Defining Haplosufficiency in Autosomal Recessive Limb-Girdle Muscular Dystrophy Using Molecular Markers in Disease Carriers.

In heterozygous carriers of CAPN3, FKRP or SGCA, residual protein expression measured on muscle biopsy defines the haplosufficiency threshold compatible with normal strength.

Autosomal recessive limb-girdle muscular dystrophies result from biallelic loss of function, heterozygous carriers being assumed haplosufficient. The authors measured relative protein expression in carriers and affected individuals across three subtypes, LGMDR1 (CAPN3), LGMDR9 (FKRP) and LGMDR3 (SGCA), from a muscle biopsy obtained after physical examination, using RNA-seq for gene expression and western blotting for protein quantification. LGMDR1 carriers had approximately 25% of total calpain-3 compared with non-carrier controls and LGMDR9 carriers nearly 50% of FKRP functionality, whereas LGMDR3 carriers had alpha- and beta-sarcoglycan levels comparable to controls. Since these carriers retain normal muscle strength, the authors propose that this level of expression represents the threshold to be restored by gene replacement therapy.

Synthesis written by Geno'X. For the full original abstract, please refer to the source publication.

The idea is useful for gene therapy trials, since it provides an expression target measured in humans rather than inferred from mouse models, with a counter-intuitive result: 25% of calpain-3 is enough for normal strength. The weakness is the very small number of participants imposed by muscle biopsy, with numbers not detailed in the abstract. Keep it as a biological dose benchmark, not as an argument to reassure a carrier in clinic, since the question of long-term muscle risk remains open.

Analysis by Dr Thibaut Benquey