Analysis of Dystrophin Gene Deletions Indicates that the Hinge III Region of the Protein Correlates with Disease Severity

Abstract

We have investigated the frequency of deletions in the dystrophin gene in 108 unrelated Duchenne and Becker muscular dystrophy (DMD/BMD) patients from southern Italy (DMD, n. 47; BMD, n. 61) and identified 89 deletions. The de novo mutation rate (about 30%), and the preferentially maternal origin of deletional mutations, analysed in families in which the maternal grandparents were available or their haplotypes could be unequivocally reconstructed, are in agreement with data reported for other populations. The correlation between BMD phenotype and type of deletion suggests that, in the distal rod domain region, the deletion size may not be as crucial as the particular combination of missing exons. In fact, we provide immunohistochemical and clinical evidence that in-frame deletion of the hinge III region in the distal rod domain results in a milder phenotype as compared with shorter deletions that do not include the hinge III region. Our data obtained in BMD patients, by confirming inferences arising from minigene transfection experiments in mdx mice, represent an important contribution to gene therapy approaches.

Functional changes in Becker muscular dystrophy: implications for clinical trials in dystrophinopathies

“…The exception was represented by the 45–55 deletion, associated with ~90% dystrophin and a mild phenotype, as previously reported29. Similar to “x-51” deletions, the absence of the hinge III domain from the internally deleted protein may stabilize dystrophin in patients with del 45–558.…”

“…Although this analysis is hindered by low patient numbers, challenges in accurate dystrophin quantification28, and sampling variability in muscle biopsies, there does not seem to exist such a linear correlation within homogeneous mutation groups. On the other hand, differences in both dystrophin quantity and phenotype severity across different mutation groups, which are consistently observed38911, suggest that the molecular properties of different internally deleted dystrophin proteins dictate disease severity, by causing varying degrees of downstream pathologic phenomena, such as altered costamere resistance to mechanical stress, membrane hyperpermeability, and disrupted interaction with other proteins (e.g. dystrophin-associated glycoproteins, nitric oxide synthase, syntrophin, and dystrobrevin).…”

“…There was a double rationale to this grouping criterion: first, “del 45-x” BMD mutations have been described in the literature to be associated with a more severe phenotype than “del x-51”89101112; second, functional measure changes in these groups may be taken as a model of successful exon skipping in corresponding DMD populations9. The relatively frequent “del 48” was grouped separately, and all remaining patients were included in a miscellaneaous “other” mutation group.…”

“…Deletions including in-frame exons in the proximal rod domain3, or the hinge 3 domain encoded by exons 50–5189, have been associated to mild phenotypes; while deletions situated in the exon 45–53 mutational hotspot10, but not including exons 50–51, usually cause “typical” BMD111213. Moreover, linear or threshold correlations between dystrophin quantity in skeletal muscle tissue and BMD severity have been described39131415.…”

A Single CRISPR-Cas9 Deletion Strategy that Targets the Majority of DMD Patients Restores Dystrophin Function in hiPSC-Derived Muscle Cells

“…Large deletions in the rod domain of dystrophin often produce a more functional (more like wild type) protein, than even very small deletions (Harper et al, 2002). Larger deletions, which remove hinge III (exons 50–51), are believed to lead to a milder BMD phenotype than smaller deletions, or those which retain hinge III (Carsana et al, 2005). Thus, in many cases larger deletions are more therapeutically beneficial than smaller ones, due to the way they affect the secondary structure of the protein.…”