Immune-mediated myositis following gene therapy for Duchenne muscular dystrophy: a case report

Abstract

Clinical data were from the authors' observations and the electronic medical records of Children’s Health. Pre-treatment histologic samples were provided by Sarepta Therapeutics. The muscle biopsy was processed at the University of Texas Southwestern Medical Center. A 9-year-old ambulatory male patient, diagnosed with DMD at 2.5 years of age presented with acute-onset bulbar and limb girdle weakness 1 month post-treatment with delandistrogene moxeparvovec GT in ENDEAVOR (SRP-9001-103, NCT04626674) cohort 2 (Fig. 1 A). This patient has a deletion of exons 3–43 in the DMD gene, which encodes part of the N-terminal, part of the central rod domain, and hinge 1 and hinge 2. Parents described onset of symptoms 4 days earlier with swelling of the lips and lower face, and change in voice; 2 days later, he was unable to climb stairs (Fig. 1 B). Pre-GT, the patient was maintained on daily deflazacort (27 mg/day [0.9 mg/kg/day]) with good motor function, independent ambulation, and no history of respiratory insufficiency or heart disease. Daily prednisone (30 mg/day [1 mg/kg/day]) was added to his regimen 24-h pre-GT and was continued without any missed doses at the time of presentation. Other daily medications included vitamin D (1,000 IU), lisinopril (5 mg), fish oil (1200 mg), two multivitamins, coenzyme Q10 (100 mg), and melatonin (3 mg). He took alendronate (35 mg) weekly. Deletion of exons 3–43 of the patient’s DMD gene and timeline of the IMM event. A Deletion of exons 3–43 of the patient’s DMD gene. B Timeline of the IMM event. Abbreviations: ABD = actin-binding domain; CR = cysteine-rich domain; CT = C-terminal domain; DMD, Duchenne muscular dystrophy; DYS = dystrophin; H = hinge domain; IMM = immune-mediated myositis; MRI = magnetic resonance imaging; N = N-terminal; NGT = nasogastric tube; PICU = pediatric intensive care unit; R = spectrin‑like repeat domain; SAE = serious adverse event; TPE = therapeutic plasma exchange The patient was lethargic with angioedema, had increased work of breathing, hypophonic voice, and difficulty swallowing. He required help sitting and could only bear weight with assistance. Weakness was most severe in shoulder abductors and hip flexors. Deep tendon reflexes were intact at the Achilles bilaterally and absent at biceps, triceps, brachioradialis, and patella. He had no observed sensory deficit, muscle tenderness, or skin rash. Initial laboratory data showed no abnormality in complete blood count, erythrocyte sedimentation rate, venous capillary gases, electrolytes or troponin-I level (24 pg/ml), or creatine kinase (27,903 U/L); additional data collected at admission are shown in Online Resource Table 1. Due to worsening bulbar weakness, he was transferred to the intensive care unit and a nasogastric tube was placed for feeding. Heated high flow nasal cannula, then bilevel positive airway pressure, was administered for ventilation. Six rounds of therapeutic plasma exchange (TPE) were initiated [ 9 ]. After round four, he showed objective improvement in voice quality, could count to 10 without taking a breath, and could sit independently; thereafter, he showed slow but steady improvement. Following TPE, muscle biopsy was obtained from the left quadriceps (51 days post-GT; Fig. 1 B). Three days before discharge (52 days post-GT), maintenance immunosuppression with oral tacrolimus was initiated and maintained for 27 months before tapering to cessation over 6 weeks. At discharge (20 days post-admission and 55 days post-GT), his creatine kinase level was 1,218 U/L and he could ambulate with slight assistance, tolerate all intake by mouth, and required no respiratory support. Motor function assessments were conducted bi-annually during standard of care visits (Online Resource Table 2). During the last clinic visit (22 months post-GT), he was unable to rise independently and could ambulate for short distances with frequent falls. Pulmonary evaluation indicated normal respiratory muscle strength. Manual motor testing revealed lower proximal muscle strength compared with 1 year prior (10 months post-GT). Muscle biopsy showed minimal micro-dystrophin expression (12 months post-GT; Online Resource Table 3). In the biopsy obtained 1 day pre-treatment, dystrophin epitope immunohistochemical staining showed that a substantial subset (30–40%) of muscle fibers expressed a weak sarcolemmal C-terminal (dystrophin 2 [DYS2]; Fig. 2 C) while rod domain (dystrophin 1 [DYS1]) and N-terminal (dystrophin 3 [DYS3]; Fig. 2 B) were completely absent, suggesting that a truncated dystrophin protein containing the C-terminal was present pre-GT. In the post-treatment biopsy obtained after TPE, there was an abundance of CD8- and CD4-positive T cells and CD68-positive macrophages in the endomysium that surrounded and invaded individual myofibers (Fig. 3 ). These inflammatory cells appeared to selectively attack myofibers expressing the rod domain (Fig. 2 D [arrows]) and transgene-derived N-terminal (Fig. 2 E [arrows]), but not those expressing the native C-terminal (Fig. 2 F). Dystrophin epitope staining in pre- and post-treatment muscle biopsies. Pre-treatment muscle demonstrated absence of A DYS1, B DYS3, and C Partial expression of DYS2 in some myofibers. In the post-treatment muscle biopsy, D DYS1 and E DYS3 were expressed in a subset of myofibers, the majority of which were under attack by inflammatory cells. F DYS2 was expressed in a subset of myofibers similar to pre-treatment muscle. Images of D – F were taken from the same field on serial sections. The color-coded arrows (black, blue, and red) point out three myofibers on serial sections with different epitope stains. Scale bar: 100 µm. Abbreviation: DYS = dystrophin Post-treatment muscle histology. A–I Low magnification; scale bar: 100 µm. A H&E showed a muscle with chronic active dystrophic changes and endomysial mononuclear inflammation surrounding and invading viable myofibers (arrow). B MHC class I immunostain highlighted inflammatory cells and patchy MHC1 upregulation in myofibers. C . C5b-9 immunostain highlighted scattered necrotic fibers. D–F CD markers. D CD8. E CD4. F CD68. The mononuclear inflammation was composed of mixed CD8+ T cells, CD4+ T cells, and CD68+ macrophages. CD20+ B cells were absent (data not shown). G – I Immunostain of dystrophin epitopes. G′–I′ High magnification; scale bar: 50 µm. G , G ′ DYS1 (rod domain) was only expressed in a small subset of fibers, the vast majority of which were either dying (arrows) or under attack by inflammatory cells (arrow heads and G′ ). H , H ′ DYS3 (N-terminal) was expressed in a small subset of myofibers, most of which were under attack by inflammatory cells (arrow); rare ones had intact sarcolemmal expression (arrowhead). (I, I’) DYS2 (C-terminal) was variably expressed in a significant subset of myofibers, all of which had an intact sarcolemmal membrane. Abbreviations: DYS = dystrophin; H&E = hematoxylin and eosin; MHC, major histocompatibility complex

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