What are the genetics of SMA?
What is Spinal Muscular Atrophy? SMA is a deadly and relatively common genetic disease and is the leading genetic cause of death in infants and toddlers. It is the absence/defect in the Survival Motor Neuron Gene (SMN1) that causes Spinal Muscular Atrophy. The SMN1 gene codes for survival of motor neuron (SMN) protein, and this protein is critical to the survival and health of motor neurons, nerve cells in our spinal cord that our brain uses to control our muscles. Without sufficient SMN protein, motor neurons shrink and die. As the motor neuron network breaks down, the ability of the brain to control muscles diminishes and with less control and use, muscles weaken and waste away.
In the majority of SMA cases, a mutation in a gene termed SMN1 causes the disease Spinal Muscular Atrophy. Usually when SMA happens, both copies of the gene are mutated, which means that both parents are probably carriers. Other children in the family may also carry one or two ‘bad copies.’ 1 in every 35-40 Americans are carriers of SMA, which means that instead of having two healthy copies of the SMN1 gene, they have one healthy copy and one defective, mutated copy. As a result, a carrier only has one effective gene directing the production of SMN protein, but that is more than sufficient for motor neuron health. However, if two carriers have a child, that child randomly gets one gene from each parent. If the child is lucky, he/she will randomly get at least one ‘good’ gene from one parent and the child will be healthy.
In the majority of SMA cases, a mutation in a gene termed SMN1 causes the disease Spinal Muscular Atrophy. Usually when SMA happens, both copies of the gene are mutated, which means that both parents are probably carriers. Other children in the family may also carry one or two ‘bad copies.’ 1 in every 35-40 Americans are carriers of SMA, which means that instead of having two healthy copies of the SMN1 gene, they have one healthy copy and one defective, mutated copy. As a result, a carrier only has one effective gene directing the production of SMN protein, but that is more than sufficient for motor neuron health. However, if two carriers have a child, that child randomly gets one gene from each parent. If the child is lucky, he/she will randomly get at least one ‘good’ gene from one parent and the child will be healthy. If the child is unlucky however, (25% chance) he/she will randomly get the defective SMN1 gene from both parents, and be left with two defective copies of the SMN1 gen
