Molecular modifiers in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked progressive neuromuscular disease affecting 1:3500 –1/5000 boys at birth. It is caused by the absence of dystrophin, a subsarcolemmal protein that confers membrane stability linking cytoskeletal actin to the extracellular matrix. Dystrophin is part of a multi-protein complex called dystrophin associated protein complex (DAPC), which contains, among the other components, β-dystroglycan and nitric oxide synthase (NOS). The consequences of the absence of dystrophin are: deregulation of calcium homeostasis, tissue necrosis, and progressive accumulation of fat and fibrosis and loss of contractile muscle fibers. The ensuing muscle weakness leads to progressive and severe disability, with loss of independent ambulation around the early teens, and cardiac and respiratory failure leading to patient’s death, usually around the age of 20-30 years.Despite all patients having a complete lack of dystrophin in muscle fibers, a relevant inter-patient variability in disease severity is observed (e.g. loss of ambulation may range from 8 to beyond 15 years of age). Emerging evidence points to genetic modifiers, i.e. polymorphisms in genes different form the disease gene, as one of the causes of this variability, but little is yet known about the underlying molecular mechanisms.My PhD work can be divided into 4 aims: Aim 1: To characterize the molecular mechanism underlying the modifier effect of the rs28357094 T>G SNP in the SPP1gene, encoding osteopontin (OPN) the first identified genetic modifier of DMD. I treated dystrophic and healthy cell line with two different concentrations of deflazacort (DFZ), one of the glucocorticoids mainly used to treat DMD patients, in order to analyze osteopontin expression in relation to genetic background at rs24357094. The results obtained revealed: (I) a developmental regulated expression pattern of OPN; (II) no difference of osteopontin expression are observed related to rs28357094 genotype; (III) an increase in OPN expression only in TG DFZ-treated myotubes, suggesting a possible interaction between glucocorticoid responsive elements (GRs) in the promoter of the SPP1gene and the glucocorticoid.Aim 2: To investigate the possible roles of SPP1splicing isoforms in DMDmuscle biopsies and cells. Three SPP1isoforms, named a, b and c, were analyzed. SPP1mRNA studies revealed that all three isoforms are overexpressed in DMD muscle compared to controls, but not in myogenic cell cultures. Moreover, SPP1isoforms expression was directly correlated withage in DMD patients’ muscle biopsies. Finally, muscle biopsies carrying the rs24357094 TT genotype showed an increased expression of all three SPP1isoforms compared to TG genotype. Aim 3: To validate the known DMD geneticmodifiers in novel cohorts of DMD patients utilizing different outcome measures. First, we asked if SPP1genotype and LTBP4haplotype (the second identified modifier of DMD) can modulate the cardiac involvement in DMD. LTBP4haplotye and the SPP1rs28357094 were genotyped in 168 DMD patients. LTBP4haplotype is composed of 4 polymorphisms in perfect linkage disequilibrium (LD). The genotype at rs10880 resulted, as expected, to be associated to a delay at age of loss of ambulation (LoA) and, as novel finding, also to a delay in cardiomyopathy onset. The SPP1minor G allele at rs28357094 resulted also associated to a later cardiomyopathy onset.Finally, I participate to the identification of the third genetic modifier in DMD: CD40. CD40was identified through a GWAS approach in a large cohort of DMD patients.The CD40rs1883832 C>T polymorphism is located within the Kozak sequence of the gene and it causes a decrease of transcriptional activity of the promoter resulting in an increase of the CD40 secreted isoform. In order to validate CD40 as a genetic modifier in DMD in an independent cohort from the discovery cohort, rs1883832 was genotyped in 96 DMD patients.DMD patients carrying the minor T allele lost ambulation earlier compared to patients carrying the C allele. Moreover, in order to study the functional role of CD40 in DMD, RT-PCR and immunoblot were performed in a subset of patients’ muscle biopsies stratified according to rs1883832 genotype. Our results reveal that the minor T allele is associated to an increase of the transcript and a decrease of the protein compared to C genotype.Taken together these data contribute to clarify some aspects of the molecular mechanisms underlying the downstream consequences of genetic modifiers in DMD. Further studies are needed to fully translate the knowledge acquired in thefield of genetic modifiers in DMD to the clinic, e.g. to implement patient genotyping for genetic counseling, prognosis, planning of treatments, and stratification in clinical trials