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case-control·genetics, skeletal dysplasia, bone development, endoplasmic reticulum stress, ophthalmology·PMC10624854

Specific heterozygous variants in MGP lead to endoplasmic reticulum stress and cause spondyloepiphyseal dysplasia

Nature Communications · 12 authors, 10 centres

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FIDELITY 100%

POPULATIONFour individuals from two unrelated families with spondyloepiphyseal dysplasia and heterozygous MGP variants (C19F or C19Y); also Mgp+/56G>T mice

INTERVENTIONNot applicable (observational and mechanistic study)

COMPARISONWild-type MGP (in vitro and in vivo); Mgp−/− and MgpS3mut/S3mut mice

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TL;DR

This study identifies heterozygous variants in the MGP gene (Cys19Phe and Cys19Tyr) as the cause of a novel autosomal dominant spondyloepiphyseal dysplasia (SED, MGP type) in four individuals from two families. The C19F variant impairs signal peptide processing, leading to intracellular accumulation of mutant MGP, endoplasmic reticulum stress, and apoptosis of growth plate chondrocytes, resulting in short stature, platyspondyly, and epiphyseal anomalies. These findings establish a distinct disorder from Keutel syndrome and suggest potential therapeutic targets such as ER stress inhibitors.

Full summary

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**Background:** Skeletal dysplasias are heterogeneous genetic disorders affecting bone and cartilage development. While biallelic loss-of-function variants in MGP cause autosomal recessive Keutel syndrome (KS), this study reports four individuals from two families with a previously undescribed spondyloepiphyseal dysplasia (SED) and heterozygous MGP variants affecting residue Cys19 (C19F and C19Y). MGP is a secreted inhibitor of extracellular matrix mineralization, and its post-translational modifications are critical for function. The authors hypothesized that these heterozygous variants cause a dominant-negative or gain-of-function effect distinct from KS.

**Methods:** Exome sequencing was performed on three affected individuals from Family 1 (c.56G>T, p.C19F) and one from Family 2 (c.56G>A, p.C19Y), with segregation confirmed by Sanger sequencing. A CRISPR/Cas9 knock-in mouse model (Mgp+/56G>T) was generated to introduce the C19F variant. Phenotypic analyses included radiography, micro-CT, histology (von Kossa, safranin O, toluidine blue, TRAP), calcein double labeling, and TUNEL assays. In vitro studies used ATDC5 chondrogenic cells and HEK-293 cells transfected with FLAG- or GFP-tagged WT or C19F MGP constructs. Protein secretion was assessed by immunoprecipitation, Western blotting, and mass spectrometry. ER stress was evaluated by immunofluorescence for calnexin, CHOP, ATF6-N, p-eIF2α, and sXBP1. Cell death was quantified using ethidium homodimer and TUNEL assays, with 4-PBA treatment to inhibit ER stress.

**Key Results:** All four affected individuals had short stature (height SD from −2.2 to −5), disproportionate short trunk, platyspondyly with biconcave vertebral bodies, brachytelephalangism, midface retrusion, and epiphyseal anomalies. Individual 4 also had premature growth plate closure and corneal ulceration. The variants were absent from gnomAD and had CADD scores of 27 (C19F) and 25 (C19Y). Mgp+/56G>T mice showed reduced body weight after 2 weeks, shorter long bones (femur and tibia significantly shorter at 6 weeks), midface retrusion, osteopenia, and hypermineralized tracheal cartilage. Micro-CT revealed significantly reduced trabecular bone volume/tissue volume (BV/TV) in lumbar vertebrae (p<0.01) and distal femur (p<0.01), with decreased trabecular number and increased spacing. Histomorphometry showed a 67% decrease in trabecular BV/TV, reduced osteoblast numbers (N.Ob/T.Ar and N.Ob/B.Pm, p<0.01), and reduced osteoclast numbers (N.Oc/T.Ar and N.Oc/B.Pm, p<0.01) in mutant mice. Growth plates were thinner, prematurely mineralized, and had almost complete loss of type X collagen-positive hypertrophic zone. In vitro, C19F MGP accumulated intracellularly with a ~17 kDa band (unprocessed signal peptide) and was poorly secreted compared to WT. Mass spectrometry confirmed retention of signal peptide residues in mutant MGP. C19F MGP-expressing cells showed upregulation of ER stress markers calnexin and CHOP, colocalization with calnexin, and increased apoptosis (TUNEL-positive cells). In ATDC5 cells, 4-PBA treatment significantly reduced cell death (p<0.01). In vivo, growth plate chondrocytes of Mgp+/56G>T mice showed increased calnexin, ATF6-N, p-eIF2α, and CHOP, but not sXBP1, and markedly increased TUNEL-positive cells compared to controls, Mgp−/−, or MgpS3mut/S3mut mice.

**Clinical Implications:** This study defines a novel autosomal dominant skeletal dysplasia (SED, MGP type) caused by heterozygous Cys19 variants in MGP, distinct from KS. The mechanism involves impaired signal peptide cleavage, ER retention, ER stress, and chondrocyte apoptosis, leading to growth plate abnormalities and low bone mass. The findings highlight ER stress as a potential therapeutic target; 4-PBA, an FDA-approved ER stress inhibitor, reduced cell death in vitro. Future studies should explore ER stress-modulating drugs or caspase inhibitors as treatments for this and related skeletal dysplasias. The mouse model recapitulates key human features and will be valuable for mechanistic and therapeutic research.

PICO

PPOPULATION

Four individuals from two unrelated families with spondyloepiphyseal dysplasia and heterozygous MGP variants (C19F or C19Y); also Mgp+/56G>T mice

IINTERVENTION

Not applicable (observational and mechanistic study)

OOUTCOME

Skeletal phenotype (short stature, platyspondyly, epiphyseal dysplasia, growth plate abnormalities); intracellular MGP retention; ER stress markers (calnexin, CHOP, ATF6-N, p-eIF2α); apoptosis (TUNEL); bone histomorphometry (BV/TV, osteoblast/osteoclast counts)

STUDY TYPE

case-control

SPECIALTY

genetics

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FIDELITY CHECK

100% · A

ORIGINAL PAPER

PMC10624854

Specific heterozygous variants in MGP lead to endoplasmic reticulum stress and cause spondyloepiphyseal dysplasia

Nature Communications

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ABSTRACT

Abstract Matrix Gla protein (MGP) is a vitamin K-dependent post-translationally modified protein, highly expressed in vascular and cartilaginous tissues. It is a potent inhibitor of extracellular matrix mineralization. Biallelic loss-of-function variants in the MGP gene cause Keutel syndrome, an autosomal recessive disorder characterized by widespread calcification of various cartilaginous tissues and skeletal and vascular anomalies. In this study, we report four individuals from two unrelated families with two heterozygous variants in MGP , both altering the cysteine 19 residue to phenylalanine or tyrosine. These individuals present with a spondyloepiphyseal skeletal dysplasia characterized by short stature with a short trunk, diffuse platyspondyly, midface retrusion, progressive epiphyseal anomalies and brachytelephalangism. We investigated the cellular and molecular effects of one of the heterozygous deleterious variants (C19F) using both cell and genetically modified mouse models. Heterozygous ‘knock-in’ mice expressing C19F MGP recapitulate most of the skeletal anomalies observed in the affected individuals. Our results suggest that the main underlying mechanism leading to the observed skeletal dysplasia is endoplasmic reticulum stress-induced apoptosis of the growth plate chondrocytes. Overall, our findings support that heterozygous variants in MGP altering the Cys19 residue cause autosomal dominant spondyloepiphyseal dysplasia, a condition distinct from Keutel syndrome both clinically and molecularly. Subject terms: Diseases, Gene expression, Bone development, Cartilage development, CRISPR-Cas9 genome editing

CITATION EXPORT

Ophélie Gourgas, Gabrielle Lemire, Alison J Eaton et al.. Specific heterozygous variants in MGP lead to endoplasmic reticulum stress and cause spondyloepiphyseal dysplasia. Nature Communications. (2023). https://doi.org/10.1038/s41467-023-41651-6

AUTHORS

12 · 10 CENTRES

Ophélie Gourgas, Gabrielle Lemire, Alison J Eaton, Sultanah Alshahrani, Angela L Duker, Jingjing Li, Ricki S Carroll, Stuart Mackenzie, Sarah M Nikkel, Michael B Bober, Kym M Boycott, Monzur Murshed

1Department of Medicine, McGill University, Montreal, QC Canada
2Children’s Hospital of Eastern Ontario Research Institute, University of Ottawa, Ottawa, ON Canada
3Department of Genetics, Children’s Hospital of Eastern Ontario, Ottawa, ON Canada
4Broad Center for Mendelian Genomics, Program in Medical and Population Genetics, Broad Institute of MIT and Harvard, Cambridge, MA USA
5University of Alberta, Edmonton, AB Canada
6Faculty of Dental Medicine and Oral Health Sciences, McGill University, Montreal, QC Canada
7Department of Oral and Maxillofacial Surgery, Faculty of Dentistry, King Abdulaziz University, Jeddah, Saudi Arabia
8Nemours Children’s Health, Wilmington, DE USA
9University of British Columbia, Vancouver, BC Canada
10Shriners Hospitals for Children ˗ Canada, Montreal, QC Canada