SMA

This is a pretty good overview of how SMA is inherited. The last paragraph describes what SMA does to the human body. It's really hard to believe that this is what I have with no family history. Both of my parents need to be carriers.

Source: http://www.fsma.org/exp_answer_19.shtml

Question: What are the dominant and recessive traits of SMA? How does someone get it?

Answer: Dominant and recessive traits are specific terms used to describe different genetic disorders and refer to the way that a genetic disease is passed on from one generation to the next or inherited.

How are traits inherited? Each cell in our body contains a nucleus that houses DNA on structures that we call chromosomes. Each chromosome comes in two copies, one copy was passed down from our Dad (from the sperm cell) and one copy is passed down from our Mom (from the egg cell). This means that we have two chromosomes 1, 2, 3 etc. Genes, which are contained on our chromosomes, code for all the material (proteins) in a cell that tells it what to do. In general, we have two normal copies of each gene (one inherited from each of our parents). This basic information will help me explain the difference between a dominant and a recessive trait. For some genes, we absolutely need the two copies to be functional. For other genes, we are OK even if only one copy is functional. Mutations are changes in DNA that renders a gene semi-functional or completely non-functional. A dominant trait is one where we need both genes to be functional. For example, Amyotrophic Lateral Sclerosis (ALS) is an autosomal dominant trait. Individuals with ALS have one normal ALS gene and one ALS gene that has a mutation. Thus, all individuals that have a mutation in the ALS gene will most likely, at some point in their lives, display the symptoms associated with ALS even though they have one normal gene.

Childhood-onset SMA is a recessive trait. SMA is caused by mutations in a gene that is called SMN (for survival motor neuron gene). We only need one normal SMN gene for our motor neurons to develop and function normally. Individuals with SMA have two mutant SMN genes because they inherited one mutant gene from each of their parents. Because their parents each had one normal and one mutant SMN gene (they were carriers), they did not have SMA because one copy of SMN is sufficient. Thus SMA is a recessive tait, the fact that we have a SMN mutation stays hidden (recessive) until we produce a child that inherited the SMN mutation from each of his/her biological parents. So SMA is an inherited disease that is passed on from parent to child through their SMN gene. Both parents have to be carriers, that is, have one normal and one mutant SMN gene. In a recessive trait, carriers are asymptomatic (do not have any SMA symptoms). About 1 in 50 individuals are SMA carriers. This means that (in terms of probabilities) in about 1 in 2,500 couples, both would be SMA carriers. Each time two carriers have a baby, there is a 25% chance that the baby inherited both SMN mutations and will have SMA, 50% chance that the baby will be a carrier, and 25% chance that the baby inherited both normal SMN genes. Thus, for a recessive trait, each time that two carriers produce a child, there is a 1 in 4 chance that the baby will have the genetic disease in question.

SMA stands from spinal muscular atrophy. Because SMN is very important for normal functioning of motor neuron cells in the spinal cord, when SMN is almost completely absent, spinal cord motor neuron cells die. When these cells die, they stop sending messages to the muscles they are responsible for. Motor neurons are the batteries of muscle and tell muscle when to move and when to rest. Because the muscle is no longer receiving these messages, it wastes away or atrophies. I hope that this answers your questions.

Louise R. Simard, Ph.D.
Associate Professor
Centre de Recherche de l'Hôpital Sainte-Justine
January 2005