Generalized arterial calcification (AC) of infancy (GACI) is an autosomal recessive disorder that features deposition of hydroxyapatite (HA) within the internal elastic lamina of medium and large arteries, leading to intimal fibrous tissue proliferation and blood vessel stenosis.1–4 In 2008, a review of GACI found fewer than 200 reported patients.5 Most cases (GACI1; OMIM# 208000) represent loss-of-function mutations within the gene that encodes ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1; OMIM# *173335),6 but a similar phenotype (GACI2; OMIM# 614473) results from inactivating mutations within the gene for ATP-binding cassette transmembrane transporter, subfamily C, member 6 (ABCC6; OMIM# *603234).7, 8 ENPP1 is expressed on osteoblasts, chondrocytes, and vascular smooth muscle cells9 and hydrolyzes extracellular nucleotide triphosphates to liberate inorganic pyrophosphate (PPi), a potent inhibitor of mineralization.10, 11 Hence, ENPP1 deficiency in GACI1 reduces extracellular PPi levels and predisposes to ectopic calcification (EC).12 The precise function of ABCC6 is unclear.8
The age at presentation of GACI is bimodal.5 In the perinatal period, hydrops, heart failure, respiratory distress, or cyanosis typically occurs.13 Alternatively, presentation at approximately 3 months of age features refusal to feed, irritability, or heart failure.5 GACI is usually diagnosed from radiographic evidence of diffuse AC plus EC of other soft tissues including joints and kidneys.1
Without treatment, approximately 85% of GACI patients die by 6 months of age from cardiac ischemia or heart failure.14, 15 Treatment of the AC includes etidronate (EHDP; ethane-1-hydroxy-1,1-diphosphonic acid, also known as 1-hydroxyethylidene-bisphosphonate).16 Sufficient doses of this first-generation bisphosphonate (BP), an analogue of PPi, inhibit mineralization.17 In 2008, a retrospective, multicenter, observational analysis of 55 patients showed survival beyond infancy with EHDP therapy.18 That same year, Chong and Hutchins5 reported that 15 of 22 GACI survivors had received EHDP.
EHDP treatment of GACI can eliminate the radiographically evident AC within 2 weeks to 2 years, and is then typically stopped.16, 19 Recurrence of AC has not been reported,20, 21 but its long-term sequelae can include persistent arterial luminal narrowing from fibrosis.19, 22, 23 Generalized skeletal disease has not been reported in early GACI. However, hypophosphatemic rickets has occurred in a few surviving GACI patients at varying ages both with and without bisphosphonate treatment.18, 19, 24, 25
We report a 7-year-old boy with GACI and severe skeletal toxicity from protracted EHDP therapy including rickets that mimicked severe hypophosphatasia (HPP) and bone modeling abnormalities found in osteopetrosis (OPT).
Patient and Methods
Informed written consent for all studies was obtained as approved by the Human Research Protection Office, Washington University School of Medicine, St Louis, MO, USA.
This boy was referred for skeletal deformity acquired during EHDP treatment for GACI. He was admitted to our facility at age 6–11/12 years, returned as an outpatient 6 and 12 weeks later, and was readmitted at 7–4/12 years of age.
He was born at term to a 29-year-old gravida 4, para 2 woman by cesarean delivery for breech position. The prenatal course was uncomplicated.
During the first week of life, a “knot” at his left wrist was evaluated by an orthopedic surgeon with concern for a fibrous lesion. Accordingly, a chest radiograph was performed at age 24 days for the associated risk of sarcomatous pulmonary metastases. Instead, “irregular calcification” was found superior to the manubrium. Contrast-enhanced computed tomography (CT) at 30 days of age showed extensive periosteal reaction of unclear etiology involving the medial clavicles and several lower cervical vertebrae (see Radiological Findings).
At 38 days of age, a radiographic skeletal survey demonstrated EC both in and around multiple joints, and AC within multiple arteries. Mild cardiomegaly was also noted. Scrotal ultrasound showed hydroceles, and testicular microlithiasis. Noncontrast CT demonstrated extensive AC, cardiomegaly, and cardiac hypokinesis. Echocardiogram revealed left-ventricular dysfunction. GACI was diagnosed, and EHDP (Didronel; Norwich Pharmaceuticals, Inc., North Norwich, NY, USA) therapy commenced at 13 mg/kg/d orally using a compounded suspension.
At 6 months of age, radiographic AC was reported as still present. At 18 months of age, the dose of EHDP was increased to 20 mg/kg/d. Hypertension and left-ventricular dysfunction were managed with captopril and digoxin.
By 1–8/12 years of age, he no longer required digoxin. However, bone and joint pain required opioids, and he had mild scoliosis. He never walked.
At 2–9/12 years of age, cardiac anatomy and function by echocardiography were normal. Multiple carious teeth were removed by a dentist who found the roots to be untethered and without resorption (a clinical hallmark of HPP).
Over the subsequent years, cardiac function remained stable, and his care focused primarily on pain control, because skeletal deformities developed and gradually progressed.
At 3–3/12 years of age, EHDP treatment continued at 20 mg/kg/d (200 mg/d). At 4 years of age, previously evident AC of the aorta and coronary arteries was no longer visible on cardiac CT scan.
At almost 7 years of age, he was admitted to the Center for Metabolic Bone Disease and Molecular Research, Shriners Hospital for Children, St. Louis, MO, USA (Research Center). We learned of his intractable pain and tenderness of virtually all bones and joints, limited mobility, joint contractures, dysphagia, constipation, and declining visual acuity. He was wheelchair-bound with delayed motor milestones, but social and receptive language seemed normal. His daily oral medications included 200 mg of EHDP (12 mg/kg/d), frequent opioids, captopril, ferrous sulfate, famotidine, baclofen, and polyethylene glycol. His diet consisted of dairy products and pureed table food because throat pain precluded swallowing solids.
His parents were from different regions of Mexico. There was no family history of early death or skeletal deformity.
He was small and irritable, with dysmorphic facial features and joint abnormalities (Fig. 1). Vital signs were normal. Weight and length were less than the third centile for age. Head circumference was in the fifth centile for age. Plagiocephaly included a small pulsatile defect at the occiput. His eyes were proptotic, with left exotropia (Fig. 1A), but without band keratopathy. He was edentulous except for 4 carious primary molars. There was left-sided torticollis with a rigid neck and left sternoclavicular joint prominence. Cardiopulmonary, abdominal, and genitourinary exams were normal. Musculoskeletal examination was limited by his bone and joint pain. His spine was rigid and scoliosis accompanied mid-thoracic lordosis and compensatory lumbar kyphosis. Left shoulder motion was limited. The left elbow was enlarged and immobile (Fig. 1A). A subcutaneous mass protruded from the volar aspect of the right wrist, and another mass limited motion of the right third metacarpal-phalangeal (MCP) joint (Fig. 1B). The hips were rigid with left-sided dislocation causing a short limb (Fig. 1A). The right thigh bowed anteriorly (Fig. 1C). The right knee was enlarged, immobile, tender, indurated, and fluctuant. Valgus bowing with saber-shin deformity affected the left tibia. His ankles were enlarged. Hallux valgus was present bilaterally.
His diet reportedly contained 1500 mg calcium daily, estimated from a 7-day food record (2011 Recommended Daily Allowance [RDA] for age = 1000 mg/d). We matched this calcium intake until his first urine collection showed hypercalciuria (see Biochemical Findings). Fasting blood biochemical profiles (Dade Behring Dimension Xpand instrument; Siemens Health Care Diagnostics, Inc., Los Angeles, CA, USA) and key parameters of bone and mineral homeostasis were compared to values from fasting sera obtained at our facility in 2006 and 2007 from 34 healthy children.26 Mineral homeostasis assessment included timed urine collections.
Skeletal remodeling was evaluated using bone turnover markers (BTMs) including serum osteocalcin (OCN: Kit #LKON1; Siemens Health Care Diagnostics) and bone-specific alkaline phosphatase (BAP) by ELISA (Quidel Comp., San Diego, CA, USA) and urine free deoxypyridinoline (DPD: Immulite 1000 Pyrilinks – D Kit; Siemens Medical Solutions Diagnostics Ltd., Lianberis, Gwynedd, UK).
To search for biochemical evidence of HPP (see Radiological Findings), we assayed plasma pyridoxal 5′-phosphate (PLP)27 and serum PPi28 levels. Similarly, serum levels of the brain isoenzyme of creatine kinase (CK-BB) (Kit #K20; Sebia, Norcross, GA, USA),29 tartrate-resistant acid phosphatase, TRAP-5b (Kit #8033, Quidel; Los Angeles, CA, USA), lactate dehydrogenase (LDH),26 and aspartate aminotransferase (AST)26 were assayed for evidence of OPT. The methods, results, and interpretation of our molecular studies to determine the genetic basis and biochemical investigations to understand the pathogenesis of our patient's GACI are detailed in the Supplementary Appendix.
All radiological studies were reviewed. Because of EC, lumbar spine, but not total hip, dual-energy X-ray absorptiometry (DXA) (Hologic QDR 4500-A instrument; Hologic Corp., Waltham, MA, USA) could be interpreted. Renal and testicular ultrasonography were performed at St. Louis Children's Hospital, St. Louis, MO, USA.
We confirmed absence of AC in our patient by radiologic studies and stopped his EHDP treatment because of possible iatrogenic mucosal injury, odynodysphagia, and profound rachitic skeletal disease mimicking severe HPP (see Results). For his hypercalciuria (see Biochemical Findings), we recommended limiting his excessive daily calcium intake to the RDA and monitored him for possible “hungry bone” syndrome.
The general radiological findings are discussed here, and most details are provided in the figure legends. Additional observations are given in the Supplementary Results. The CT of our patient's neck, chest, and abdomen from age 6 weeks, radiograph of his left wrist at age 6 months, and radiographic skeletal surveys from ages 38 days and 33 months (Figs. 2–4) were compared to our studies spanning ages 6–11/12 to 7–4/12 years (Figs. 5, 6).
The CT demonstrated diffuse calcium deposits within the cardiovascular tissue before EHDP treatment (Fig. 2A, B). There was also ventricular wall thickening, coarctation of the aorta, aortic narrowing that progressively worsened in the distal abdominal aorta, and cardiac hypokinesia.
Skeletal survey at age 38 days, before EHDP treatment, confirmed AC extensively involving the cardiovascular system (Fig. 2C) with cardiomegaly. EC was also present over the medial left clavicle, with intraarticular and periarticular EC at the left elbow, bilateral wrists (Fig. 3E), hips (especially left), right knee and ankle, and great toes. Sonography showed testicular microlithiasis as well as hydroceles, but nephrocalcinosis was not seen. However, the skeleton appeared normal (Fig. 3A, E; Fig. 4A; Supplementary Results).
At age 6 months, the left wrist showed no residual AC in the radial or ulnar artery following 5 months of EHDP treatment. Peri- and intraarticular EC had decreased (Fig. 2D).
At age 33 months (after more than 2.5 years of EHDP treatment), the skeletal survey showed no AC and the heart appeared normal, but severe skeletal disease had developed (Fig. 3B, F; Fig. 4B; Supplementary Results). The right distal third metacarpal and third proximal phalanx were irregular and associated with a large MCP swelling with intra- and periarticular EC. Additionally, there were some features of OPT including vertebral sclerosis (Fig. 4B) and the distal femurs were “under-tubulated” with the “Erlenmeyer flask” appearance of OPT. Provisional zones of calcification were dense. The growth plate changes resulted in long-bone shortening and metaphyseal flaring. Multiple radiographic changes mimicked pediatric HPP (Fig. 3).
Upon referral, at age 6–11/12 years, a skeletal survey confirmed the absence of AC. However, the skeletal abnormalities had progressed markedly (Figs. 3C, 4C). HPP-like rachitic changes were present in most major long bones. Considerable disruption in physeal development was seen at the wrists and hands (Fig. 3G). Right hand deformities had worsened (Fig. 5A). Hip joint EC had increased, as had abnormalities of the right femur, knee, and tibia (Fig. 5D). In the left distal femur, the Erlenmeyer flask deformity had progressed (Fig. 4D). The physes at the knees and ankles were wider, more irregular, and associated with sclerosis and lucent defects in the provisional zones of calcification. The changes of defective mineralization seemed incongruent with the intra- and extraarticular EC (Fig. 3G). Hence, the changes resembling HPP and OPT were worse, and the skeleton was now profoundly deformed. Renal sonography to screen for nephrocalcinosis and nephrolithiasis from hypercalciuria revealed increased echogenicity of the renal cortex and the columns of Bertin.
Odynodysphagia explained why our patient's diet was primarily dairy products and pureed table food. He consumed an excessive 1575 mg/d of calcium during the first day of his 5-day admission.
Hemogram revealed a normal hemoglobin and hematocrit, but the leukocyte count was 18.8 K/µL (normal [Nl]: 3.8–9.8), and the platelet count was 598 K/µL (Nl: 140–440). The erythrocyte sedimentation rate was 44 mm/h (Nl: 0–15).
A serum basic metabolic panel and biochemical parameters of mineral homeostasis were unremarkable except for mild elevations in serum and urine calcium and Pi (Table 1). Then, during dietary mineral reduction, 2 daily timed urine samples (collected while consuming 1100 and then 1050 mg calcium/d) showed normal calcium/creatinine and phosphorous/creatinine ratios (Table 1). BTMs were unremarkable except for a low serum OCN and slightly high TRAP5b level (Table 1). Our search for biochemical changes of HPP showed a normal serum PPi level of 7.6 µM (Nl: 1.9–8.9) and unremarkable plasma PLP level of 32 nM (Nl: 5–107). His serum total creatine kinase (CK) activity was 105 IU/L (Nl: 31–152), but the brain isoenzyme of CK (BB-CK) was significantly elevated at 43 IU/L (Nl: 0–4), representing 41% of total CK, and was therefore consistent with an OPT.29 Serum TRAP-5b was slightly increased at 28.2 IU/L (Nl: 6.8–26.7), but LDH and AST were normal at 208 IU/L (Nl: 141–237) and 30 IU/L (Nl: 0–36), respectively.
EHDP = etidronate (ethane-1-hydroxy-1,1-diphosphonic acid, also known as 1-hydroxyethylidene-bisphosphonate); Nl = normal; ND = not done; ALP = alkaline phosphatase; BAP = bone-specific alkaline phosphatase; PTH = parathyroid hormone; TRAP-5b = tartrate-resistant acid phosphatase 5b; CK-BB = brain isoenzyme of creatine kinase; PLP = pyridoxal 5′-phosphate; PPi = inorganic pyrophosphate; DPD = deoxypyridinoline.
Calcium (Nl: 9.0–10.1 mg/dL)
Ionized calcium (Nl: 4.5–5.3 mg/dL)
Inorganic phosphate (Nl: 3.7–5.4 mg/dL)
ALP (Nl: 218–499 IU/L)
BAP (Nl: 26–259 IU/L)
PTH (Nl: 10–69 pg/mL)
1,25(OH)2 Vitamin D (Nl: 10–75 pg/mL)
TRAP5b (Nl: 6.8–26.7 IU/L)
CK-BB (Nl: 0–4 IU)
Osteocalcin (Nl: 37–119 ng/mL)
PLP (Nl: 5–107 nM)
PPi (Nl: 1.9–8.9 µM)
DPD/nM creatinine (Nl: 13.7–41.0)
Calcium/creatinine ratio (Nl: 75–250 mg/g)
261, 184, 181
Phosphorus/creatinine ratio (Nl: 0.8–1.8 g/g)
2.2, 2.3, 1.6
Findings after cessation of EHDP therapy
Six weeks after stopping EHDP, remarkable improvement occurred in our patient's pain, mobility, demeanor, and social expression (including Wong Baker Faces Pain Scale). His speech was understandable, he was more physically interactive, and requests for opioids had diminished. The erythema and swelling of the volar mass at his right wrist had markedly decreased. Radiographs of the wrists showed a reversal of the disordered mineral deposition (Fig. 6B, F). Similar improvements were noted throughout the skeleton.
Three months after stopping EHDP, he was animated, talkative with clear speech, and crawling in a quadruped position. Analgesic dosing had been decreased by 50–70%. Mobility of all joints had improved, except for limited knee and hip extension. The right wrist mass was gone, with only some residual induration. The enlarged right knee was much smaller. Improved range-of-motion of the upper limbs enabled him to propel his wheelchair. Dramatic skeletal improvement continued (Fig. 5; Fig. 6C, G) and resembled the responses observed in infants and young children receiving bone-targeted, enzyme-replacement therapy for severe HPP.30 Renal and testicular ultrasound studies revealed persistent, but surprisingly improved, testicular microlithiasis when compared to the postnatal images. Neither medullary nephrocalcinosis nor nephrolithiasis was present. Increased echogenicity in the renal cortex and columns of Bertin resembled the findings on referral. AC was not evident in the abdominal aorta, celiac artery, or superior mesenteric vessels.
During his readmission 6 months after withdrawal of EHDP, he was 0.9 kg heavier, talked enthusiastically, and “danced” in his wheelchair. The right third finger was more functional, and the subcutaneous masses at the right third MCP joint were also gone. The occipital defect had healed. Pill counts showed a 60% and 75% reduction of hydrocodone and methadone, respectively. Radiographs showed continued improvement in physeal changes. Remarkably, the peri- and intraarticular EC had almost resolved. Erlenmeyer-flask deformities of the distal left femur were also resolving (Supplementary Results). Bone mineral density (BMD) of his lumbar spine on DXA had increased from a Z-score of +5.7 to +6.2, in keeping with mineralization of excessive osteoid associated with rickets. Fusion of the carpal bones, posterior elements of the spine, and cranial sutures persisted (Figs. 5, 6).
Six weeks after EHDP withdrawal and reduction of dietary calcium, key biochemical parameters of mineral and skeletal homeostasis were normal except serum OCN remained low at 14 pg/mL (Nl: 37–119) (Table 1). CK-BB had decreased to 21 IU (Nl: 0–4), but TRAP-5b had increased to 45.7 IU/L (Nl: 6.8–26.7). Three consecutive nighttime urine collections showed normal calcium/creatinine and phosphorus/creatinine ratios (Table 1). Three months after stopping EHDP, serum alkaline phosphatase (ALP) had increased to 384 IU/L (Nl: 218–499).
At his readmission, 6 months off EHDP, serum OCN remained low at 13 pg/mL (Nl: 37–119), CK-BB activity was now nearly normal at 5 IU/L (Nl: 0–4), and TRAP-5b was normal at 22 IU/L (Nl: 6.8–26.7). Serum ALP and BAP had both risen steadily in keeping with ongoing healing of rickets as well as perhaps reduced inhibition of ALP by EHDP (see Discussion). The plasma PLP level remained normal at 57 nM (Nl: 5–107) as did the urine calcium/creatinine ratio and DPD/creatinine ratio. We found no biochemical changes concerning for hungry bone syndrome (Table 1).
Our 7-year-old patient with GACI was given high-dose EHDP therapy nearly lifelong. Although his AC quickly resolved, spinal as well as intra- and periarticular EC worsened. Additionally, he developed severe skeletal toxicity, with some radiographic findings mimicking HPP while others resembled OPT. After withdrawal of EHDP, his EC and rickets rapidly corrected. Below, we discuss BP therapy for GACI, the mechanisms likely responsible for our patient's unique skeletal disturbance, and the remarkable healing that followed cessation of EHDP exposure.
BPs are typically prescribed for skeletal disease because they inhibit osteoclasts (OCs).31 However, their hydrolysis-resistant P-C-P motif resembles the tissue nonspecific alkaline phosphatase (TNSALP) susceptible P-O-P core of PPi, a potent inhibitor of mineralization.11 EHDP was shown in the 1960s to block precipitation of calcium phosphate salts,32 and has been known for decades to inhibit crystallization of these minerals in vitro and in vivo.33 EHDP binds to the surface of matrix vesicles, and impairs their crystallization of calcium and phosphate.34 Hence, EHDP was used to treat GACI16 long before low levels of PPi were identified in its pathogenesis35, 36 and its genetic basis was discovered in 2003.6 EHDP treatment for GACI was supported by two clinical observations:1 hydroxyapatite (HA) was found in specimens of arteries from GACI patients postmortem,3 and2 EHDP seemed promising as an inhibitor of ectopic bone formation in fibrodysplasia ossificans progressiva (FOP).17, 37, 38 Fortunately, the first use of EHDP for GACI proved successful.12 Recent studies have associated BP therapy (predominantly EHDP) with improved survival in GACI.18 However, rickets or osteomalacia can develop during protracted EHDP administration for any disorder, although few reports are in the medical literature.23, 24, 38–41 Amino-BPs were formulated to be more potent antiresorptive agents, and therefore less likely to interfere with mineralization at therapeutic levels.42 Nevertheless, in 2009, treatment with an intravenous infusion of the nitrogen-containing BP, pamidronate (PMD), seemed successful for a GACI patient.43 Also in 2009, other investigators reported their plan to stop risedronate therapy after 3 years of successful treatment of a GACI patient.44 Hence, PPi-like effects of all BPs might reverse the AC of GACI. Of interest, nitrogen-containing BPs are now being assessed for calcific aortic stenosis.45
In 2003, the discovery in GACI that the cell-surface PPi-generating enzyme, ENPP1, has diminished activity from loss-of-function mutations in its gene6 provided rationale for “replenishment” of PPi effects using BPs. Nevertheless, concerns for EHDP treatment included: (i) expectation that it would not ameliorate any vascular fibroblast proliferation,4 which could be irreversible,46 and (ii) BPs incorporate into non-hydrolyzable adenosine-containing compounds,47, 48 which are toxic to cells.49, 50 Thus, BP excesses in non-osseous tissues could have unpredictable and undesirable effects.51 Accordingly, EHDP remains a mainstay of therapy for GACI, but doses are typically tapered and then discontinued after the AC resolves.19, 24
HPP is the inborn error of metabolism caused by loss-of-function mutation(s) within the gene that encodes TNSALP.52 PPi is a natural substrate for this cell-surface enzyme,53 and consequently PPi accumulates extracellularly in HPP and leads to rickets or osteomalacia.54, 55 Elevated plasma levels of PLP in HPP reflect extracellular excesses of this TNSALP substrate.54, 55 The radiographic features of pediatric HPP can include severe rachitic disease with widened physes, irregular provisional zones of calcification, lucent defects, and intervening areas of sclerosis within flared metaphyses, focal “tongues” of radiolucency that extend into metaphyses from physes, bowed long bones, increased digital markings with a “beaten copper” appearance of the skull from premature bony fusion of cranial sutures, and a widened diploic space.30, 53 Enlarged pulp chambers in teeth are sometimes observed.53 HPP in adults features osteomalacia and sometimes complications of calcium pyrophosphate dihydrate (CPPD) deposition, or calcific periarthritis from seemingly paradoxical formation of HA near joints.53 Our patient's elevated erythrocyte sedimentation rate, thrombocytosis, and leukocytosis suggested systemic inflammation, which can occur from intraarticular calcium-containing crystal deposition.56 However, other tissues seemed unaffected and, therefore, further rheumatologic study was not undertaken. After discontinuing EHDP, these parameters normalized within 6 months.
All of the principal radiographic characteristics of pediatric HPP30, 53 were acquired by our patient during EHDP therapy. Additionally, he suffered skeletal pain and premature loss of teeth without tooth root resorption consistent with HPP.53 Of note, EHDP can irreversibly inhibit purified ALP.57 The similarity of the clinical and skeletal manifestations, as well as the hyperphosphatemia from enhanced renal reclamation of Pi, of EHDP toxicity to HPP is recognized.58 However, our studies seeking biochemical hallmarks of HPP were unremarkable, including normal circulating levels of ALP, BAP, PLP, and PPi, despite the mild abnormalities of mineral homeostasis. Furthermore, mutation analyses of TNSALP and PHOSPHO1 were negative, showing no genetic basis for HPP (Supplementary Appendix). Protracted EHDP administration to our patient seemed to have its toxic effects directly within the skeleton and dentition. Perhaps, we did not find elevated plasma PLP levels because our patient's TNSALP gene was intact and bone and liver ALP were not compromised.
Despite resolution of our patient's AC while rickets developed during EHDP treatment, seemingly paradoxical EC advanced near his spine and joints until EHDP treatment stopped. Deficiency of PPi in GACI1 could cause EC at sites in addition to AC (Supplementary Appendix). Vertebral fusion is a major feature of mice genetically deficient in ENPP1.59, 60 Interestingly, adults with HPP and elevated extracellular PPi levels sometimes manifest seemingly paradoxical calcific periarthritis involving HA deposition in periarticular areas, and also Forestier disease (calcification of the anterior and posterior longitudinal ligaments in the spine).61 Therefore, the fusion of the posterior elements of the spine of our patient may be from his GACI1 but refractory to EHDP therapy, or perhaps from his EHDP toxicity that mimicked HPP. Periarticular EC in GACI typically resolves during EHDP therapy.16, 62, 63 Our patient's joint-centered EC worsened with EHDP exposure, and seemed to explain why EHDP treatment was continued. However, within 6 weeks of stopping EHDP, this EC diminished, and was nearly gone after 6 months. Alternatively, perhaps his unique genetic constitution (Supplementary Appendix) altered tissue-specific mineral homeostasis in unexpected ways, or the paradox of rickets with EC reflected differences in tissue-specific EHDP penetration. BPs can form insoluble aggregates with divalent cations.64 In one report, 200 mg/d EHDP for 2 weeks in three repeating 12-week cycles resulted in CPPD deposition in the joint fluid of an adult.65 We wonder if our patient's lifelong, high-dose EHDP exposure resulted in skeletal saturation and EHDP precipitation within joints, mimicking CPPD crystal deposition. However, we did not biopsy his joint-centered EC because of concern for infection, etc.
Our patient developed features of OPT from failure of OC action during growth.66 Different BPs inhibit OCs by different biochemical mechanisms, but theoretically all can cause OPT in toxic doses in children.31 In fact, we have reported a boy with PMD-induced OPT.67 The clinical features of OPT can include cranial nerve palsies because cranial foramina do not widen during growth, and possibly cranial nerve palsy explained our patient's exotropia. The radiographic characteristics of OPT include generalized osteosclerosis with diminished metaphyseal modeling (shaping) featuring undertubulation of metaphyses.68 The biochemical abnormalities of OPT can reflect impaired OC function and increased numbers of OCs, including elevations in serum of CK-BB and TRAP-5b, which emanate from the OCs.26, 29 Our patient's radiographic features of OPT included vertebral endplate sclerosis, lumbar spine DXA Z-score of +5.7, and Erlenmeyer flask deformity of his distal femurs. His elevated serum CK-BB and TRAP-5b levels were also consistent with genetic or BP-induced OPT, and corrected with cessation of EHDP exposure. His normal serum LDH and AST levels gave no indication of Albers-Schönberg disease (chloride channel 7 deficiency OPT).26 His precarious clinical situation at presentation precluded iliac crest biopsy to assess for any accumulation of calcified primary spongiosa from OPT.67
Features of both HPP and OPT underscore the profound skeletal toxicity our patient experienced from protracted EHDP therapy. On referral, he was receiving a dose given for 6 months to some adults with Paget's disease of bone.69 His mild hypercalcemia, hypercalciuria, and hyperphosphatemia probably reflected his excessive intake of dairy products as well as a block in skeletal uptake of mineral from EHDP effects. These biochemical findings excluded rickets from nutritional deficiencies, and rapidly corrected when dairy product consumption decreased. Some GACI patients develop hypophosphatemic rickets,18, 25, 70 perhaps related to the disturbance of PPi metabolism.71 Acquired hypophosphatemia and rickets (OMIM# 613312) caused by his two altered ENPP1 alleles was an unlikely cause of his bone problems because his serum Pi level fell below the normal range just once as we followed him and without hyperphosphaturia. Theoretically, any predisposition to hypophosphatemia for our patient may have been masked by EHDP treatment, which can cause hyperphosphatemia.64 We note the interesting contrast of acquired hypophosphatemia with PPi deficiency in GACI compared to the hyperphosphatemia with PPi excess in HPP.53 Our patient's serum PTH level remained normal after EHDP was stopped, indicating that despite his lowered dietary calcium and phosphorus intake mineral absorption was adequate to keep up with the hungry bones documented radiographically. Serum ALP and BAP increased steadily and became distinctly elevated, consistent with healing rickets.72 His low serum OCN level at baseline suggested global suppression of bone turnover,73 but his normal urine DPD level at baseline perhaps somehow reflected his high skeletal mass. Although bone mineralization increased after EHDP withdrawal, inexplicably serum OCN levels remained low.
In conclusion, our experience shows that infants with GACI who receive lifesaving EHDP treatment warrant close monitoring for resolution of AC so that this BP can be discontinued at the earliest opportunity. Protracted EHDP therapy for GACI can cause severe skeletal toxicity.
All authors state that they have no conflicts of interest.
We thank the nursing, laboratory, and dietary staff of the Center for Metabolic Bone Disease and Molecular Research, Shriners Hospital for Children, St Louis, MO, USA for making this report possible. Xiafang Zhang sequenced the ENPP1 gene, and Margaret Huskey the TNSALP and ACVR1 genes. Vivienne McKenzie and Sharon McKenzie helped to create the manuscript. Supported in part by Shriners Hospitals for Children, The Clark and Mildred Cox Inherited Metabolic Bone Disease Research Fund, The Hypophosphatasia Research Fund, The Frederick S. Upton Foundation, and The Barnes-Jewish Hospital Foundation.
Authors' roles: All of the authors helped to draft this manuscript or to revise it for intellectual content, and all approved the submitted version. Clinical oversight and investigation: JEO, GSG, KLM, and MPW. Interpretation of diagnostic imaging: WHM. Mutation analyses: SM, CS, JLM. Kinetic analysis: TK-M, JLM. Site-directed mutagenesis and expression: CS, JLM. Vitamin B6 analysis: KLE. Manuscript completion: MPW.