Identification of ENPP1 Haploinsufficiency in Patients With Diffuse Idiopathic Skeletal Hyperostosis and Early-Onset Osteoporosis

Abstract

Homozygous ENPP1 mutations are associated with autosomal recessive hypophosphatemic rickets type 2 (ARHR2), severe ossification of the spinal ligaments, and generalized arterial calcification of infancy type 1. There are a limited number of reports on phenotypes associated with heterozygous ENPP1 mutations. Here, we report a series of three probands and their families with heterozygous and compound heterozygous ENPP1 mutations. The first case (case 1) was a 47-year-old male, diagnosed with early-onset osteoporosis and low-normal serum phosphate levels, which invoked suspicion for hypophosphatemic rickets. The second and third cases were 77-and 54-year-old females who both presented with severe spinal ligament ossification and the presumptive diagnosis of diffuse idiopathic skeletal hyperostosis (DISH). Upon workup, fibroblast growth factor 23 (FGF23) was noted to be relatively high in case 2 and serum phosphorous was low-normal in case 3, and the diagnoses of X-linked hypophosphatemic rickets (XLH) and ARHR2 were considered. Genetic testing for genes related to congenital hypophosphatemic rickets was therefore performed, revealing heterozygous ENPP1 variants in cases 1 and 2 (case 1, c.536A>G, p.Asn179Ser; case 2, c.1352A>G, p.Tyr451Cys) and compound heterozygous ENPP1 variants in case 3 constituting the same variants present in cases 1 and 2 (c.536A>G, p.Asn179Ser and c.1352A>G, p.Tyr451Cys). Several in silico tools predicted the two variants to be pathogeneic, a finding confirmed by in vitro biochemical analysis demonstrating that the p.Asn179Ser and p.Tyr451Cys ENPP1 variants possessed a catalytic velocity of 45% and 30% compared with that of wildtype ENPP1, respectively. Both variants were therefore categorized as pathogenic loss-of-function mutations. Our findings suggest that ENPP1 mutational status should be evaluated in patients presenting with the diagnosis of idiopathic DISH, ossification of the posterior longitudinal ligament (OPLL), and early-onset osteoporosis.

Catalysis-Independent ENPP1 Protein Signaling Regulates Mammalian Bone Mass

“…ENPP1 haploinsufficiency is associated with early onset osteoporosis in humans, a finding also noted in ENPP1-deficient murine models. (2,12) The loss of ENPP1 catalytic activity in ENPP1 deficiency has been proposed to induce the osteopenia in ENPP1 deficiency, but decreased PPi should result in increased and not decreased bone mass because PPi inhibits hydroxyapatite formation, which constitutes the hard mineralized matrix of bone. The low bone mass induced by ENPP1 deficiency cannot therefore be explained by an understanding of the enzyme's catalytic function alone, emphasizing that other mechanisms are likely to be involved.…”

“…Patients with CKD‐MBD exhibit significant fracture risk with high subsequent mortality, and one of the primary physiologic mechanism driving these mineralization abnormalities is secondary hyperparathyroidism induced by imbalances in Ca, Pi, and vitamin D due to renal failure. Hyperparathyroidism has also been noted in patients with ENPP1 haploinsufficiency ( 12 ) and in humans and mice with homozygous ENPP1 deficiency. ( 2,14,29,33 ) ENPP1 and/or PPi deficiency may therefore exacerbate the secondary hyperparathyroidism present in CKD‐MBD, further exacerbating the aberrant bone mineralization present in renal failure.…”

“…(10,11) The phenotype present in GACI is readily apparent from our understanding of the physical-chemical properties of ENPP1's enzymatic products-reduced extracellular PPi leads to hydroxyapatite deposition in soft tissues, resulting in the observed lethal arterial calcifications in GACI. What is not apparent is why middle-aged adults with ENPP1 haploinsufficiency exhibit early onset osteoporosis, (2,12) or why homozygous deficiency of ENPP1 in children who survive GACI exhibit low bone mass that progressively worsens, even following treatment of their hypophosphatemic rickets with phosphate supplementation. (13) GACI/ARHR2 patients commonly experience repeated long-bone fractures, rachitic skeletal deformities, and impaired growth and development as children, as well as painful enthesopathies due to calcifications of tendons and ligaments during adulthood.…”

Adult Presentation of X-Linked Hypophosphatemia

“…It has been recognized that enthesopathy is also frequently present in patients with other inherited FGF23-related hypophosphatemic rickets, including autosomal recessive hypophosphatemic rickets 1 and 2 caused by homozygous mutations in DMP1 and ENPP1, respectively, although other types of inherited or acquired FGF23-related hypophosphatemia are not associated with the development of enthesopathy [28,29]. Recently, we reported that haploinsufficiency of ectonucleotide pyrophosphatase/phosphodiesterase (ENPP1) with heterozygous or compound heterozygous mutations of ENPP1 is also associated with milder phenotypes of enthesopathy, manifested by OPLL and diffuse idiopathic skeletal hyperostosis (DISH) [30]. This means that the enthesopathy present in patients with XLH and ARHR1/2 is not a consequence of chronic hypophosphatemia or high levels of serum FGF23, and there is a common mechanism to develop enthesopathy among XLH and ARHR1/2.…”

“…As in the case with enthesopathy, conventional therapy did not improve or prevent the development of osteoarthritis in the adults with XLH, while the effect of burosumab on this complication is indecisive. [30]. This means that the enthesopathy present in patients with XLH and ARHR1/2 is not a consequence of chronic hypophosphatemia or high levels of serum FGF23, and there is a common mechanism to develop enthesopathy among XLH and ARHR1/2.…”