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BMC Medical Genetics
Published in: BMC Medical Genetics 1/2016

Open Access 01-12-2016 | Case report

Functional validation of novel compound heterozygous variants in B3GAT3 resulting in severe osteopenia and fractures: expanding the disease phenotype

Authors: Florian Job, Shuji Mizumoto, Laurie Smith, Natario Couser, Ashley Brazil, Howard Saal, Melanie Patterson, Margaret I. Gibson, Sarah Soden, Neil Miller, Isabelle Thiffault, Carol Saunders, Shuhei Yamada, Katrin Hoffmann, Kazuyuki Sugahara, Emily Farrow

Published in: BMC Medical Genetics | Issue 1/2016

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Abstract

Background

A new disease class of syndromes, described as linkeropathies, which are derived from defects in the glycosaminoglycan-linker region as well as glycosaminoglycan-side chains of proteoglycans is increasingly being recognized as a cause of human disease. Proteoglycans are an essential component of the extracellular matrix. Defects in the enzymatic process of proteoglycan synthesis broadly occur due to the incorrect addition of side chains. Previously, homozygous missense variants within the B3GAT3 gene encoding beta 1,3 glucuronyltransferase 3(GlcAT-I) responsible for the biosynthesis of glycosaminoglycans have been described in 7 individuals.

Case presentation

In this study, a 4-year-old patient with a severe phenotype of osteoporosis, hypotonia, joint laxity, fractures, scoliosis, biscuspid aortic valve and myopia was referred for next generation sequencing after extensive negative clinical testing. Whole exome sequencing was performed on the proband and his unaffected parents to identify the molecular basis of his disease. Sequencing revealed compound heterozygous variants in B3GAT3: c.1A > G (p.Met1?) and c.671 T > A (p.L224Q). Clinical and in vitro functional studies were then completed to verify the pathogenicity of the genotype and further characterize the functional basis of the patient’s disease demonstrating the patient had a decrease both in the protein level of B3GAT3 and in the glucuronyltransferase activity when compared to control samples. Independent in vitro assessment of each variant confirmed the B3GAT3: c.1A > G (p.Met1?) variant is functionally null and the c.671 T > A (p.L224Q) missense variant has significantly reduced glucuronyltransferase activity (~3% of control).

Conclusions

This is the first report of a patient with compound heterozygosity for a null variant in trans with a missense in B3GAT3 resulting in a severe phenotype, expanding both the genotypic and phenotypic spectrum of B3GAT3-related disease.
Appendix
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Metadata
Title
Functional validation of novel compound heterozygous variants in B3GAT3 resulting in severe osteopenia and fractures: expanding the disease phenotype
Authors
Florian Job
Shuji Mizumoto
Laurie Smith
Natario Couser
Ashley Brazil
Howard Saal
Melanie Patterson
Margaret I. Gibson
Sarah Soden
Neil Miller
Isabelle Thiffault
Carol Saunders
Shuhei Yamada
Katrin Hoffmann
Kazuyuki Sugahara
Emily Farrow
Publication date
01-12-2016
Publisher
BioMed Central
Published in
BMC Medical Genetics / Issue 1/2016
Electronic ISSN: 1471-2350
DOI
https://doi.org/10.1186/s12881-016-0344-9

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