A Study to Assess if BIIB122 Tablets Are Safe and Can Slow Worsening of Early-Stage Parkinson's Disease in Participants With Specific LRRK2 Genetic Variants Between the Ages of 30 and 80 Using the Movement Disorder Society-Unified Parkinson's Disease Rating Scale

Number of Participants With Treatment-Emergent Adverse Events (TEAEs) and Serious Adverse Events (SAEs)

An adverse event (AE) is any untoward medical occurrence in a participant or clinical investigation participant administered a pharmaceutical product and that does not necessarily have a causal relationship with this treatment. An AE can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease temporally associated with the use of a medicinal (investigational) product, whether or not related to the medicinal (investigational) product. An SAE is any untoward medical occurrence that at any dose results in death; in the view of the investigator, places the participant at immediate risk of death (a life-threatening event); requires inpatient hospitalization or prolongation of existing hospitalization; results in persistent or significant disability/incapacity; results in a congenital anomaly/birth defect or is a medically important event.

Time frame: From the first dose (Day 1) of the study drug up to the end of follow-up (up to 336 days)

Time to Confirmed Worsening in MDS-UPDRS Part II Score

Time to confirmed worsening is defined as a worsening event sustained over 2 consecutive assessments. MDS-UPDRS is a multimodal scale assessing impairment and disability consisting of 4 parts. Part II assesses motor experiences of daily living (range 0-52). It contains 13 questions which are to be completed by the participant. For each question, a numeric score is assigned between 0-4, where 0 = Normal, 1 = Slight, 2 = Mild, 3 = Moderate, 4 = Severe. MDS-UPDRS Part II score ranges from 0-52. A higher score indicates more severe symptoms of PD.

Time frame: From Week 96 to Week 180

Change From Baseline in MDS-UPDRS Parts II and III Combined Score at Week 96

MDS-UPDRS is a multimodal scale assessing impairment and disability consisting of 4 parts. Part II assesses motor experiences of daily living (range 0-52). It contains 13 questions which are to be completed by the participant. Part III assesses the motor signs of PD and is administered by the rater (range 0-132). Part III contains 33 scores based on 18 items. For each question, a numeric score is assigned between 0-4, where 0 = Normal, 1 = Slight, 2 = Mild, 3 = Moderate, 4 = Severe. MDS-UPDRS Part II and III combined score equals the sum of Parts II and III (range 0-184). A higher score indicates more severe symptoms of PD.

Time frame: Baseline, Week 96

Time to Confirmed Worsening in Modified Schwab and England Activities of Daily Living Scale (mSE-ADL) Score

Time to confirmed worsening is defined as a worsening event sustained over 2 consecutive assessments. The mSE-ADL scale reflects the speed, ease, and independence with which an individual performs daily activities or personal chores with 100% indicating total independence, falling to 0%, which indicates a state of complete dependence. The individual is asked to rate his or her function using an 11-point scale (10% increments), from 100% (completely independent; able to do all chores without slowness, difficulty, or impairment; essentially normal; unaware of any difficulty) to 0% (vegetative functions such as swallowing, bladder and bowels are not functioning; bedridden). The lower the score, the worse the functional status.

Time frame: From Week 96 to Week 180

Change From Baseline in MDS-UPDRS Parts I, II, and III Combined Score at Week 96

The MDS-UPDRS is a multimodal scale consisting of four parts. Part I assessed non-motor experiences of daily living and has 2 components (Range 0-52). Part IA contains 6 questions and are assessed by the examiner (Range 0-24). Part IB contains 7 questions on non-motor experiences of daily living which was completed by the participant (Range 0-28). Part II assessed motor experiences of daily living (Range 0-52). It contained 13 questions completed by the participant. Part III assessed the motor signs of PD and was administered by the rater (Range 0-132). Part III contained 33 scores based on 18 items. For each question a numeric score is assigned between 0-4, where 0 = Normal, 1 = Slight, 2 = Mild, 3 = Moderate, 4 = Severe. The MDS-UPDRS Total Score equals the sum of Parts I,II, and III (Range: 0-236). A higher score indicated more severe symptoms of PD.

Time frame: Baseline, Week 96