Neurodegenerative Disorders: Spinal Muscular Atrophy (SMA)

Introduction

Spinal Muscular Atrophy (SMA) is a genetic neurodegenerative disease that primarily affects motor neurons, leading to paralysis and muscle atrophy.
Unlike other neurodegenerative disorders, SMA typically presents during infancy.
Symptoms include severe muscle weakness, often referred to as "floppy baby syndrome," due to the lack of muscle tone.
This muscle weakness leads to severe feeding and respiratory issues, and failure to achieve developmental milestones like lifting the head, sitting, and walking.

Key Points

Discovery of the genetic cause of SMA.
The concept of a modifier gene.
Success stories of biomedical research, leading to three FDA-approved disease-modifying treatments for SMA.

Affected Neurons

Motor neurons are primarily affected in SMA.
These neurons are located in the ventral horn of the spinal cord and project onto muscles, forming cholinergic synapses at neuromuscular junctions.
In SMA, these neurons degenerate, leading to impaired development, denervation, synaptic dysfunction, and reduced myofibril size.
Both the health of the neuron and the muscle are affected.

Prevalence and Genetics

SMA affects approximately 1 in 11,000 babies, with 10,000 to 25,000 children in the US impacted.
It is an autosomal recessive disorder, meaning two copies of the mutated gene (one from each parent) are required for the disease to develop.
If both parents are carriers, there is a 25% chance of inheriting the disease.
Autosomal Recessive vs. Autosomal Dominant Inheritance: It's essential to understand the difference.

Identifying the Genetic Cause

In 1995, the genetic cause of SMA was identified through painstaking mapping of common genetic areas with mutations in SMA patients.
The critical region identified was on the fifth chromosome (5q13), containing four genes that are duplicated in humans.
The gene of interest is called Survival of Motor Neurons 1 (SMN1).
The duplicated gene, SMN2, differs from SMN1 by a single nucleotide (C to T), which causes splicing of exon 7.
SMA is primarily caused by mutations related to the SMN gene.

SMN1 and SMN2 Function

Every human has two copies of SMN1 and SMN2.
SMN1 transcribes into RNA, including exons 6, 7, and 8, producing the SMN protein necessary for motor neuron survival.
SMN2 has a single nucleotide change that causes skipping of exon 7 in 90% of its mRNA, leading to the production of delta 7 mRNA, which is unstable, and consequently, very little SMN protein is produced from SMN2.
Approximately 90% of the functional SMN protein comes from SMN1, and only 10% from SMN2.
In SMA patients with an SMN1 mutation, no functional protein is generated from SMN1, and the limited amount from SMN2 is insufficient to maintain motor neuron health.

Purpose of SMN Protein

The exact function of SMN protein is not fully understood.
It is believed to be essential for the biogenesis of ribonucleoproteins involved in mRNA splicing; neurons are particularly sensitive to splicing problems.
SMN is also crucial for transporting mRNAs from the soma down long axons, where they are translated and are essential for axon survival.

Clinical Variability and SMN2 Copy Number

SMA presents with varying degrees of severity, classified into subtypes from type 0/1 to 4, with lower numbers indicating more severe disease.
Severity is correlated with the number of copies of the SMN2 gene.
Individuals can have varying numbers of SMN2 copies (three, four, or more), impacting disease severity.
More copies of SMN2 result in a less severe disease manifestation.
The SMN2 gene acts as a modifier gene.

Modifier Gene

The SMN2 gene exerts its influence by modulating the severity of the disease caused by mutations in the SMN1 gene.
It influences the severity because, even though approximately 90% of SMN2 RNA undergoes degradation, the remaining functional SMN2 can produce a limited supply (10%) of SMN protein. This limited quantity has the potential to partially offset and mitigate the loss of function resulting from the SMN1 mutation.
SMN2, even with its limited functionality, can compensate for the lack of SMN1 by approximately 40% when four copies are present.

Revised Classification

The classification of SMA is now based on the number of SMN2 copies.
Individuals with three or four copies of SMN2 are classified as type 4, often diagnosed later in life, while those with two or fewer copies typically have severe SMA (types 1-3).

Therapeutic Developments

The first therapeutic agents modifying SMA were identified around 2020.

Antisense Oligonucleotides (ASOs)

ASOs are short, synthetic nucleic acid strands that modulate gene expression by binding to pre-mRNA or mRNA.
They can modify splicing, exclude certain exons, degrade mRNA, or inhibit translation.
Splice Inclusion: inclusion of exons.
Splice Exclusion: exon exclusion.
mRNA degradation: using RNase.
Translation Inhibition: prevent mRNA from getting translated.
In SMA, ASOs are used to modify SMN2 splicing to include exon 7.

Mechanism

In normal circumstances, SMN1 produces mRNA with all exons including 7, while SMN2 skips exon 7 due to a single base pair mutation.
The goal is to modify SMN2 to include exon 7, compensating for the lack of functional SMN1.
An inhibitor, the HNRNP complex, binds to a sequence at the end of exon 7, causing it to be skipped.
The drug nusinersen, developed by Biogen, uses a complementary sequence to displace the inhibitory complex, leading to the inclusion of exon 7.
Nusinersen displaces HNRNP proteins, facilitating exon 7 inclusion in SMN2 pre-mRNA.

Administration and Cost

Nusinersen is administered via direct injection into the spinal cord.
Dosing involves multiple injections in the initial phase, followed by maintenance doses every four months for life.
The drug is highly expensive, costing around $750,000 for the first year and $375,000 annually thereafter.
Recently, insurance coverage has improved access to the treatment.

Gene Therapy

Gene therapy modifies a person's genes to treat or cure disease through various methods.

Replace a disease-causing gene.
Inactivate a gene.
Introduce a new modified gene.

There are two primary approaches:

Ex vivo: Stem cells are extracted, modified genetically outside the body, selected, and then injected back into the patient.
In vivo: Genetic modification occurs inside the patient using AAV or DNA associated viral plasmids or lipid nanoparticles.

Adeno-Associated Virus (AAV)

AAV is a replication-defective, single-stranded DNA virus used for gene delivery.
It's favored due to its low pathogenicity and minimal triggering of immune responses.
AAV binds to a cell surface receptor, is endocytosed, and either degraded or enters the nucleus.
Inside the nucleus, it sheds its coat, the single-stranded DNA creates a copy of itself, becomes transcribed into mRNA, and then translated into protein.

Application to SMA

Modified AAVs are used to introduce the SMN1 gene into motor neurons.
Studies in mice have shown that postnatal delivery of SMN1 via AAV is therapeutic, improving survival rates compared to untreated mice.

Current Issues in AAV Mediated Gene Therapy

Immunogenicity: The body's immune system may react against the viral capsid.
Potency and Efficiency: The virus may be degraded before reaching the nucleus.
Genotoxicity: Random integration of the virus into the genome can cause issues.
Persistence: Once a gene is inserted, it's nearly impossible to remove.

Zolgensma

Zolgensma, developed by Novartis, uses an AAV9 vector to deliver the SMN1 gene.
It's administered as a single dose intravenously.
Clinical trial results showed significant improvement in motor skills, especially when administered early (0-1 months).
The therapy has a very high cost, approximately $2,000,000.

Controversies

Novartis allegedly hid data during FDA approval regarding the virus's effects on animal health.
Deaths have been reported due to liver toxicity, as the virus accumulates in the liver.
Potential long-term toxic effects of AAV9-mediated overexpression are being investigated, suggesting a risk of toxic function from overexpression.

Small Molecule Approach: Risdiplam

Discovery

Risdiplam was identified through a screen for compounds that increase luciferase activity, indicating increased SMN2 expression.
A startup called PTC Therapeutics initially made this discovery, which was later acquired by Roche.

Mechanism of Action

It binds to sites on either side of exon 7, stabilizing its inclusion in the mRNA and increasing total SMN protein.
SMN protein levels in muscle correlate with those in the brain, this is a useful biomarker.

Clinical Use

FDA approved in 2020 for children older than two months.

Summary of FDA-Approved Therapeutics

Nusinersen: An ASO that targets SMN2 to increase exon 7 inclusion.
Zolgensma: A viral-mediated gene therapy that introduces SMN1.
Risdiplam: A small molecule that increases SMN production by modifying SMN2 splicing.

Overall Summary

SMA is an autosomal recessive disease caused by biallelic loss of function in the SMN1 gene.
SMN1 and SMN2 can be targeted to restore SMN protein levels.
Therapies including ASOs, gene therapy, and small molecules have been identified to target SMA.
SMN2 creates similar proteins that compensate for what is created by SMN1. Overexpressing SMN2 allows compensation for the loss of function in SMN1.