Phase 1/2 Study of MZ-1866, an AAV-9 Gene Therapy Delivered by Intracerebroventricular Injection to Participants With Pitt Hopkins Syndrome

The goal of this clinical trial is to learn if MZ-1866 is a safe and tolerable treatment for children and adults with Pitt Hopkins Syndrome. To evaluate the safety of MZ-1866, the following will be evaluated: * frequency and severity of adverse events * physical exam, laboratory results and electrocardiogram findings Participants will: * receive a single dose of MZ-1866 by intracerebroventricular injection * be seen by the study physician and site staff periodically to assess changes to their health status * be periodically evaluated using neurodevelopmental tools Caregivers will: * be interviewed periodically about the health status and development of the participant * keep diaries and complete periodic questionnaires regarding participant symptoms

Inclusion Criteria: * The participant has a TCF4 gene mutation confirmed as "pathogenic" or "likely pathogenic" by whole exome sequencing, whole genome sequencing, gene panel, single gene testing, or microarray, performed at an accredited laboratory * Clinical phenotype consistent with Pitt Hopkins Syndrome, in the opinion of the investigator * The participant, or the participant's parent or legal guardian, is willing to provide access to prior medical records for the collection of demographics and diagnostic and treatment history Exclusion Criteria: * A deletion that includes the TCF4 gene that is over 12 Mbp in size * Another genetic mutation or clinical comorbidity not associated with Pitt Hopkins Syndrome that could potentially confound interpretation of the study data * A central nervous system structural or vascular abnormality that is a contraindication to the ICV administration procedure, including but not limited to: signs or symptoms of increased intracranial pressure, history of a space-occupying lesion, or presence of a ventricular shunt that would preclude ICV procedures or safety assessments, or increase risk to the participant * Not able to receive prophylactic corticosteroids due to a medical contraindication or participant has a history of a condition that could worsen with corticosteroid therapy as assessed and determined by the Investigator * Not able to undergo MRI procedures * Cannot be anesthetized for the ICV injection