Molecular therapies exploit the understanding of pathogenic mechanisms to reconstitute impaired gene function or manipulate flawed RNA expression. These therapies include (1) RNA interference by antisense oligonucleotides, (2) mRNA modification using small molecules, and (3) gene replacement therapy, the viral-mediated intracellular delivery of exogenous nucleic acids to reverse a genetic defect. Several molecular therapies are approved for treating spinal muscular atrophy (SMA), a recessive genetic disorder caused by survival motor neuron (SMN)1 gene alterations. SMA involves degeneration of lower motor neurons, which leads to progressive muscle weakness, hypotonia, and hypotrophy. Onasemnogene abeparvovec is a gene replacement therapy for SMA that uses adeno-associated virus delivery of functional SMN1 cDNA to motor neurons. Two other molecular therapies modulate SMN2 transcription: nusinersen, an antisense oligonucleotide, and risdiplam, a small molecule designed to modify faulty mRNA expression. The most suitable individualized treatment for SMA is not established. Here, we describe remarkable clinical improvement in a 4-month-old patient with SMA type 1 who received onasemnogene abeparvovec therapy. This case represents an explanatory bridge from bench to bedside with regard to therapeutic approaches for genetic disorders in neurology. Knowledge of the detailed mechanisms underlying genetic neurologic disorders, particularly monogenic conditions, is paramount for developing tailored therapies. When multiple disease-modifying therapies are available, early genetic diagnosis is crucial for appropriate therapy selection, highlighting the importance of early identification and intervention. A combination of drugs, each targeting unique genetic pathomechanisms, may provide additional clinical benefits.
- antisense oligonucleotides;
- adeno-associated virus;
- bilevel positive airway pressure;
- Duchenne muscular dystrophy;
- spinal muscular atrophy;
- survival motor neuron
Genetic molecular therapies are developed to restore gene function or modulate RNA expression related to diseases arising from genetic defects. Three currently available therapeutic approaches in clinical neurology are classified based on the mechanism of action: (1) RNA interference by antisense oligonucleotides (ASOs) or small-interfering RNAs (siRNAs), (2) mRNA splicing modification by small molecules, and (3) gene replacement therapy, which is the delivery of exogenous DNA encoding a functional version of the target gene.1 Therapies under investigation or currently available include RNA interference drugs (inotersen and patisiran) for hereditary transthyretin amyloidosis and 4 ASOs (casimersen, viltolarsen, golodirsen, and eteplirsen) for Duchenne muscular dystrophy (DMD) related to specific exon alterations.1 Three spinal muscular atrophy (SMA) therapies based on these technologies (onasemnogene abeparvovec, nusinersen, and risdiplam) and targeting the respective genetic mechanism have shown remarkable clinical benefits.
A 4-month-old female child presented with diffuse muscle hypotonia and no head control during a routine pediatric consultation. She was referred at age 4.5 months to our tertiary care center for persistent hypotonia, motor delay, failure to thrive, and meteorism. The patient’s parents reported a reduction in spontaneous movements beginning as early as age 1 month. The family history was negative for known genetic neurologic disorders, and she was born at term (weight 3,440 g) following a normal pregnancy and delivery.
Neurologic examination revealed poor head control, diffuse appendicular and truncal hypotonia, and severe lower limb weakness (Medical Research Council 1/5), weak hand prehension, and areflexia. Tongue fasciculations were absent. She presented no signs of chest deformity or respiratory distress. Electromyography showed a neurogenic pattern with active denervation in lower and upper limb muscles. Considering the motor deficits, neurologic examination, and electrophysiologic findings, our diagnosis was SMA type 1. Genetic testing confirmed a homozygous survival motor neuron (SMN)1 deletion and 2 copies of SMN2. The child scored a 22 on the Children’s Hospital of Philadelphia Infant Test of Neuromuscular Disorders (CHOP-INTEND), a reliable scale for assessing motor development in the first 2 years of life.2 This scale consists of 16 items, each rated 0 (no response) to 4 (complete response), based on spontaneous upper and lower limb, truncal and pelvic movements, and head control and strength. The total score range is 0–64, with better motor function reflected in higher scores.
Testing for adeno-associated virus (AAV) serotype 9 antibodies was negative. The parents were counseled regarding therapeutic alternatives, and the patient received an intravenous dose of onasemnogene abeparvovec (1.1 × 1014 vector genomes/kg) at age 5 months. Per therapeutic protocol, she also received prednisolone (1 mg/kg) starting the day before gene therapy began and continuing for 8 weeks. She was monitored weekly for laboratory abnormalities, including liver enzymes and platelets. Because of a slight transient increase in aspartate aminotransferase (51 U/L), the corticosteroid dose was increased temporarily to 2 mg/kg for 1 week and subsequently reduced progressively until cessation. No side effects were noted. The CHOP-INTEND score at 4 weeks postinfusion was 32, indicating clinical improvement. During the hospitalization, prophylactic bilevel positive airway pressure (BiPAP) was prescribed for overnight use.
Two months after therapy administration, the patient was hospitalized with viral pneumonia, requiring escalation of respiratory support with continuous BiPAP. She recovered after 4 weeks with ongoing respiratory improvement noted up to 2 months after discharge. At 8 weeks postdischarge, her BiPAP requirement was downgraded to overnight only. Six months postgene therapy treatment, the patient showed moderate head control and improved hypotonia, lower limb strength, and hand prehension. She had preserved spontaneous breathing and swallowing and could sit without assistance for 5 seconds, with a CHOP-INTEND score of 42.
This child is in a recent cohort of patients with SMA type 1 receiving onasemnogene abeparvovec gene therapy in a real-world setting. She showed motor improvement based on parent-reported functional skills and serial CHOP-INTEND assessment. Apart from a transient aspartate aminotransferase increase controlled by corticosteroids, we detected no other laboratory abnormalities or major side effects. Comprehensive follow-up data collection is ongoing to assess long-term efficacy and any delayed-onset side effects of onasemnogene abeparvovec.
SMA as a Neurologic Genetic Disease
SMA, a leading genetic cause of infant mortality,3 is archetypal in that knowledge of the underlying genetic mechanisms has allowed for use of gene-specific approaches to alter the disease course. Most SMA cases (95%–98%) are caused by homozygous deletions in SMN1, which lies in the telomeric region of chromosome 5q13(Figure 1A).4 The remaining cases involve small intragenic SMN1 alterations.5 These alterations lead to progressive limb muscle weakness, hypotonia, and atrophy with motor function decline and the development of bulbar difficulties and respiratory insufficiency.6 A centromeric SMN2 variant7 may be present in variable copy numbers on 5q13 and is an almost identical homolog of SMN1 except for a few nucleotides.3 A crucial C-to-T substitution in exon 7 at position 6 alters SMN2 splicing so that ∼90% of transcripts lack exon 7, and the resulting truncated SMN protein is rapidly degraded. The remaining 10% of protein produced is insufficient for motor neuron survival,3 and SMN2 copy number is inversely correlated with phenotypic severity. Current advanced molecular treatments for SMA target repletion of SMN in motor neurons by replacing SMN1 or acting on SMN2 genes transcription.
Molecular Therapies in Neurology
Molecular therapies target the underlying disease mechanism with the aim of reconstituting gene function or manipulating RNA expression. Three techniques are currently available in clinical neurology1 (additional data listed in eTable 1 in the Supplement, links.lww.com/WNL/C379). Their mechanism of action depends on the underlying gene alterations, which may cause partial or complete loss of function, gain of toxic function, or both.
When loss of function is the prevalent mechanism, the aim of treatments is restoring expression of the affected protein. If the prevailing mechanism is gain of toxic function, the therapy goal is to suppress expression of the disease-related protein. Gene replacement with viral vectors (Figure 1B) or ASOs/miRNAs can be tailored to downregulate or upregulate target gene/mRNA expression. For instance, when protein expression is reduced or absent, as in SMA or DMD, gene replacement therapy and ASOs increase the expression of the missing protein. The viral vector delivers cDNA encoding a functional SMN protein or dystrophin, and ASOs modulate splicing of SMN2 or the mutant dystrophin gene to increase overall levels of functional protein. Conversely, with hereditary transthyretin amyloidosis or SOD1-related amyotrophic lateral sclerosis,8 the underlying pathomechanism is associated with a toxic gain of function alteration, and viral vectors, siRNAs, or ASOs are used to reduce levels of the mutated protein.
Molecular Therapies in SMA
EU and US regulatory bodies have approved 3 therapies for SMA: onasemnogene abeparvovec (Zolgensma), nusinersen (Spinraza), and risdiplam (Evrysdi) (Figure 1C and eTable 1, links.lww.com/WNL/C379). All 3 increase levels of functional SMN by re-expressing a healthy copy of SMN1 cDNA or by modifying SMN2 splicing. Onasemnogene relies on nonreplicating, recombinant AAV9 that can cross the blood-brain barrier, at least in infants, to deliver a functional SMN cDNA transgene to motor neurons9 (eTable 1, mechanism 1, links.lww.com/WNL/C379). It is given once intravenously and indicated for children younger than age 2 years with SMA type 1.9 Nusinersen is a short single-stranded oligonucleotide that is delivered intrathecally and alters SMN2 pre-mRNA splicing to ensure exon 7 inclusion, boosting production of the functional protein10 (eTable 1, links.lww.com/WNL/C379, mechanism 2). The small molecule risdiplam, an oral therapy for SMA, binds SMN2 pre-mRNA in 2 regions, increasing the quantity of functional SMN9 (eTable 1, links.lww.com/WNL/C379, mechanism 3). The most suitable treatment for each patient, best initial approach, and possible advantages of combination therapies remain unclear.9,11
Early diagnosis is paramount for access to and benefit from molecular therapies in neurology, yet diagnostic delays persist.12 Implementation of prenatal genetic testing or neonatal screening for SMA could lead to earlier diagnosis and improve access to personalized therapies.13 Our patient showed a remarkable clinical amelioration despite being symptomatic with a low CHOP-INTEND score at treatment initiation. Although the diagnosis was made at a tertiary care center when she was age 4.5 months, symptoms may have manifested 3 months earlier. Our case highlights the need for neonatal screening to treat patients as soon as possible to yield maximum benefit from molecular treatments.
Other gene replacement therapies approaches are under investigation for a wide range of neurologic diseases. In most cases, these experimental therapies target loss-of-function alteration diseases such as DMD, lysosomal disorders (e.g., Pompe, Gaucher, and Fabry diseases),14 idiopathic Parkinson disease, Leber hereditary optic neuropathy, and Charcot-Marie-Tooth 1A.1 Less frequently, viral-mediated delivery of microRNAs or short hairpin RNAs to silence disease genes is used for disorders involving gain-of-function mechanisms, such as amyotrophic lateral sclerosis (SOD1, C9orf72, and FUS alterations) and Huntington disease.8 In addition, genome editing with mRNA CRISPR-Cas9 is being explored for Huntington disease13 and hereditary transthyretin amyloidosis.15
The efficacy:safety ratio must be carefully evaluated for classical gene transfer approaches with AAV or other viruses and with genome editing. Overall, genetic molecular strategies offer new perspectives and hope for patients affected by genetic and acquired neurologic diseases with known molecular mechanisms.
This study was supported by the Italian Ministry of Health Foundation IRCCS Ca’ Granda Ospedale Maggiore Policlinico Ricerca Corrente 2022 to GPC.
S. Corti has participated in Scientific Advisory Board meetings of AveXis and Scholar Rock and is the Principal Investigator on a nonprofit grant from Roche to the University of Milan; A. Govoni has participated in a Working group of Novartis and a Co-creation workshop organized by Roche; G. Costamagna and A. Wise are part of the editorial staff of the Resident & Fellow Section of Neurology. Go to Neurology.org/N for full disclosures.
The support of Associazione Centro Dino Ferrari is gratefully acknowledged.
Go to Neurology.org/N for full disclosures. Funding information and disclosures deemed relevant by the authors, if any, are provided at the end of the article.
Submitted and externally peer reviewed. The handling editor was Roy Strowd III, MD, Med, MS.
- Received April 24, 2022.
- Accepted in final form August 10, 2022.
- © 2022 American Academy of Neurology