Understanding Spinal Muscular Atrophy
Spinal Muscular Atrophy (SMA) is a genetic disorder that affects the nerve cells responsible for controlling voluntary muscle movement. These specialized nerve cells, known as motor neurons, are located in the spinal cord and brainstem. When these motor neurons are compromised, they cannot effectively send signals to muscles, leading to muscle weakness and atrophy.
The primary cause of SMA is a deficiency in a protein called Survival Motor Neuron (SMN) protein. This deficiency arises from a genetic mutation, most commonly in the SMN1 gene. Without sufficient SMN protein, motor neurons gradually deteriorate and eventually die.
The loss of motor neurons directly impairs the brain’s ability to control muscle movements, including those for walking, breathing, and swallowing. SMA presents across a spectrum of severity, affecting individuals from infancy through adulthood.
SMA Prevalence: Is There a Difference Between Males and Females?
Spinal Muscular Atrophy affects males and females with equal frequency. This equal distribution is a direct result of the genetic location of the gene responsible for SMA.
The gene primarily associated with SMA, the SMN1 gene, is located on an autosome, specifically chromosome 5. Autosomes are non-sex chromosomes, meaning they are present in two copies in all individuals, regardless of whether they are male or female. This distinguishes SMA from sex-linked genetic disorders, which show different patterns of inheritance and prevalence based on biological sex.
The Genetics Behind SMA Inheritance
SMA follows an autosomal recessive inheritance pattern, which explains why it affects males and females equally. For an individual to develop SMA, they must inherit two copies of the mutated SMN1 gene. One mutated copy comes from the biological mother, and the other mutated copy comes from the biological father.
Parents who carry one normal copy and one mutated copy of the SMN1 gene are known as carriers. These individuals typically do not exhibit any symptoms of SMA because their single functional SMN1 gene produces enough SMN protein to maintain motor neuron health. Carriers are generally unaware of their status until they have a child affected by SMA or undergo genetic testing.
When two carriers have a child, there is a 25% chance with each pregnancy that the child will inherit two mutated copies of the SMN1 gene and develop SMA. There is also a 50% chance the child will be a carrier, inheriting one normal and one mutated gene. Lastly, there is a 25% chance the child will inherit two normal copies of the gene and be unaffected.