Loss of survival motor neuron (SMN) protein in muscles may contribute directly to the development of spinal muscular atrophy (SMA), in addition to its lack in motor neurons, an early study in mice suggested.
Lowering SMN levels solely in muscle was seen to be enough to cause SMA-like symptoms and premature death in mice, with severity depending on the number of copies of the SMN2 gene.
Optimal SMA therapies, the researchers said, should aim to restore SMN protein not only in the nervous system but also in skeletal muscle.
The study, “Muscle-specific SMN reduction reveals motor neuron–independent disease in spinal muscular atrophy models” was published in the Journal of Clinical Investigation.
The most common types of SMA (0, 1, 2, 3, and 4) are caused by mutations in the SMN1 gene and are characterized by a loss of motor neurons, which control voluntary muscles. This leads to progressive muscle weakness and muscle wasting (atrophy), affecting such basic abilities as sitting, walking, swallowing, and breathing.
The SMN1 gene provides instructions for making the survival motor neuron (SMN) protein. Typically, people with SMA produce only residual levels of the protein, and this deficiency — specifically the loss of the protein in motor nerve cells — is considered to be the disease’s root cause.
But the SMN protein is produced in several cell types throughout the body, and plays multiple and fundamental roles. One of its more important functions is to help in the assembly of the machinery (spliceosome) needed for processing messenger RNA molecules (mRNA), which serve as genetic blueprints for making proteins. This is a process that can impact a wide range of proteins and their functions within cells.
SMN also plays a part in the development of specialized outgrowths of nerve cells through which neurons communicate with each other and with muscles.
While for some time SMN deficiency was thought to result solely in the selective death of motor neurons, numerous recent studies challenge that notion, indicating that SMA may be a multi-system disorder affecting cell types and tissues other than motor neurons, and reaching beyond the central nervous system (CNS, brain and spinal cord).
Given the variety of SMN functions and its widespread presence in the body, it is debated whether SMA symptoms and signs derive uniquely from motor neuron loss or if they stem from broader effects.
Researchers at Columbia University Medical Center decided to weigh in on this debate. Specifically, they wanted to understand the effects of low SMN on skeletal, or voluntarily controlled, muscles.
“That CNS cells and, in particular, spinal motor neurons are especially vulnerable to low SMN, and therefore must express adequate SMN levels to thwart disease onset is widely accepted,” the scientists wrote. “These cells are efficiently targeted by intrathecal [spinal cord] administration of SMN-restoring agents such as Spinraza. … Yet, lingering questions about restricted intrathecal delivery of the drug and the long-term effects of depriving the periphery of adequate SMN remain.”
To do so, they used a mouse model genetically engineered to selectively inactivate SMN protein production only in skeletal muscle and mimic a severe form of SMA.
These mice also carried one to two copies of SMN2, a gene related to SMN1 that can make a small amount of working SMN protein. The amount of this gene’s copies is directly related to disease severity (the more copies, the less severe).
Researchers observed that low levels of SMN were “profoundly damaging” to the animals’ limb and respiratory muscles.
Disease onset was rapid in mice with a single SMN2 copy, leading to muscle degeneration, motor and breathing disabilities within a few days after birth, and a shortened lifespan.
“We conclude that the[se] mutants expressed approximately 25% of WT [wild type; normal] SMN protein and that the overt disease phenotype observed was indeed a consequence of selectively reducing protein in the muscle tissue of the animals,” the team wrote.
In comparison, a late-onset disease was observed in mice with two copies of SMN2.
Even though symptoms were not immediately obvious, taking months to appear, the persistent low levels of SMN eventually resulted in a broad range of defects in muscle fibers and neuromuscular junctions (NMJs) — the sites at which motor neurons and muscle fibers communicate — as well as a poor motor performance and premature death.
Researchers then decided to test if it was possible to reverse these damaging effects by restoring SMN protein using an antisense oligonucleotide. Treatment with the agent, injected systemically (body-wide) after disease onset, raised the levels of SMN in skeletal muscle and reversed the muscle defects.
“We conclude that muscle is a critical cellular site of action of the SMN protein and that expressing protein from 1 or 2 copies of the SMN2 gene is insufficient to prevent the onset of a severe, cell-autonomous muscle phenotype [trait],” the researchers wrote.
“SMN restorative therapies that deprive [the muscle] of adequate protein are unlikely to achieve maximum benefit,” they added. “Rather, our results have raised the prospect of intrathecally [through the spinal canal] treated subjects possibly developing severe and potentially life-threatening myopathies over time.”
These results, they continued, “imply that the most optimal SMN-enhancing treatment regimens for SMA will be those that restore the protein to skeletal muscle.”
The nine researchers responsible for this study reported no conflicts of interest, such as pharmaceutical company funding, employment, or individual investments.
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