الرئيسية
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من طرف al_zoubi الثلاثاء يوليو 08, 2008 12:57 pmCause 
The region of chromosome 5 that contains the SMN (survival motor neuron) gene has a large duplication. A large sequence that contains several genes occurs twice in adjacent segments. There are thus two copies of the gene, SMN1 and SMN2. The SMN2 gene has an additional mutation that makes it less efficient at making protein, though it does so in a low level. SMA is caused by loss of the SMN1 gene from both chromosomes. The severity of SMA, ranging from SMA 1 to SMA 3, is partly related to how well the remaining SMN 2 genes can make up for the loss of SMN 1. Often there are additional copies of SMN2, and an increasing number of SMN2 copies causes less severe disease.
A severe infantile form of the disease can also occur in boys because of missense and synonymous variants mutations in gene UBE1 on the X chromosome. Sons of women with this genetic mutation generally have a 50% chance of suffering the disease, and daughters of women with the mutation have a 50% chance of being carriers, while unaffected by the disease itself.[1]
All forms of SMN-associated SMA have a combined incidence of about 1 in 6,000. SMA is the most common cause of genetically determined neonatal death. The gene frequency is thus around 1:80, and approximately one in 40 persons are carriers. There are no known health consequences of being a carrier, and the only way one might know to consider the possibility is if a relative is affected.
[edit] Types
The region of chromosome 5 that contains the SMN (survival motor neuron) gene has a large duplication. A large sequence that contains several genes occurs twice in adjacent segments. There are thus two copies of the gene, SMN1 and SMN2. The SMN2 gene has an additional mutation that makes it less efficient at making protein, though it does so in a low level. SMA is caused by loss of the SMN1 gene from both chromosomes. The severity of SMA, ranging from SMA 1 to SMA 3, is partly related to how well the remaining SMN 2 genes can make up for the loss of SMN 1. Often there are additional copies of SMN2, and an increasing number of SMN2 copies causes less severe disease.
A severe infantile form of the disease can also occur in boys because of missense and synonymous variants mutations in gene UBE1 on the X chromosome. Sons of women with this genetic mutation generally have a 50% chance of suffering the disease, and daughters of women with the mutation have a 50% chance of being carriers, while unaffected by the disease itself.[1]
All forms of SMN-associated SMA have a combined incidence of about 1 in 6,000. SMA is the most common cause of genetically determined neonatal death. The gene frequency is thus around 1:80, and approximately one in 40 persons are carriers. There are no known health consequences of being a carrier, and the only way one might know to consider the possibility is if a relative is affected.
[edit] Types
