A Case of Vitamin D-Dependent Rickets Type 1A with a Novel Mutation in the Uzbek Population

A toddler with severe vitamin D-dependent rickets type 1 A (VDDR1A), hungry bone syndrome, and severe RSV infection: presentation and therapeutic challenges

BACKGROUND

Vitamin D-dependent rickets type 1 A (VDDR1A) is an autosomal recessive disorder due to mutations in the CYP27B1 gene which result in inability to generate 1,25(OH)2D.

CASE PRESENTATION

An 18-month-old boy with VDDR1A presented with hypotonia and respiratory distress. He had been diagnosed 2 months earlier, having been evaluated for stunted growth, hypotonia, and delayed developmental milestones. He was stabilized with oxygen and bronchodilators for his bronchiolitis and high doses of alfacalcidol, calcium, and phosphate supplements for his hungry bone syndrome. Of note, the patient sustained upper limb fractures after a fall from his bed during admission. Overall, he had a protracted disease course; however, his bone profile gradually improved and he steadily recovered.

CONCLUSION

VDDR1A causes failure to thrive, hypotonia, and increased fracture risk and may complicate the clinical course of lower respiratory tract infections. Furthermore, management of hungry bone syndrome requires supraphysiologic doses of vitamin D metabolites and calcium.

Siblings with vitamin D-dependent rickets type 1A: Importance of genetic testing and a review of genotype-phenotype correlations

Vitamin D-dependent rickets type 1A (VDDR1A) is a rare condition caused by biallelic pathogenic variants in CYP27B1, which encodes 25-hydroxyvitamin D3-1-α-hydroxylase. Inadequate activity of this enzyme results in deficient 1α-hydroxylation of inactive 25-hydroxyvitamin D to biologically active 1,25-dihydroxyvitamin D, with consequent adverse effects on calcium and phosphate metabolism. A female child was clinically diagnosed at 18 months old with hypophosphatemic rickets based on phenotype and biochemical testing, with neither parent affected. A subsequent affected male sibling led to the reconsideration of the diagnosis. Exome sequencing showed a homozygous CYP27B1 c.1040T>A (p.Ile347Asn) variant for both children. No variants were found in genes associated with hypophosphatemic rickets. A review of published cases of VDDR1A with homozygous CYP27B1 variants indicates variable clinical presentation, lack of genotype-phenotype correlation, and low serum phosphate at diagnosis in most cases. These findings emphasize the clinical importance of molecular testing as part of the diagnostic evaluation for cases of non-nutritional rickets.

Vitamin D-Dependent Rickets Type 1A in Two Siblings with a Hypomorphic CYP27B1 Variant Frequent in the African Population

Vitamin D-dependent type 1A rickets (VDDR-1A) is a rare autosomal recessive disease due to the inability to convert 25-hydroxyvitamin D [25(OH)D] to the active form 1.25-dihydroxyvitamin D [1.25(OH) 2 D] by the enzyme 25(OH)D-1α-hydroxylase leading to low or low-normal serum levels of [1.25(OH) 2 D]. We report two sisters with rickets in whom the diagnosis of VDDR-1A was a challenge. They had normal 1.25(OH)2D levels, which are unusual with this condition but may be explained by the identified genotype. Both have compound heterozygous for two, most likely, hypomorphic CYP27B1 alleles: the novel p.(Arg117Gly) variant, and p.(Ala129Thr), which are present in 0.43% of the African population. This report illustrates the variability of clinical, laboratory, and radiological presentation between two sisters with the same genotype, during phases of faster or slower growth. Genetic testing was crucial for establishing the diagnosis that optimized the management and genetic counseling.

Molecular Diagnoses of X-Linked and Other Genetic Hypophosphatemias: Results From a Sponsored Genetic Testing Program

X-linked hypophosphatemia (XLH), a dominant disorder caused by pathogenic variants in the PHEX gene, affects both sexes of all ages and results in elevated serum fibroblast growth factor 23 (FGF23) and below-normal serum phosphate. In XLH, rickets, osteomalacia, short stature, and lower limb deformity may be present with muscle pain and/or weakness/fatigue, bone pain, joint pain/stiffness, hearing difficulty, enthesopathy, osteoarthritis, and dental abscesses. Invitae and Ultragenyx collaborated to provide a no-charge sponsored testing program using a 13-gene next-generation sequencing panel to confirm clinical XLH or aid diagnosis of suspected XLH/other genetic hypophosphatemia. Individuals aged ≥6 months with clinical XLH or suspected genetic hypophosphatemia were eligible. Of 831 unrelated individuals tested between February 2019 and June 2020 in this cross-sectional study, 519 (62.5%) individuals had a pathogenic or likely pathogenic variant in PHEX (PHEX-positive). Among the 312 PHEX-negative individuals, 38 received molecular diagnoses in other genes, including ALPL, CYP27B1, ENPP1, and FGF23; the remaining 274 did not have a molecular diagnosis. Among 319 patients with a provider-reported clinical diagnosis of XLH, 88.7% (n = 283) had a reportable PHEX variant; 81.5% (n = 260) were PHEX-positive. The most common variant among PHEX-positive individuals was an allele with both the gain of exons 13-15 and c.*231A>G (3'UTR variant) (n = 66/519). Importantly, over 80% of copy number variants would have been missed by traditional microarray analysis. A positive molecular diagnosis in 41 probands (4.9%; 29 PHEX positive, 12 non-PHEX positive) resulted in at least one family member receiving family testing. Additional clinical or family member information resulted in variant(s) of uncertain significance (VUS) reclassification to pathogenic/likely pathogenic (P/LP) in 48 individuals, highlighting the importance of segregation and clinical data. In one of the largest XLH genetic studies to date, 65 novel PHEX variants were identified and a high XLH diagnostic yield demonstrated broad insight into the genetic basis of XLH. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Genotype and phenotypic spectrum of vitamin D dependent rickets type 1A: our experience and systematic review

BACKGROUND

Vitamin D dependent rickets type 1 (VDDR1) is a rare disease due to pathogenic variants in 1-α hydroxylase gene. We describe our experience with systematic review of world literature to describe phenotype and genotype.

METHODS

Seven patients from six unrelated families with genetically proven VDDR1 from our cohort and 165 probands from systematic review were analyzed retrospectively. The clinical features, biochemistry, genetics, management, and long-term outcome were retrieved.

RESULTS

In our cohort, the median age at presentation and diagnosis was 11(4-18) and 40(30-240) months. The delayed diagnoses were due to misdiagnoses as renal tubular acidosis and hypophosphatemic rickets. Four had hypocalcemic seizures in infancy whereas all had rickets by 2 years. All patients had biochemical response to calcitriol, however two patients diagnosed post-puberty had persistent deformity. Genetic analysis revealed two novel (p.Met260Arg, p.Arg453Leu) and a recurring variant (p.Phe443Profs*24). Systematic review showed that seizures as most common presentation in infancy, whereas delayed motor milestones and deformities after infancy. Diagnosis was delayed in 27 patients. Patients with unsatisfactory response despite compliance were >12 years at treatment initiation. Inappropriately normal 1,25(OH)2D may be present, however suppressed ratio of 1,25(OH)2 D/25(OH)D may provide a clue to diagnosis. Various region specific and hot-spot recurrent variants are described. Patients with truncating variants had higher daily calcitriol requirement and greatly suppressed ratio of 1,25(OH)2D/25(OH)D.

CONCLUSION

Delayed diagnosis may lead to permanent short stature and deformities. Truncating variants tend to have severe disease as compared to non-truncating variants. Diagnostic accuracy of 1,25(OH)2 D/25(OH)D ratio needs further validation.

Vitamin D-Resistant Rickets Diagnostics and Treatment Challenges at Muhimbili National Hospital, Tanzania

Introduction. Rickets is softening of bones caused by defective mineralization of the cartilage in the epiphyseal growth plate, causing widening of the ends of long bones, growth retardation, and skeletal deformities in children. It can be classified into calciopenic and phosphopenic, each type with various subclasses. Case Presentations. We presented 2 cases, first of a 1 year and 4-month-old male, with a history of recurrent episodes of cough for 8 months and bowing of the legs 6 months prior to admission. Clinical and laboratory investigation was suggestive of vitamin D-dependent rickets, and he started vitamin D treatment with minimal response. The second case is of a 4 years and 7-month-old male who presented with developmental delay, poor weight gain, and recurrent chest infection and worsening of bone pain since 9 months of age. Laboratory investigation was suggestive of phosphopenic rickets, and he was started on treatment at 9 months of age with little improvement and at 4 years, he sustained multiple fractures and succumbed to severe respiratory tract infection and died at 4 years and 7 months of age.

CONCLUSION

Rickets pose a diagnostic and treatment challenge in resource-limited countries, and clinical judgment and early initiation of treatment are important.