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Banerjee RR, Spence T, Frank SJ, Pandian R, Hoofnagle AN, Argiropoulos B, Marcadier JL. Very Low Vitamin D in a Patient With a Novel Pathogenic Variant in the GC Gene That Encodes Vitamin D-Binding Protein. J Endocr Soc 2021; 5:bvab104. [PMID: 34589658 PMCID: PMC8362819 DOI: 10.1210/jendso/bvab104] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/20/2021] [Indexed: 12/18/2022] Open
Abstract
Circulating plasma vitamin D metabolites are highly bound to vitamin D-binding protein
(DBP), also known as group-specific component or
Gc-globulin. DBP, encoded by the GC gene, is a member of the albumin
family of globular serum transport proteins. We previously described a homozygous
GC gene deletion in a patient with apparent severe vitamin D
deficiency, fragility fractures, and ankylosing spondylitis. Here, we report an unrelated
patient free of fractures or rheumatologic disease, but with very low 25-hydroxyvitamin D
and 1,25-hydroxyvitamin D, as well as undetectable DBP measured by liquid
chromatography–tandem mass spectrometry. A whole gene deletion was excluded by microarray,
and Sanger sequencing of GC revealed a homozygous pathogenic variant
affecting a canonical splice site (c0.702-1G > A). These findings indicate that loss of
function variants in GC that eliminate DBP, and severely reduced total
circulating vitamin D levels, do not necessarily result in significant metabolic bone
disease. Together with our previous report, these cases support the free-hormone
hypothesis, and suggest free vitamin D metabolites may serve as preferable indicators of
bone and mineral metabolism, particularly when clinical suspicion of DBP deficiency is
high.
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Affiliation(s)
- Ronadip R Banerjee
- Division of Endocrinology, Department of Medicine, University of Alabama at Birmingham School of Medicine, Birmingham, AL 35294, USA.,Endocrinology Section, Medical Service, Veterans Affairs Medical Center, Birmingham, AL 35233, USA
| | - Tara Spence
- Department of Genetics and Genomics, Alberta Precision Laboratories, Calgary, Alberta T3B 6A8, Canada
| | - Stuart J Frank
- Division of Endocrinology, Department of Medicine, University of Alabama at Birmingham School of Medicine, Birmingham, AL 35294, USA.,Endocrinology Section, Medical Service, Veterans Affairs Medical Center, Birmingham, AL 35233, USA
| | | | - Andrew N Hoofnagle
- Departments of Laboratory Medicine and Pathology, Kidney Research Institute, University of Washington, Seattle, WA 98195, USA
| | - Bob Argiropoulos
- Department of Genetics and Genomics, Alberta Precision Laboratories, Calgary, Alberta T3B 6A8, Canada
| | - Julien L Marcadier
- The University of Calgary, and the Division of Medical Genetics, Alberta Children's Hospitals, Calgary, Alberta T3B 6A8, Canada
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Berg AH, Tavasoli M, Lo AS, Burnett-Bowie SAM, Bhan I, Karumanchi SA, Kalim S, Zhang D, Zhao S, Thadhani RI. Development and analytical validation of a novel bioavailable 25-hydroxyvitamin D assay. PLoS One 2021; 16:e0254158. [PMID: 34242315 PMCID: PMC8270209 DOI: 10.1371/journal.pone.0254158] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/12/2020] [Accepted: 06/21/2021] [Indexed: 11/18/2022] Open
Abstract
BACKGROUND Bioavailable 25-hydroxyvitamin D (25OHD) may be a better indicator of vitamin D sufficiency than total 25OHD. This report describes a novel assay for measuring serum bioavailable 25OHD. METHODS We developed an assay for 25OHD % bioavailability based on competitive binding of 25OHD tracer between vitamin D-binding protein (DBP)-coated affinity chromatography beads and serum DBP. Bioavailable 25OHD, total 25OHD, albumin, and DBP protein concentrations were measured in 89 samples from hospitalized patients and 42 healthy controls to determine how the DBP binding assay responds to differences in concentrations of DBP and compares to calculated bioavailable 25OHD values. RESULTS DBP binding assay showed a linear relationship between DBP-bound 25OHD tracer recovered from bead supernatant and DBP calibrator concentrations (y = 0.0017x +0.731, R2 = 0.9961, p<0.001). Inversion of this relationship allowed interpolation of DBP binding equivalents based upon 25OHD tracer recovered. The relationship between DBP binding equivalents and % bioavailability fits a non-linear curve, allowing calculation of % bioavailable 25OHD from DBP binding equivalents (y = 10.625x-0.817, R2 = 0.9961, p<0.001). In hospitalized patient samples, there were linear relationships between DBP protein concentrations and DBP binding equivalents (y = 0.7905x + 59.82, R2 = 0.8597, p<0.001), between measured vs. calculated % bioavailability (y = 0.9528 + 0.0357, R2 = 0.7200, p<0.001), and between absolute concentrations of measured vs. calculated bioavailable 25OHD (y = 1.2403 + 0.1221, R2 = 0.8913, p<0.001). CONCLUSIONS The DBP-binding assay for bioavailable 25OHD shows expected changes in 25OHD % bioavailability in response to changes in DBP concentrations and concordance with calculated bioavailable 25OHD concentrations.
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Affiliation(s)
- Anders H. Berg
- Department of Pathology, Cedars Sinai Medical Center, Los Angeles, California, United States of America
| | - Mahtab Tavasoli
- Department of Pathology, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts, United States of America
| | - Agnes S. Lo
- Department of Medicine and Biomedical Sciences, Cedars-Sinai Medical Center, Los Angeles, California, United States of America
- Division of Nephrology and Center for Vascular Biology Research, Department of Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts, United States of America
| | - Sherri-Ann M. Burnett-Bowie
- Endocrine Division, Department of Medicine, Massachusetts General Hospital, Boston, Massachusetts, United States of America
| | - Ishir Bhan
- Division of Nephrology, Department of Medicine, Massachusetts General Hospital, Boston, Massachusetts, United States of America
| | - S. Ananth Karumanchi
- Department of Medicine and Biomedical Sciences, Cedars-Sinai Medical Center, Los Angeles, California, United States of America
- Division of Nephrology and Center for Vascular Biology Research, Department of Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts, United States of America
| | - Sahir Kalim
- Division of Nephrology, Department of Medicine, Massachusetts General Hospital, Boston, Massachusetts, United States of America
| | - Dongsheng Zhang
- Department of Medicine and Biomedical Sciences, Cedars-Sinai Medical Center, Los Angeles, California, United States of America
- Division of Nephrology and Center for Vascular Biology Research, Department of Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts, United States of America
| | - Sophia Zhao
- Division of Nephrology, Department of Medicine, Massachusetts General Hospital, Boston, Massachusetts, United States of America
| | - Ravi I. Thadhani
- Division of Nephrology, Department of Medicine, Massachusetts General Hospital, Boston, Massachusetts, United States of America
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