Collagen VI-related limb-girdle syndrome caused by frequent mutation in COL6A3 gene with conflicting reports of pathogenicity

Advances in Molecular Function and Recombinant Expression of Human Collagen

Collagen is the main protein found in skin, bone, cartilage, ligaments, tendons and connective tissue, and it can exhibit properties ranging from compliant to rigid or form gradients between these states. The collagen family comprises 28 members, each containing at least one triple-helical domain. These proteins play critical roles in maintaining mechanical characteristics, tissue organization, and structural integrity. Collagens regulate cellular processes such as proliferation, migration, and differentiation through interactions with cell surface receptors. Fibrillar collagens, the most abundant extracellular matrix (ECM) proteins, provide organs and tissues with structural stability and connectivity. In the mammalian myocardial interstitium, types I and III collagens are predominant: collagen I is found in organs, tendons, and bones; collagen II is found in cartilage; collagen III is found in reticular fibers; collagen IV is found in basement membranes; and collagen V is found in nails and hair. Recombinant human collagens, particularly in sponge-like porous formats combined with bone morphogenetic proteins, serve as effective scaffolds for bone repair. Due to their biocompatibility and low immunogenicity, collagens are pivotal in tissue engineering applications for skin, bone, and wound regeneration. Recombinant technology enables the production of triple-helical collagens with amino acid sequences identical to human tissue-derived collagens. This review summarizes recent advances in the molecular functions and recombinant expression of human collagens, with a focus on their biomedical applications.

Segregation of the COL6A2 Variant (c.1817-3C>G) in a Consanguineous Saudi Family with Bethlem Myopathy

Introduction: Bethlem myopathy is a rare genetic disease caused by a variant mapped to 21q22, which harbors the collagen type VI alpha 2 chain (COL6A2) and collagen type VI alpha 1 chain (COL6A1) genes, and 2q37, which harbors the collagen type VI alpha 3 chain (COL6A3) gene. Disease onset can occur at any age, and the symptoms are related to those of muscular dystrophy. Since Bethlem myopathy is a rare disease, no previous studies have been conducted in Arab countries, including Saudi Arabia. Its variable presentation of nonspecific muscular contractions and severity represents a diagnostic dilemma. Case presentation: Here, we report a Saudi pediatric patient, who is 9 years old (proband), brought to the pediatric clinic of King Saud's Hospital by his mother. The boy presented with difficulty standing, walking, and running with his classmates and unaffected siblings. He has a younger sibling, aged 6 years old, who reported having a limping gait and difficulty bending his right knee. Laboratory results for the proband were unremarkable except for a slight increase in creatine kinase (CK). Whole-exome sequencing (WES) was performed for five family members, including the proband and his symptomatic brother, their mother and two asymptomatic siblings. A very rare 3' splice site acceptor intronic variant, NM_001849.4: c.1817-3C>G, located three nucleotides before exon 25, was identified in COL6A2. Bioinformatics tools (SpliceAI, dbscSNV, FATHMM-MKL, and MaxEntScan) predicted this variant as pathogenic. The proband and his 6-year-old sibling presented a homozygous genotype for the variant, whereas the mother and one asymptomatic sibling were heterozygous, and the other sibling carried homozygous wild-type alleles. Conclusions: This is the first study to report a case of Bethlem myopathy confirmed by WES in Saudi Arabia and all Arab nations. The identified variant is rare, and its segregation pattern suggests autosomal recessive inheritance. The segregation pattern and bioinformatics tool results may qualify this variant to be annotated as pathogenic, addressing the reported uncertainty of its classification. Our findings contribute to linking and filling the knowledge gap of diagnosing and managing patients with collagen VI-related myopathies, providing greater clinical and genetic understanding to the existing knowledge.

A novel variant of COL6A3 c.6817-2(IVS27)A>G causing Bethlem myopathy: A case report

Bethlem myopathy (BM) is a disease that is caused by mutations in the collagen VI genes. It is a mildly progressive disease characterized by proximal muscle weakness and contracture of the fingers, the wrist, the elbow, and the ankle. BM is an autosomal dominant inheritance that is mainly caused by dominant COL6A1, COL6A2, or COL6A3 mutations. However, a few cases of collagen VI mutations with bilateral facial weakness and Beevor's sign have also been reported. This study presents a 50-year-old female patient with symptoms of facial weakness beginning in childhood and with the slow progression of the disease with age. At the age of 30 years, the patient presented with asymmetrical proximal muscle weakness, and the neurological examination revealed bilateral facial weakness and a positive Beevor's sign. Phosphocreatine kinase was slightly elevated with electromyography showing myopathic changes and magnetic resonance imaging (MRI) of the lower limb muscles showing the muscle MRI associated with collagen VI (COL6)-related myopathy (COL6-RM). The whole-genome sequencing technology identified the heterozygous mutation c.6817-2(IVS27)A>G in the COL6A3 gene, which was in itself a novel mutation. The present study reports yet another case of BM, which is caused by the recessive COL6A3 intron variation, widening the clinical spectrum and genetic heterogeneity of BM.

Clinical and Molecular Spectrum Associated with COL6A3 c.7447A>G p.(Lys2483Glu) Variant: Elucidating its Role in Collagen VI-related Myopathies

BACKGROUND

Dominant and recessive autosomal pathogenic variants in the three major genes (COL6A1-A2-A3) encoding the extracellular matrix protein collagen VI underlie a group of myopathies ranging from early-onset severe conditions (Ullrich congenital muscular dystrophy) to milder forms maintaining independent ambulation (Bethlem myopathy). Diagnosis is based on the combination of clinical presentation, muscle MRI, muscle biopsy, analysis of collagen VI secretion, and COL6A1-A2-A3 genetic analysis, the interpretation of which can be challenging.

OBJECTIVE

To refine the phenotypical spectrum associated with the frequent COL6A3 missense variant c.7447A>G (p.Lys2483Glu).

METHODS

We report the clinical and molecular findings in 16 patients: 12 patients carrying this variant in compound heterozygosity with another COL6A3 variant, and four homozygous patients.

RESULTS

Patients carrying this variant in compound heterozygosity with a truncating COL6A3 variant exhibit a phenotype consistent with COL6-related myopathies (COL6-RM), with joint contractures, proximal weakness and skin abnormalities. All remain ambulant in adulthood and only three have mild respiratory involvement. Most show typical muscle MRI findings. In five patients, reduced collagen VI secretion was observed in skin fibroblasts cultures. All tested parents were unaffected heterozygous carriers. Conversely, two out of four homozygous patients did not present with the classical COL6-RM clinical and imaging findings. Collagen VI immunolabelling on cultured fibroblasts revealed rather normal secretion in one and reduced secretion in another. Muscle biopsy from one homozygous patient showed myofibrillar disorganization and rimmed vacuoles.

CONCLUSIONS

In light of our results, we postulate that the COL6A3 variant c.7447A>G may act as a modulator of the clinical phenotype. Thus, in patients with a typical COL6-RM phenotype, a second variant must be thoroughly searched for, while for patients with atypical phenotypes further investigations should be conducted to exclude alternative causes. This works expands the clinical and molecular spectrum of COLVI-related myopathies.