Gene Transfer Clinical Study in X-Linked Myotubular Myopathy

This study will evaluate safety and preliminary efficacy of gene transfer in X-Linked
Myotubular Myopathy. Subjects will receive a single dose of AT132 delivered intravenously. A
maximum of 3 dose levels of AT132 are planned for evaluation in this study. Four subjects
will be enrolled at each dose level, including 1 subject at each dose level randomized to
control with delayed administration of the investigational medicinal product. Dose escalation
to the next dose level will be considered after evaluation of at least 4 weeks of data from
all subjects dosed at the current dose level. One of the dose levels will be chosen for dose
expansion, and the chosen dose will be administered to all delayed-treatment control
subjects.

The primary efficacy endpoint measures will be assessed at Week 48. Subjects will be followed
for a total of 5 years after administration of AT132.

This study will utilize an independent Data Monitoring Committee (DMC) that will monitor
subject safety and provide recommendations to Audentes regarding dose escalation, dose
expansion, and safety matters.

Key Inclusion Criteria:

- Subject has a diagnosis of XLMTM resulting from a genetically confirmed mutation in
the MTM1 gene as assessed by a Sponsor-approved testing facility.

- Subject is male.

- Subject is aged less than 5 years old at Day 1 and/or participated in the ATX-MTM-009
(INCEPTUS) study.

- Subject requires some mechanical ventilator support (eg, ranging from 24 hours per day
full-time mechanical ventilation, to noninvasive support such as continuous positive
airway pressure (CPAP) or bilevel positive airway pressure BiPAP during sleeping
hours).

- Subject requiring invasive mechanical ventilator support is fitted with or willing to
be fitted with a cuffed tracheostomy tube for some respiratory assessments.

- Subject has ventilator maximum positive end-expiratory pressure (PEEP) <8 cm H2O at
screening.

Key Exclusion Criteria:

- Subject is participating in an interventional study designed to treat XLMTM.

- Subject born <35 weeks gestation who is still not term as per corrected age.

- Subject tests positive for AAV8 neutralizing antibody with titers above protocol
specified threshold.

- Subject had recent surgery (<3 months before Day 1) or has planned surgery that may
confound data collection during the first 48 weeks of the study.

- Subject has a clinically important condition other than XLMTM in the opinion of the
investigator.

- Subject has a clinically significant underlying liver disease.

- Subject is currently experiencing a clinically important respiratory infection or
other active infection.

- Subject has received pyridostigmine or any medication to treat XLMTM within 3 months
before Day 1.

- Other than as required per protocol, subject has received immune-modulating agents
within 3 months before Day 1 (use of inhaled corticosteroids to manage chronic
respiratory conditions is allowed); use of other concomitant medications to manage
chronic conditions must have been stable for at least 4 weeks before dosing.

- Subject has a contraindication to prednisolone.

- Subject has a contraindication to study drug or ingredients.