Mutation update: Variants of the ENPP1 gene in pathologic calcification, hypophosphatemic rickets, and cutaneous hypopigmentation with punctate keratoderma.

ENPP1 encodes ENPP1, an ectonucleotidase catalyzing hydrolysis of ATP to AMP and inorganic pyrophosphate (PPi), and an endogenous plasma protein physiologically preventing ectopic calcification of connective tissues. Mutations in ENPP1 have been reported in association with a range of human genetic diseases. In this mutation update, we provide a comprehensive review of all the pathogenic variants, likely pathogenic variants, and variants of unknown significance in ENPP1 associated with three autosomal recessive disorders-generalized arterial calcification of infancy (GACI), autosomal recessive hypophosphatemic rickets type 2 (ARHR2), and pseudoxanthoma elasticum (PXE), as well as with a predominantly autosomal dominant disorder-Cole disease. The classification of all variants is determined using the latest ACMG guidelines. A total of 140 ENPP1 variants were curated consisting of 133 previously reported variants and seven novel variants, with missense variants being the most prevalent (70.0%, 98/140). While the pathogenic variants are widely distributed in the ENPP1 gene of patientsgen without apparent genotype-phenotype correlation, eight out of nine variants associated with Cole disease are confined to the somatomedin-B-like (SMB) domains critical for homo-dimerization of the ENPP1 protein.

Hyperpigmentation in a Chinese family with autosomal dominant Cole disease.

Cole disease (OMIM 615522), caused by mutations in ENPP1, is a rare autosomal dominant or recessive genodermatosis characterized by guttate hypopigmentation and punctate palmoplantar keratoderma. To date, a few cases with autosomal recessive inheritance had been reported with hyperpigmentation. The aim of this case report was to investigate the molecular basis of individuals with hyperpigmentation, hypopigmentation and punctate keratoderma in a Chinese family. A Chinese pedigree of suspected Cole disease with hyperpigmentation was subjected to mutation detection in the ENPP1 gene. All exons of the ENPP1 gene and adjacent exon-intron border sequences were amplified using polymerase chain reaction and directly sequenced. Three-dimensional (3D) models of the wild-type and mutated ENPP1 proteins were predicted by PyMOL viewer. Both of the proband and his affected father carried a heterozygous missense mutation p.C176R in ENPP1. In silico modelling of the ENPP1 wild-type and ENPP1 with the p.C176R mutation showed the residue Arg-176 disturbed the fold of the loop conformation. Based on clinical and genetic findings, a diagnosis of Cole disease was made. We identified a heterozygous mutation, p.C176R, in the ENPP1 gene in a Chinese family with Cole disease. This study clearly showed that hyperpigmentation could also occur in Cole disease in cases with autosomal dominant inheritance. Our data expand the phenotypic spectrum of ENPP1 mutations underlying Cole disease.

A novel ENPP1 mutation identified in a multigenerational family affected by Cole disease.

Cole disease is a rare autosomal dominant genodermatosis with only five cases published in literature since its first description in 1976. We report a case of a 3-year-old boy of Italian ancestry who presented with hypopigmented skin patches on the upper extremities and multiple yellowish, firm papules and small plaques on his palms and soles. There were similar findings in the family, extending back at least four generations. Whole exome sequence analysis revealed a novel variant of the ENPP1 gene mutation, which has not been previously reported to be associated with Cole disease. Although there is no extracutaneous involvement associated with this condition, accurate diagnosis and variant identification is nevertheless important so that appropriate medical and genetic counseling can be offered to affected individuals and their at-risk relatives.

Quantitative correlation of ENPP1 pathogenic variants with disease phenotype.

Ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) codes for a type 2 transmembrane glycoprotein which hydrolyzes extracellular phosphoanhydrides into bio-active molecules that regulate, inter alia, ectopic mineralization, bone formation, vascular endothelial proliferation, and the innate immune response. The clinical phenotypes produced by ENPP1 deficiency are disparate, ranging from life-threatening arterial calcifications to cutaneous hypopigmentation. To investigate associations between disease phenotype and enzyme activity we quantified the enzyme velocities of 29 unique ENPP1 pathogenic variants in 41 patients enrolled in an NIH study along with 33 other variants reported in literature. We correlated the relative enzyme velocities with the presenting clinical diagnoses, performing the catalytic velocity measurements simultaneously in triplicate using a high-throughput assay to reduce experimental variation. We found that ENPP1 variants associated with autosomal dominant phenotypes reduced enzyme velocities by 50 % or more, whereas variants associated with insulin resistance had non-significant effects on enzyme velocity. In Cole disease the catalytic velocities of ENPP1 variants associated with AD forms trended to lower values than those associated with autosomal recessive forms - 8-32 % vs. 33 % of WT, respectively. Additionally, ENPP1 variants leading to life-threatening vascular calcifications in GACI patients had widely variable enzyme activities, ranging from no significant differences compared to WT to the complete abolishment of enzyme velocity. Finally, disease severity in GACI did not correlate with the mean enzyme velocity of the variants present in affected compound heterozygotes but did correlate with the more severely damaging variant. In summary, correlation of ENPP1 enzyme velocity with disease phenotypes demonstrate that enzyme velocities below 50 % of WT levels are likely to occur in the context of autosomal dominant disease (due to a monoallelic variant), and that disease severity in GACI infants correlates with the more severely damaging ENPP1 variant in compound heterozygotes, not the mean velocity of the pathogenic variants present.