Rare Diseases Report 2022

Spinal muscular atrophy: Patient care in the age of genetically targeted therapy


 

In 2016, the U.S. Food and Drug Administration approved nusinersen, the first treatment for spinal muscular atrophy (SMA). Until then, SMA had a mortality rate nearly double that of the general population.1 Two-thirds of patients were symptomatic within 6 months of birth and, in the absence of mechanical ventilation and other support, had a nearly 100% mortality rate by age 2.2

Five years later, there are three approved treatments for SMA, all of which have been shown to slow or even halt disease progression in many patients. These new therapies, coupled with expanded newborn screening and advancements in optimizing patient care, are changing the natural history of the disease and offering a prognosis that extends well beyond adolescence. Neurologists, whose SMA patient population once consisted almost entirely of children, are now treating more adults with the disease. Indeed, more than half of all people alive with SMA in the United States today are adults, according to Cure SMA.

“Managing SMA used to be clinic follow-ups where we were doing our best supportive care and watching people fall apart before our eyes,” said John Brandsema, MD, a physician and neuromuscular section head at the Children’s Hospital of Philadelphia. “Today, what we see in the vast majority of people is that they are either the same as they were before – which is completely against the natural history of this disease and something to be celebrated – or that people are really better with their function. It totally changes everything in the clinic.”

Among those changes are a more proactive approach to rehabilitation and an even greater emphasis on personalized medicine and multidisciplinary care. But there is also a need for updated treatment guidelines, a new classification system to measure disease severity, specific biomarkers to guide therapy choices, more data on long-term efficacy of existing therapeutics, new medications to complement those therapies, and a deeper understanding of a disease that may have treatment options but still has no cure.

Advances in early diagnosis

Patients with SMA lack a working copy of the survival motor neuron 1 (SMN1) gene, which provides instructions for producing a protein called SMN that is critical for the maintenance and function of motor neurons. Without this protein, motor neurons eventually die, causing debilitating and progressive muscle weakness that affects the ability to walk, eat, and breathe. SMA is rare, affecting about 1 in 10,000 newborns.

In approximately 96% of patients, SMA is caused by homozygous loss of the SMN1 gene. People with SMA have at least one copy of the SMN2 gene, sometimes called a “backup” gene, that also produces SMN protein. However, a single nucleotide difference between SMN2 and SMN1 causes about 90% of the protein produced by SMN2 to be truncated and less stable. Even with multiple copies of SMN2 present, as is the case with many infants with SMA, the amount of functional protein produced isn’t enough to compensate for the loss of SMN1.3

All three approved medications are SMN up-regulators and work to increase the amount of functional SMN protein. Starting these medications early, even before symptoms present, is critical to preserve motor function. Early treatment depends on early diagnosis, which became more widespread after 2018 when SMA was added to the federally Recommended Uniform Screening Panel for newborns. As of July 1, 2022, 47 states have incorporated SMA newborn screening into their state panel, ensuring that 97% of all infants born in the United States undergo SMA screening shortly after birth. Screening in the remaining states – Hawaii, Nevada, and South Carolina – and Washington, D.C. is expected by mid-2023.

SMA newborn screening is a PCR-based assay that detects homozygous SMN1 gene deletion found in about 95% of all people with SMA. The remaining 5% of cases are caused by various genetic mutations that can only be detected with gene sequencing. In these cases, and in children who don’t undergo SMA newborn screening, the disease is usually identified when symptoms are noticed by a parent, pediatrician, or primary care provider. But a study found that in 2018 only 52.7% of pediatricians correctly identified genetic testing as a requirement for a definitive diagnosis of SMA; in 2019, with a larger sample size, that number decreased to 45%.4 The lack of awareness of diagnostic requirements for SMA could contribute to delays in diagnosis, said Mary Schroth, MD, chief medical officer for Cure SMA and a coauthor of the study.

“In our world, suspicion of SMA in an infant is an emergency situation,” Dr. Schroth said. “These babies need to be referred immediately and have genetic testing so that treatment can begin as soon as possible.”

Based on the study findings, Dr. Schroth and others with Cure SMA launched a new tool in 2021 designed to help pediatricians, primary care physicians, and parents identify early signs of SMA, so that a referral to a pediatric neurologist happens quickly. Called SMArt Moves, the educational resource features videos and a checklist to help increase early detection in infants who had a negative SMA newborn screening result or did not receive SMA screening at birth.5

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