Startle responses in Duchenne muscular dystrophy: a novel biomarker of brain dystrophin deficiency

Abstract

AbstractDuchenne muscular dystrophy (DMD) is characterised by loss of dystrophin in muscle. Patients affected by DMD also have variable degree of intellectual disability and neurobehavioural co-morbidities. In contrast to muscle, in which a single full-length isoform (Dp427) is produced, multiple dystrophin isoforms are produced in the brain, and their deficiency accounts for the variability of CNS manifestations, with increased risk of comorbidities in patients carrying mutations affecting the 3’ end of gene, disrupting the shorter Dp140 and Dp71 isoforms. The mdx mouse model of DMD lacks Dp427 in muscle and CNS and exhibits exaggerated startle responses to threat, linked to the deficiency of dystrophin in limbic structures such as the amygdala, which normalise with postnatal brain dystrophin-restoration therapies. A pathological startle response is not a recognised feature of DMD, and its characterisation has implications for improved clinical management and translational research.To investigate startle responses in DMD, we used a novel fear-conditioning task in an observational study of 56 males aged 7-12 years (31 DMD, mean age 9.7±1.8 years; 25 Controls, mean age 9.6±1.4 years). Trials of two neutral visual stimuli were presented to participants: one ‘safe’ cue presented alone; one ‘threat’ cue paired with an aversive noise to enable conditioning of physiological startle responses (skin conductance response, SCR; heart rate, HR). Retention of conditioned physiological responses was subsequently tested with presentation of both cues without the aversive noise in an ‘extinction’ phase. Primary outcomes were the magnitude of the initial unconditioned SCR and HR change responses to the aversive ‘threat’ and acquisition and retention of conditioned responses after conditioning. Secondary outcomes were neuropsychological measures and genotype associations.The initial (unconditioned) mean SCR to threat was greater in DMD than Controls (Mean difference 3.0 µS (95% CI 1.0, 5.1), P=.004), associated with a significant threat-induced bradycardia only in the DMD group (mean difference -5.6 bpm (95% CI 0.51, 16.9); P=.04). DMD participants found the task more aversive than Controls, consequently early termination during the extinction phase occurred in 26% of the DMD group (vs. 0% Controls; P=.007).This study provides the first evidence that boys with DMD show increased unconditioned startle responses to threat, similar to the mdx mouse phenotype that also responds to brain dystrophin restoration. Our study provides new insights into the neurobiology underlying the complex neuropsychiatric co-morbidities in DMD and defines an objective measure of this CNS phenotype, which will be valuable for future CNS-targeted dystrophin-restoration studies.