Study to Learn If Tofersen (BIIB067) Can Delay ALS Symptoms and About Its Safety in Adults With a SOD1 Gene Mutation Who Have Not Yet Developed ALS Symptoms

In this study, researchers will learn more about the effects of a drug called tofersen, also known as BIIB067 and QALSODY®, in adults with amyotrophic lateral sclerosis (ALS) who have a superoxide dismutase 1 (SOD1) gene mutation. One of the causes of ALS is a change (or mutation) in the SOD1 gene, which causes abnormal SOD1 protein to be made. This abnormal protein can cause the motor neurons in people with ALS to break down and die. When neurons are damaged, they release a protein called neurofilament light chain, also known as NfL. Previous studies have shown that NfL levels are higher in people with SOD1-ALS, even before any ALS symptoms appear. In this study, researchers will look at the effect of tofersen treatment in the earlier stages of the disease, even before ALS symptoms appear. Researchers believe that giving tofersen to patients as early as possible may slow the onset of ALS symptoms and the progression of the disease. This study will have 4 parts. In Part A, participants will have their health monitored and NfL levels tested on a monthly basis, but they will not receive any study drug. Tofersen or placebo will be given to participants in Parts B, while all participants will receive tofersen in Parts C and D. The main question that researchers want to answer is: • How long does it take for ALS symptoms to appear? Researchers will also learn more about: * How many participants develop ALS symptoms within 24 months of the start of Part B * Changes in participants' ability to perform their daily activities * Changes in participants' ability to breathe * How many participants die or need long-term breathing support * Changes in NfL levels in the blood * Changes in SOD1 protein levels in the fluid around the brain and spinal cord * How many participants have adverse events or serious adverse events. Adverse events are health problems that may or may not be caused by the study drug. The study will be done as follows: * Participants will first be scr