Data from the literature have established the cause of SMA: 95% of people have a homozygous loss of SMN1 (no copy of the SMN1 gene) and 5% a heterozygous loss of SMN1 (absence of the SMN1 gene on one of the chromosomes and mutation on the other).
A retrospective study was carried out in Brazil on a population of 450 people with SMA composed of:
- 23,3% type I SMA
- 34% type II SMA
- 40,7% type III SMA
- 2% type IV SMA
In this Brazilian population, 89.3% of people have a homozygous loss of the SMN1 gene and 10.7% a heterozygous loss of the SMN1 gene. In the latter case, the most frequent anomalies were localized in exons 3 or 6 of the SMN1 gene, which leads the authors to recommend looking first for anomalies in these two exons during prenatal diagnosis in Brazil.
In most cases, people with type I SMA had 2 copies of the SMN2 gene, people with type II had 3 copies, and people with type III had 3 or 4 copies. Although some people with type I SMA had 3 copies of the SMN2 gene, and others with type III or IV SMA had only 2 copies, the inverse correlation between the number of copies of the SMN2 genes and the severity of the condition. disease was observed in most situations.
