Hypophosphatemia, hyperphosphaturia, and bisphosphonate treatment are associated with survival beyond infancy in generalized arterial calcification of infancy

Abstract

Background: Generalized arterial calcification of infancy has been reported to be frequently lethal, and the efficiency of any therapy, including bisphosphonates, is unknown. A phosphate-poor diet markedly increases survival of NPP1 null mice, a model of generalized arterial calcification of infancy.

Methods and results: We performed a multicenter genetic study and retrospective observational analysis of 55 subjects affected by generalized arterial calcification of infancy to identify prognostic factors. Nineteen (34%) patients survived the critical period of infancy. In all 8 surviving patients tested, hypophosphatemia due to reduced renal tubular phosphate reabsorption developed during childhood. Eleven of 17 (65%) patients treated with bisphosphonates survived. Of 26 patients who survived their first day of life and were not treated with bisphosphonates only 8 (31%) patients survived beyond infancy. Forty different homozygous or compound heterozygous mutations, including 16 novel mutations in ENPP1, were found in 41 (75%) of the 55 patients. Twenty-nine (71%) of these 41 patients died in infancy (median, 30 days). Seven of the 14 (50%) patients without ENPP1 mutations died in infancy (median, 9 days). When present on both alleles, the mutation p.P305T was associated with death in infancy in all 5 cases; otherwise, no clear genotype-phenotype correlation was seen.

Conclusion: ENPP1 coding region mutations are associated with generalized arterial calcification of infancy in approximately 75% of subjects. Except for the p.P305T mutation, which was universally lethal when present on both alleles, the identified ENPP1 mutations per se have no discernable effect on survival. However, survival seems to be associated with hypophosphatemia linked with hyperphosphaturia and also with bisphosphonate treatment.

Keywords: genetics; mortality; pediatrics; prognosis; survival.

Figure 1

Characteristic manifestations of GACI. A through E, Patient 6 in his first week of age. A, Echocardiogram showing bright echogenic walls of the aortic arch and descending aorta (arrows), consistent with aortic calcifications. B, Increased echogenicity of the aortic root (arrowhead) and left (LCA) and right (RCA) coronary arteries (arrows). C, Increased echogenicity of abdominal aorta (arrows) and celiac trunc (CT) of the same patient. D, Periarticular calcifications (arrows) of the carpal joint and around carpal bones of the left hand. E, Chest x-ray showing moderate cardiomegaly and periarticular calcifications (arrows) of both shoulders. F, Cross-section through the aorta from patient 41, who died at the age of 6 weeks, showing calcification at the level of the internal elastic lamina (arrows) and marked intima proliferation (arrowheads) (hematoxylin–eosin, ×20).

Figure 2

Schematic representation of the human ENPP1 gene and protein with mutations identified in 55 GACI patients. Numbered boxes represent the 25 exons; patterned boxes represent functional domains. Novel mutations are shown in boldface. Insert at the left top of the figure shows allele frequency of most common ENPP1 mutations in our study cohort. SO Domain indicates somatomedin B-like domain. *Splice site mutations c.430+2T>C and 556-2A>G result most likely in exon skipping and hence in frameshifts. †The mutation P365fsX15 was previously shown to result from skipping of exon 11 caused by the mutation c.1164+2T>A.

Figure 3

Cumulative survival in GACI patients of our study cohort. A, Overall survival in all 55 patients, including 12 patients who presented as stillbirths or died before the age of 1 day. B, Survival beyond infancy in 43 patients, who survived their first day of life, including 17 patients treated with bisposphonates compared with 26 patients, who did not receive bisphosphonate therapy. Patients treated with bisphosphonates showed significantly increased likelihood of survival (P=0.026; log-rank test).

Figure 4

Serum phosphate levels and maximal renal tubular phosphate reabsorption in patients with GACI surviving beyond infancy. A, Serum phosphate levels available from 13 surviving patients. B, TmP/GFR levels available from 11 surviving patients. Normal serum phosphate levels in children between 1 and 3 years is 1.00 to 1.95 mmol/L, between 4 and 6 years is 1.05 to 1.80 mmol/L, and between 7 and 9 years is 0.95 to 1.75 mmol/L. TmP/GFR was calculated according to Brodehl et al using the formula TmP/GFR=S_p-(U_p×S_crea)/U_crea. Normal TmP/GFR in children between 2 and 15 years is 1.15 to 2.44 mmol/L. The means of serum phosphate levels and the means of TmP/GFR levels of each surviving patient were significantly lower than the lowest reference values in patients older than 3 years of age (P=0.031 for serum phosphate and P=0.004 for TmP/GFR levels; Wilcoxon test for 2 paired samples).