Clinical and Biochemical Phenotypes in a Family With ENPP1 Mutations

Abstract

Inactivating mutations of the ENPP1 gene are associated with generalized arterial calcification of infancy (GACI) and less often autosomal-recessive hypophosphatemic rickets type 2 (ARHR2). We aimed to investigate the spectrum of phenotypes in a family with monoallelic and biallelic mutations of ENPP1 after identification through whole exome sequencing of a 54-year-old female with biallelic mutation of ENPP1, c.323G > T; p.Cys108Phe and c.1441C > T; p.Arg481Trp. Including the proband, 2 subjects had biallelic mutations, 5 had monoallelic mutations, and 2 had no mutation of ENPP1. The maternal mutation, a known pathogenic variant associated with GACI, was found in 3 subjects with monoallelic mutations, while the paternal mutation, which was not previously reported, was present in 2 subjects with monoallelic mutations. Both subjects with biallelic mutations had bowing of bilateral femurs, periarticular mineral deposition, normocalcemic primary hyperparathyroidism with multigland parathyroidectomy, increased carotid intima-media thickness, and enthesopathy was also noted in one subject. Intact FGF23 was elevated in both subjects with biallelic mutations, while C-terminal FGF23 was only elevated in one and PPi was reduced in one. Subjects with monoallelic mutations did not have periarticular calcifications or bone deformities. To conclude, patients with biallelic GACI causing mutations can survive well into adulthood, and despite the same biallelic ENPP1 pathogenic variants, clinical and biochemical manifestations can significantly differ, and include enthesopathy and primary hyperparathyroidism, which have not been previously described. Although carriers of monoallelic ENPP1 variants appear unaffected by classic disease manifestations, there may be subtle biochemical and clinical findings that warrant further investigation. © 2019 American Society for Bone and Mineral Research.

Keywords: ENPP1; GENERALIZED ARTERIAL CALCIFICATION; GENETIC; HYPOPHOSPHATEMIA; RICKETS.

Fig. 1.

Bowing deformity of both femurs in subject 1 with biallelic ENPP1 mutations.

Fig. 2.

Right-hand radiographs in subject 1 with biallelic ENPP1 mutations demonstrating areas of soft tissue calcinosis (thin arrows) in the hand and wrist from 4 years prior (left panel) compared with current evaluation (right panel). A prominent area of calcinosis along the radial aspect of the right 4th MCP joint (thick arrow) has nearly completely resolved, while other areas of calcinosis have increased slightly.

Fig. 3.

ENPP1 structure providing insights into potential mechanism of dysregulation. (A) ENPP1 model colored by domain. Disulfide bonds between cysteine residues are shown in sticks representation. The region around p.Arg481 is boxed. (B) Zoom-in on the two SMB domains. Each has four disulfide bonds. Residues with AD variants are marked by red spheres. The N-terminal residue is marked by a cyan sphere. The transmembrane domain is further N-terminal than our model. P.Cys108 is the first residue involved in a disulfide bond, after the transmembrane domain. (C) Zoom-in on the region around p.Arg481. Polar contacts are represented by dashed lines. P.Arg481 makes multiple interactions within the phosphodiesterase domain and is nearby the nuclease domain. (D) Comparison of the enzymatic velocity rates of WT, p.Arg481Trp, and p.Cys108Phe ENPP1 variants sequenced in the patients. The data are scaled relative to the WT velocity rate. The p.Arg481Trp mutation was relatively mild, reducing the enzymatic velocity by 45%, and the p. Cys108Phe mutation was more severe, reducing the enzymatic velocity rate by 80%.

Fig. 4.

ENPP1 mutation family pedigree. Arrowhead indicates the proband. *Subjects genetically tested.

Fig. 5.

Anteroposterior radiograph of the lumbar spine, pelvis, and hips demonstrates enthesopathy of both innominate bones and proximal femurs (thin arrows). In addition, there is ossification of the iliolumbar ligaments at L₅ (thick arrows).

Fig. 6.

Lateral radiograph of the thoracic spine demonstrates hypertrophic changes at all of the thoracic and lumbar interspaces (thick arrow) and ankylosis of the sternomanubrial joint (thin arrow).