The ENPP1 protein coded by the ENPP1 gene is a member of the nucleotide pyrophosphatase/phosphodiesterase family and plays a role in regulating pyrophosphate levels. ENPP1 functions in bone mineralization and soft tissue calcification (Rutsch et al. 2001. PubMed ID: 11159191). To date, almost 70 unique pathogenic variants in the ENPP1 gene have been reported. They are: missense (67%), truncating including nonsense, small frameshift del/dup and indels (23%), splicing (10%) and one large deletion involving exons 24-25 (Rutsch et al. 2008. PubMed ID: 20016754; Lorenz-Depiereux et al. 2010. PubMed ID: 20137773, Human Gene Mutation Database). Pathogenic variants in ENPP1 have also been reported in patients with autosomal dominant Cole disease (Eytan et al. 2013. PubMed ID: 24075184), and autosomal recessive hypophosphatemic rickets (Steichen-Gersdorf et al. 2015. PubMed ID: 25741938).

Pathogenic variants in the ABCC6 gene have been reported in 88% of patients with diagnosed autosomal recessive Pseudoxanthoma elasticum (Legrand et al. 2017. PubMed ID: 28102862). The ABCC6 protein (ATP-binding cassette subfamily C member 6) coded by ABCC6 gene is a member of ATP-binding cassette (ABC) protein and serves as an efflux transporter and regulates plasmatic inorganic pyrophosphate (Jansen et al. 2014. PubMed ID: 24969777). To date, ~ 350 unique pathogenic variants in the ABCC6 gene have been reported. In one study, pathogenic variants in the ABCC6 gene have been 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 patients of European American descent (Legrand et al. 2017. PubMed ID: 28102862). Pathogenic variants have also been reported in patients with autosomal dominant pseudoxanthoma elasticum, forme fruste, and autosomal recessive generalized arterial calcification of infancy/ pseudoxanthoma elasticum.

In one study, pathogenic variants in the ENPP1 gene were reported in 75% of patients with clinically diagnosed generalized arterial calcification of infancy (Rutsch et al. 2008. PubMed ID: 20016754). Only one large deletion involving deletion of exons 24-25 was reported (Lorenz-Depiereux et al. 2010. PubMed ID: 20137773).

In another 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% of cases from all ethnic backgrounds, and a large deletion involving exons 23 to 29 was found in 28% of patients of European American descent (Legrand et al. 2017. PubMed ID: 28102862).

This test is performed using Next-Gen sequencing with additional Sanger sequencing as necessary.

This test provides 100% coverage of all coding exons of the genes plus 10 bases of flanking noncoding DNA in all available transcripts along with other non-coding regions in which pathogenic variants have been identified at PreventionGenetics or reported elsewhere. We define full coverage as >20X NGS reads or Sanger sequencing. This test also identifies the ABCC6 c.1780-29T>A and c.2248-12_2248-11del variants.

Due to the homologous sequence in 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).

Dependent on the sequencing backbone selected for this testing, discounted reflex testing to any other similar backbone-based test is available (i.e., PGxome panel to whole PGxome; PGnome panel to whole PGnome).