Here we report two adult brothers presenting with the classic phenotype of Bethlem myopathy, including childhood onset, expected progression, patterns of weakness and contractures, as well as typical skin findings and changes on muscle imaging. Within the Bethlem phenotype these patients can be considered to be affected fairly typically. Both siblings are compound heterozygous for mutations in the COL6A2 gene, while each parent is a clinically unaffected carrier of one of these mutations.
The single nucleotide deletion in exon 23 (c.1770delG) on the maternal allele is an leads to the in-frame skipping of exon 23 at the N-terminal end of the triple-helical domain. In-frame deletions in this location of the triple helical domain render the chain incapable of incorporation into the heterotrimer, leading to complete exclusion of the mutant allele from the assembly process [22
]. Thus, the mutation effectively results in a recessively acting null allele [21
]. Our non-quantitative RT-PCR analysis suggests that this exon skipping occurs at a high rate (), but even if the skipping were to occur at a less complete rate, the single nucleotide deletion would result in a frameshift and premature stop, since the nucleotide is part of the coding region. Carriers for null mutations in COL6A2
are clinically unaffected [13
]. In the homozygous state the mutation would be predicted to lead to the total lack of assembly-competent alpha2 (VI) chain allowing for no collagen VI basic heterotrimer to form [21
], which would lead to the phenotype of Ullrich congenital muscular dystrophy [7
The missense mutation p.R830W on the paternal allele thus acts on this virtual null background from the maternal allele. The fact that this mutation leads to the less severe phenotype of Bethlem myopathy suggests that it must be considerably milder in terms of its effect on collagen VI function compared to a null mutation. Pathogenic missense mutations in the COL6A2 C2 domain have been observed only rarely (L837P [15
], R876S [23
]). R830W has not been reported before as a mutation or as a polymorphism in the available databases (www.dmd.nl
), nor has it been seen in our own unpublished experience. The arginine at position 830 is conserved between species () and localizes at the beginning of the C2 domain, in close proximity to a modified metal ion-dependent adhesion site (MIDAS) motif (position 839–843) (), which is of importance in metal-ion mediated protein-protein interactions, likely coordinating the formation of the collagen VI antiparallel dimer [24
]. The p.R830W amino acid change is nonconservative, changing the polarity of the protein in the region (Suppl Figure 1
), conceivably changing the functionality of the MIDAS motif, although we have not formally demonstrated this. The missense change p.R784H coinherited in cis
with c.1770delG on the maternal allele will not be part of assembled collagen VI because the preceding exon skipping renders the chain incompetent for assembly. It likely qualifies as a polymorphism as it has been observed before without clear disease association [17
] (and personal observation).
The patients presented here provide evidence that classic Bethlem myopathy can be inherited in an autosomal recessive manner. Thus, Bethlem myopathy joins Ullrich congenital muscular dystrophy in that both dominant and recessive genetic mechanisms have now been established as underlying these two classic disorders which lie at the ends of the phenotypic spectrum of collagen VI related myopathies. The fact that an “Ullrich mutation” in compound heterozygosity with a presumably milder “Bethlem mutation” results in a Bethlem myopathy phenotype leads to the conclusion that the collagen VI related myopathies are best understood as existing along a clinical and genetic spectrum, in which the severity of the mutations and the resulting functional abnormality of the collagen VI matrix dictate a phenotypic spectrum, ranging from severe Ullrich to mild Bethlem with phenotypes of intermittent severity connecting the two. Consequently, a fundamentally different genetic or biochemical mechanism distinguishing Ullrich congenital muscular dystrophy from Bethlem myopathy can no longer be assumed.
This observation significantly impacts genetic counseling and the interpretation of genetic testing in a patient with Bethlem myopathy, since the finding of one missense mutation of unclear significance now should prompt consideration of the possibility of an additional second mutation on the other allele, consistent with recessive inheritance. Furthermore, studies in the extended family are imperative to determine the mode of inheritance as well as disease association of a given genetic change of unclear significance. Obviously, the determination of recessively acting mutations in a patient with Bethlem myopathy drastically reduces the risk of disease recurrence in the patients own offspring.