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

Duchenne muscular dystrophy is characterised by loss of dystrophin in muscle, however patients also have variable degree of intellectual disability and neurobehavioural co-morbidities. In contrast to muscle, in which a single full-length dystrophin isoform (Dp427) is produced, multiple 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, which disrupt expression of shorter Dp140 and Dp71 isoforms. A mouse model (mdx mouse) 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 Duchenne muscular dystrophy, we used a novel fear-conditioning task in an observational study of 56 males aged 7-12 years (31 affected boys, 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 and heart rate). Retention of conditioned physiological responses was subsequently tested by presenting both cues without the aversive noise in an 'Extinction' phase. Primary outcomes were the initial unconditioned skin conductance and change in heart rate responses to the aversive 'threat' and acquisition and retention of conditioned responses after conditioning. Secondary and exploratory outcomes were neuropsychological measures and genotype associations. The mean unconditioned skin conductance response was greater in the Duchenne group than Controls (mean difference 3.0µS (1.0, 5.1); P = .004), associated with a significant threat-induced bradycardia only in the patient group (mean difference -8.7bpm (-16.9, -0.51); P = .04). Duchenne participants found the task more aversive than Controls, with increased early termination rates during the Extinction phase (26% in Duchenne group vs. 0% Controls; P = .007). This study provides the first evidence that boys with Duchenne muscular dystrophy show similar increased unconditioned startle responses to threat to the mdx mouse, which in the mouse respond to brain dystrophin restoration. Our study provides new insights into the neurobiology underlying the complex neuropsychiatric co-morbidities in Duchenne muscular dystrophy and defines an objective measure of this CNS phenotype, which will be valuable for future CNS-targeted dystrophin-restoration studies