The goal of this observational study is to learn about brain development in Juvenile-onset Huntington's Disease (JoHD). The main questions it aims to answer are:

• Is brain development different in JoHD than Adult-onset Huntington's Disease (AoHD)?
• Can reliable biomarkers for JoHD be found in brain structure and function?

Participants will be asked to complete cognitive tests, behavioral assessments, physical and neurologic evaluation, and MRI. Data collected will be compared to populations who are at-risk for HD and who have been diagnosed with HD as adults.

Huntington's disease (HD) is a genetic neurodegenerative disorder caused by an abnormal expansion of a trinucleotide CAG repeat region of the huntingtin gene (HTT). The majority of patients with HD do not present with symptoms until the age of 40-50 years old, on average, which is referred to as Adult-onset HD (AoHD). A much smaller percentage of patients with HD receive a motor diagnosis prior to the age of 21, which is referred to as Juvenile-onset HD (JoHD). Although patients with JoHD have the same core triad of cognitive, behavior, and motor symptoms, there are unique clinical characteristics that are distinct from AoHD. Specifically, patients with JoHD have less chorea compared to patients with AoHD, often presenting with rigidity and bradykinesia. However, due to the rarity, there is a lack of data regarding symptom characterization, neurobiology and progression of JoHD. Large-scale observational studies have been performed in AoHD, which have broadened our understanding of HD and opened the doors for the development and conduct of clinical trials. Patients with JoHD have been excluded from clinical trials, leaving patients and their families feeling hopeless and abandoned by the scientific community. Large-scale, longitudinal studies in patients with JoHD are critical to bettering our understanding of this devastating disease and providing hope to patients who have felt left behind as therapeutic strategies advance in AoHD.

In an effort to better understand the developmental aspects of this brain disease, the current study proposes to evaluate brain structure and function in children, adolescents, and young adults (ages 6-30) who have been diagnosed with JoHD. Brain structure will be evaluating using Magnetic Resonance Imaging (MRI) with quantitative measures of the entire brain, cerebral cortex, as well as white matter integrity via Diffusion Tensor Imaging. Brain function will be assessed by cognitive tests, behavioral assessment, and physical and neurologic evaluation. This study will test the hypothesis that comprehensive and longitudinal assessments of brain function and brain structure may produce reliable biomarkers of disease progression in JoHD.

Inclusion Criteria:

• Clinical diagnosis of JoHD
• Aged 4-30

Exclusion Criteria:

• Metal in body
• History of head trauma, brain tumor, seizures or epilepsy unrelated to JoHD
• History of major surgery or serious chronic medical conditions other than JoHD

Children, adolescents and young adults ages 4-30 who have been clinically diagnosed with Juvenile-onset Huntington's Disease

Aggregate, de-identified data may be shared with approved collaborators, but individual participant data will not be shared.