INCEPTUS Natural History, Run-in Study for Gene Replacement Clinical Trial in X-Linked Myotubular Myopathy

INTRODUCTION

X-linked myotubular myopathy (XLMTM) is a life-threatening congenital myopathy caused by variants in the MTM1 gene, leading to absence or insufficiency of functional myotubularin protein. Myotubularin is a broadly expressed lipid phosphatase required for the normal development, maturation, and maintenance of skeletal muscle [1]. Natural history studies have shown that most patients with XLMTM typically experience profound muscle weakness from birth, significant respiratory insufficiency requiring chronic invasive mechanical ventilation, dysphagia requiring feeding support, absent or delayed major motor milestone attainment, and high rates of early mortality [2–6] most often related to respiratory failure [5, 7–9]. Approximately 20% of XLMTM patients have a milder phenotype in which they can walk independently and may live into adulthood, though most do ultimately require some form of ventilator support (often noninvasive) and feeding assistance [2, 10, 11]. Historically, it has been estimated that approximately half of infants died before 18 months of age, and survival is highly dependent on intensive medical intervention [3, 5, 6]. XLMTM has an estimated incidence of 1 in 40,000-50,000 newborn males [12].

There are currently no approved disease-modifying therapies for XLMTM, which is managed with multidisciplinary supportive care for manifestations of chronic, profound muscle weakness. Novel therapeutic strategies have shown promise in pre-clinical disease models, [13–16] including adeno-associated virus (AAV)-mediated gene replacement therapy [17–20].

The INCEPTUS study (NCT02704273) began in 2016 in preparation for ASPIRO (NCT03199469) —the first-in-human clinical study of an investigational gene replacement therapy in patients with XLMTM, which was anticipated to enroll young patients with a ventilator-dependent phenotype. INCEPTUS was designed to obtain longitudinal data on XLMTM natural history and prospectively characterize a control group for ASPIRO. Although prior natural history studies have characterized the clinical manifestations of XLMTM, a robust set of repeated longitudinal assessments of ventilatory dependence, respiratory muscle strength, motor milestones, and quality of life is a prerequisite for evaluating a novel disease-modifying therapy.

METHODS

RESULTS

Between August 2016 and June 2019, 34 children with XLMTM enrolled at eight sites in North America and Europe (Supplementary Table 1). Median (range) follow-up was 13.0 (0.5, 32.9) months, with 23 participants (68%) followed for at least 6 months and 18 participants (53%) for at least 12 months (Supplementary Figure 1). Baseline participant characteristics are shown in Table 2. Heights and weights were within the standard growth chart ranges, except for two participants with documented macrosomia.

Fig. 1

Motor function over time for individual participants by mutation type. (A) CHOP INTEND total scores. (B) MFM-20 scores. The thick red lines represent LS means and standard errors from a mixed-effect model with fixed effect of time.

Most participants had extensive histories of medical conditions and procedures related to XLMTM (Table 3). Half of participants had nonsense MTM1 mutations that would be expected to produce little or no functional or stable myotubularin, 35% had missense mutations and 15% in-frame exonic deletions (IFED), possibly expressing stable intact or internally deleted proteins with residual myotubularin activity (Supplementary Table 2).

Disease-related events, survival, and histopathology

Overall, 61 disease-related serious adverse events were reported in 20 (59%) participants, primarily respiratory complications (85%) (Supplementary Table 3). Serious adverse events were associated with 52 hospitalizations among 18 participants, with a mean (SD) duration of 8.1 (7.6) days.

Three deaths occurred (participants 04 [3.5 years], 14 [3.5 years], and 15 [2.3 years]) associated with serious adverse events of cardiopulmonary failure, aspiration pneumonia, and hepatic hemorrhage with evidence of diffuse hepatic peliosis, respectively. In all three participants, histopathological findings were characteristic of XLMTM in skeletal muscles, including myofiber smallness, excessive internal nucleation, and organelle mislocalization (Supplementary Figure 2) [31]. These findings affected > 80% of fibers in the muscles sampled in two of the deceased participants (14 and 15). More extensive sampling of skeletal muscles in participant 04 showed a variable but moderate degree of pathology characteristic of XLMTM. None showed myofiber degeneration, necrosis, or fatty infiltration.

Fig. 2

Respiratory function over time for individual participants by mutation type. (A) Ventilator dependence within the previous 24 hours. Higher ventilator dependence reflects more daily hours of ventilator use. The thick blue and red lines represent LS means and standard errors from a mixed-effect model with fixed effect of time for invasively ventilated participants (blue) and noninvasively ventilated participants (red). (B) Maximal inspiratory pressure (MIP). The thick red line represents LS means and standard errors from a mixed-effect model with fixed effect of time.

DISCUSSION

INCEPTUS is the most comprehensive study thus far to quantify the extensive disease burden of XLMTM prospectively and longitudinally, including the persistence over time of ventilator dependence, low respiratory muscle strength, absent or severely delayed motor milestones, and diminished quality of life, in young boys with ventilator-dependent disease. These longitudinal data from 34 participants with XLMTM and ventilator dependency expand upon previous natural history studies showing severely compromised respiratory and motor function that fails to improve over time [2, 3, 6, 7]. In INCEPTUS, quarterly evaluations were performed to quantify the most comprehensive array of disease parameters to date. All INCEPTUS participants were permanently dependent on ventilator support, most were unable to achieve motor milestones, and most suffered severe medical events. Our data show that, although ventilator support extends the lifespan, respiratory and motor function impairments remain static.

Despite receiving high-quality, specialized, clinical care during INCEPTUS, participants had minimal to no improvement on any parameters and nearly 10% died from disease-associated complications. This unmet medical need for children with XLMTM is further underscored by the extensive care requirements demonstrated by ACEND scores, including constant monitoring by caregivers, multiple extended hospitalizations, and the need for frequent suctioning of secretions by caregivers, which also did not improve over the course of the study.

Patients requiring invasive ventilation for > 16 hours per day (80% in INCEPTUS) are unlikely to have intrinsic capacity to wean from ventilatory support [33]. Pediatric patients on mechanical ventilation due to a variety of underlying diagnoses have increased risk of mortality, [34] often from causes other than progression of the initial diagnosis, [35] and substantial physical, emotional, and financial demands associated with reliance on mechanical ventilation [36–38]. The importance of reducing ventilator dependence is highlighted by 85% of serious adverse events in INCEPTUS being respiratory in nature. Similar observations in a one-year longitudinal study demonstrated ventilator support time as a reliable, reproducible, robust, and clinically meaningful measure of disability in XLMTM patients [2].

Healthy infants typically reach the maximum CHOP INTEND score of 64 in the first 3 to 6 months of life [39] and can usually roll from back to belly and sit unassisted for > 30 seconds by 8 months of age [40]. None of the INCEPTUS participants achieved the maximum CHOP INTEND score despite being older than 6 months. CHOP INTEND score trajectories were essentially flat, highlighting a lack of expected progression in motor function in early life. Bayley-III data showed that 79% of participants were unable to sit unsupported for≥30 seconds. None achieved higher level motor skills, such as pulling to stand or walking with or without assistance, despite some being over 6 years old at their last evaluation.

Recent reports demonstrate hepatobiliary disease associated with the natural history of XLMTM (i.e., not associated with any specific treatments). A recent case series highlighted five untreated XLMTM patients with recurrent cholestasis, including one who has progressed to end-stage liver failure [41]. Furthermore, a retrospective analysis of hepatic data in a cohort of Italian patients with XLMTM indicates the presence of cholestasis [42]. Additional cases include a child with hypertransaminemia and hyperbilirubinemia (up to 40 times ULN) with hepatopathy and cholestasis [43] and another child with hyperbilirubinemia and elevated urine bile acids consistent with asymptomatic cholestasis leading to severe vitamin K deficiency, coagulopathy and fatal intracranial hemorrhage [44]. One-quarter of INCEPTUS participants had a history of hepatobiliary disease prior to enrollment; during prospective monitoring, 91% experienced hepatobiliary disease or hepatic adverse events or laboratory or ultrasonographic abnormalities (Table 4). Although serum bile acid levels were not measured in INCEPTUS, observed elevated transaminase and bilirubin levels and ultrasonographic abnormalities may have been reflective of underlying cholestasis. As there was not high awareness of intrahepatic cholestasis in XLMTM patients at the time of INCEPTUS, there was no determination of underlying reasons for isolated AST or ALT elevations (i.e., liver damage, infection, or transient intermittent increase). However, the extent of aminotransferase elevations, as well as the relative elevations of ALT compared to AST, are suggestive of a hepatogenic source. Based on emerging reports of intrahepatic cholestasis complicating XLMTM, [41–44] in combination with the high frequency of liver-related laboratory abnormalities described in this study, we postulate that monitoring of liver parameters (including potential consultation with a pediatric hepatologist) should be an important aspect of clinical care moving forward. Obtaining liver-related laboratory parameters (ALT, AST, GGT, total and direct bilirubin), plus evaluation of serum bile acid levels, may help identify cholestatic episodes in XLMTM patients, and will also enhance understanding of the incidence of this phenotype in the XLMTM population and the potential triggers for its occurrence. Liver biopsy is a useful modality for the diagnosis of cholestasis and for better defining its cause but is potentially contraindicated in XLMTM due to the risk of hepatic peliosis. Unfortunately, imaging modalities (such as ultrasound or MRI) have not demonstrated great utility for the diagnosis of intrahepatic cholestasis and the determination of its causes. Histopathologic evaluation for intrahepatic cholestasis and secondary injury was not performed, as liver biopsy is relatively contraindicated in XLMTM patients due to potential coexistent peliosis and subsequent risk for life-threatening hemorrhage [45]. Myotubularin may play a role in cellular processes outside of muscle cells, including intracellular/endosomal trafficking and bile export from hepatocytes [46]. Further investigation of myotubularin’s role in hepatobiliary function is needed. Preliminary experience with AAV-mediated gene replacement therapy in the first-in-human ASPIRO study includes serious hepatotoxicity leading to the death of 4 participants. Amongst surviving patients, clinically meaningful improvements in muscle strength and function were observed [47]. Currently, the study is on clinical hold until a benefit-risk assessment of proceeding with this therapy can be determined. Treated participants continue to be monitored and assessed per study protocol.

Despite the broad range of ages at enrollment in INCEPTUS (0.2 to 3.4 years), participants from all mutation groups had profound impairment and absence of improvement in disease parameters. Of note, patients with nonsense mutations tended to have more pronounced signs of hepatobiliary disease (Table 4), and non-truncating mutations have been associated with less pronounced decrements in respiratory function compared with truncating mutations [5].

INCEPTUS participation was restricted to patients who required some form of ventilator support at baseline, excluding a subset of patients who may have milder disease [2]. Nevertheless, XLMTM is widely recognized as a severe disease in which the vast majority of patients require ventilator support, and the INCEPTUS population was comparable to cohorts from previous natural history studies [2, 6, 7]. It is also important to note that, while the median follow-up in INCEPTUS was 13 months and 53% of participants had at least 12 months’ follow-up, one-third of participants had less than 6 months follow-up. A limitation of INCEPTUS is that, at the time of the study, there was not good general awareness of the potential for intrahepatic cholestatic disease in XLMTM patients. Thus, we did not collect data, such as serial serum bile acid levels, that would have been ideal for evaluating this.

CONCLUSIONS

Children with ventilator-dependent XLMTM have profoundly impaired respiratory and motor function that do not spontaneously improve and experience frequent medical complications leading to hospitalization and fatal events. Evidence of underlying hepatobiliary disease, present in almost all INCEPTUS participants, may represent an underappreciated aspect of XLMTM pathophysiology. In the INCEPTUS cohort, the high ventilator dependence, low respiratory capacity, and severely limited motor function remained static throughout the observation period.