Collagen XII myopathy with rectus femoris atrophy and collagen XII retention in fibroblasts

COL12A1 Gene Variant and a Review of the Literature: A Case Report of Ullrich Congenital Muscular Dystrophy

Introduction

Mutations in collagen type IV-associated genes lead to Ullrich congenital muscular dystrophy (UCMD) and Bethlem myopathy (BM). COL12A1 gene mutations have rarely been reported in patients with UCMD- and BM-like disorders not involving COL6 mutations. UCMD-2 results from homozygous mutations in the COL12A1 gene on the long arm of chromosome 6. Pathogenic variants in COL12A1 result in a rare congenital connective tissue/myopathy overlap syndrome under the heading of myopathic Ehlers-Danlos syndrome. COL12A1 dominant pathogenic variants have been rarely reported, and the phenotypic spectrum has not yet been identified.

Case Presentation

We describe a female patient aged 2 years and 10 months exhibiting a milder phenotype who presented due to pronounced joint hyperlaxity, frequent falls, and skin lesions. Genetic analysis revealed a homozygous c.8903C>T (p.Pro2968Leu) missense variant that had previously been described but concerning which there had been no clinical report, in the COL12A1 gene.

Discussion/Conclusion

This report is presented in order to raise awareness of rare mutations in the COL12A1 gene that affect muscle and connective tissue and to add to the literature in defining the phenotypic spectrum.

Collagen XII-Related Myopathy: An Emerging Spectrum of Extracellular Matrix-Related Myopathy

Collagen XII, a member of a protein family called fibril associated collagen with interrupted triple helices (FACIT), is an important component of extracellular matrix and is essential for bridging the neighbouring fibrils. Mutations in collagen XII have been recently described to cause a rare extracellular matrix-related myopathy in those whose phenotype resembles collagen VI-related dystrophies and were negative for pathogenic variants in COL6A genes. The authors report a 4-year old girl presented with a phenotype mimicking Ullrich congenital muscular dystrophy and genetically confirmed to have pathogenic variants in COL12A1 gene thus, expanding the phenotypic spectrum of COL12A1-related myopathy.

Homozygous splice site variant affecting the first von Willebrand factor A domain of COL12A1 in a patient with myopathic Ehlers-Danlos syndrome

Myopathic Ehlers-Danlos syndrome (mEDS) is a subtype of EDS that is caused by abnormalities in COL12A1. Up-to-date, 24 patients from 15 families with mEDS have been reported, with 14 families showing inheritance in an autosomal dominant manner and one family in an autosomal recessive manner. We encountered an additional patient with autosomal recessive mEDS. The patient is a 47-year-old Japanese man, born to consanguineous parents with no related features of mEDS. After birth, he presented with hypotonia, weak spontaneous movements, scoliosis, and torticollis. He had soft palms but no skin hyperextensibility or fragility. Progressive scoliosis, undescended testes, and muscular torticollis required surgery. During adulthood, he worked normally and had no physical concerns. Clinical exome analysis revealed a novel homozygous variant in COL12A1 (NM_004370.6:c.395-1G > A) at the splice acceptor site of exon 6, leading to in-frame skipping of exon 6. The patient was diagnosed with mEDS. The milder manifestations in the current patient compared with previously reported patients with mEDS might be related to the site of the variant. The variant is located in the genomic region encoding the first von Willebrand factor A domain, which affects only the long isoform of collagen XII, in contrast to the variants in previously reported mEDS patients that affected both the long and short isoforms. Further studies are needed to delineate comprehensive genotype-phenotype correlation of the disorder.

Homozygous splice variant (c.1741-6G>A) of the COL6A1 gene in three patients with Ullrich congenital muscular dystrophy

The three major collagen VI genes: COL6A1, COL6A2, and COL6A3 encode microfibrillar components of extracellular matrices in multiple tissues including muscles and tendons. Pathogenic variants in the collagen VI genes cause collagen VI-related dystrophies representing a continuum of conditions from Bethlem myopathy at the milder end to Ullrich congenital muscular dystrophy at the more severe end. Here we describe a pathogenic variant in the COL6A1 gene (NM_001848.3; c.1741-6G>A) found in homozygosity in three patients with Ullrich congenital muscular dystrophy. The patients suffered from severe muscle impairment characterised by proximal weakness, distal hyperlaxity, joint contractures, wheelchair-dependency, and use of nocturnal non-invasive ventilation. The pathogenicity was verified by RNA analyses showing that the variant induced aberrant splicing leading to a frameshift and loss of function. The analyses were in line with immunocytochemistry studies of patient-derived skin fibroblasts and muscle tissue demonstrating impaired secretion of collagen VI into the extracellular matrix. Thereby, we add the variant c.1741-6G>A to the list of pathogenic, recessive, splice variants in COL6A1 causing Ullrich congenital muscular dystrophy. The variant is listed in ClinVar as of "uncertain significance" and "likely benign" and may presumably have been overlooked in other patients.

Collagen XII mediated cellular and extracellular mechanisms in development, regeneration, and disease

Collagen XII, a fibril-associated collagen with interrupted triple helices (FACIT), influences fibrillogenesis in numerous tissues. In addition to this extracellular function, collagen XII also directly regulates cellular function. Collagen XII is widely expressed in connective tissues, particularly tendons, ligaments, and the periodontium and periosteum, where it is enriched in the pericellular regions. Mutations in the collagen XII gene cause myopathic Ehlers-Danlos syndrome (mEDS), an early-onset disease characterized by overlapping connective tissue abnormalities and muscle weakness. Patients with mEDS exhibit delayed motor development, muscle weakness, joint laxity, hypermobility, joint contractures, and abnormal wound healing. A mEDS mouse model was generated by deletion of the Col12a1 gene, resulting in skeletal and muscle abnormalities with disorganized tissue structures and altered mechanical properties. Extracellularly, collagen XII interacts with collagen I fibrils and regulates collagen fibril spacing and assembly during fibrillogenesis. Evidence for the binding of collagen XII to other EDS-related molecules (e.g., decorin and tenascin X) suggests that disruption of ECM molecular interactions is one of the causes of connective tissue pathology in mEDS. Collagen XII also has been shown to influence cell behavior, such as cell shape and cell-cell communication, by providing physical connection between adjacent cells during tissue development and regeneration. The focus of this review is on the functions of collagen XII in development, regeneration, and disease.

Ehlers-Danlos/myopathy overlap syndrome caused by a large de novo deletion in COL12A1

Autosomal dominant and recessive mutations in COL12A1 cause the Ehlers-Danlos/myopathy overlap syndrome. Here, we describe a boy with fetal hypokinesia, severe neonatal weakness, striking hyperlaxity, high arched palate, retrognathia, club feet, and pectus excavatum. His motor development was initially delayed but muscle strength improved with time while hyperlaxity remained very severe causing recurrent joint dislocations. Using trio exome sequencing and a copy number variation (CNV) analysis tool, we identified an in-frame de novo heterozygous deletion of the exons 45 to 54 in the COL12A1 gene. Collagen XII immunostaining on cultured skin fibroblasts demonstrated intracellular retention of collagen XII, supporting the pathogenicity of the deletion. The phenotype of our patient is slightly more severe than other cases with dominantly acting mutations, notably with the presence of fetal hypokinesia. This case highlights the importance of CNVs analysis in the COL12A1 gene in patients with a phenotype suggesting Ehlers-Danlos/myopathy overlap syndrome.

Dominant collagen XII mutations cause a distal myopathy

Objective

To characterize the natural history and clinical features of myopathies caused by mono‐allelic, dominantly acting pathogenic variants in COL12A1.

Methods

Patients with dominant COL12A1‐related myopathies were characterized by history and clinical examination, muscle imaging, and genetic analysis. Pathogenicity of the variants was assessed by immunostaining patient‐derived dermal fibroblast cultures for collagen XII.

Results

Four independent families with childhood‐onset weakness due to novel, dominantly acting pathogenic variants in COL12A1 were identified. Adult patients exhibited distal‐predominant weakness. Three families carried dominantly acting glycine missense variants, and one family had a heterozygous, intragenic, in‐frame deletion of exon 52 of COL12A1. All pathogenic variants resulted in increased intracellular retention of collagen XII in patient‐derived fibroblasts as well as loss of extracellular, fibrillar collagen XII deposition. Since haploinsufficiency for COL12A1 is largely clinically asymptomatic, we designed and evaluated small interfering RNAs (siRNAs) that specifically target the mutant allele containing the exon 52 deletion. Immunostaining of the patient fibroblasts treated with the siRNA showed a near complete correction of collagen XII staining patterns.

Interpretation

This study characterizes a distal myopathy phenotype in adults with dominant COL12A1 pathogenic variants, further defining the phenotypic spectrum and natural history of COL12A1‐related myopathies. This work also provides proof of concept of a precision medicine treatment approach by proposing and validating allele‐specific knockdown using siRNAs specifically designed to target a patient’s dominant COL12A1 disease allele.

Novel defects in collagen XII and VI expand the mixed myopathy/Ehlers-Danlos syndrome spectrum and lead to variant-specific alterations in the extracellular matrix

PURPOSE

To date, heterozygous or homozygous COL12A1 variants have been reported in 13 patients presenting with a clinical phenotype overlapping with collagen VI-related myopathies and Ehlers-Danlos syndrome (EDS). The small number of reported patients limits thorough investigation of this newly identified syndrome, currently coined as myopathic EDS.

METHODS

DNA from 78 genetically unresolved patients fulfilling the clinical criteria for myopathic EDS was sequenced using a next-generation panel of COL12A1, COL6A1, COL6A2, and COL6A3.

RESULTS

Among this cohort, we identified four pathogenic heterozygous in-frame exon skipping (∆) defects in COL12A1, clustering to the thrombospondin N-terminal region and the adjacent collagenous domain (Δ52, Δ53, Δ54, and Δ56 respectively), one heterozygous COL12A1 arginine-to-cysteine substitution of unclear significance (p.(Arg1863Cys)), and compound heterozygous pathogenic COL6A1 variants (c.[98-6G>A];[301C>T]) in one proband. Variant-specific intracellular accumulation of collagen XII chains, extracellular overmodification of the long isoform and near-absence of the short isoform of collagen XII, and extracellular decrease of decorin and tenascin-X were observed for the COL12A1 variants. In contrast, the COL6A1 variants abolished collagen VI and V deposition and increased tenascin-X levels.

CONCLUSION

Our data further support the significant clinical overlap between myopathic EDS and collagen VI-related myopathies, and emphasize the variant-specific consequences of collagen XII defects.