Fukutin-related protein (FKRP, MIM ID 606596) variants cause a range of muscular dystrophies
associated with hypo-glycosylation of the matrix receptor, alpha-dystroglycan. These
disorders are almost exclusively caused by homozygous or compound heterozygous missense
variants in the FKRP gene that encodes a ribitol phosphotransferase. To understand
how seemingly diverse FKRP missense mutations may contribute to disease, we examined
the synthesis, intracellular dynamics, and structural consequences of a panel of missense
mutations that encompass the disease spectrum. Under non-reducing electrophoresis
conditions, wild type FKRP appears to be monomeric whereas disease-causing FKRP mutants
migrate as high molecular weight, disulfide-bonded aggregates. These results were
recapitulated using cysteine-scanning mutagenesis suggesting that abnormal disulfide
bonding may perturb FKRP folding. Using fluorescence recovery after photobleaching,
we found that the intracellular mobility of most FKRP mutants in ATP-depleted cells
is dramatically reduced but can, in most cases, be rescued with reducing agents. Mass
spectrometry showed that wild type and mutant FKRP differentially associate with several
endoplasmic reticulum (ER)-resident chaperones. Finally, structural modelling revealed
that disease-associated FKRP missense variants affected the local environment of the
protein in small but significant ways. These data demonstrate that protein misfolding
contributes to the molecular pathophysiology of FKRP-deficient muscular dystrophies
and suggest that molecules that rescue this folding defect could be used to treat
these disorders.
