What is spinal muscular atrophy?

Spinal  muscular  atrophy,  normally  called  ‘SMA’, describes a genetic disorder with different degrees of  severity  associated  with  the  loss  of  motor neurons and disease-related complications affecting the entire body. Loss  of  motor  neurons  leads  to  a  loss  of  muscle mass,  which  is  translated  to  muscle  atrophy. Depending on the type of  SMA,  this  could  lead  to  the  loss  of  physical strength  and  the  ability  to  walk,  eat or breathe. As  a  disease  of  the  entire  body,  SMA  affects  not only muscles but also many other organs.

In humans, two genes, SMN1 and SMN2, encode SMN. SMA is caused by loss or mutation of the SMN1 gene and retention of SMN2. The SMN1 gene provides instructions for making the survival of motor neuron (SMN) protein, which is necessary for the survival of motor neurons and other cell types in the body. People  with  SMA  are, therefore, dependent  on the related SMN2 gene. However, this gene cannot produce enough functional SMN protein.

The  severity of SMA varies  among  individuals and   depends   on   the   age   of   onset   and   the number  of  SMN2 copies  they  have.  

  • Type  0, also  called  prenatal  onset  SMA,  is  the  most severe form of this disorder;  it  affects  babies that are still in the womb. These    babies    experience   severe   hypotonia and are unable to sit or roll.
  • Type 1, found in infants  under  the  age  of  2  years,  is  a  very severe  form  of  SMA.  Just  like  Type  0,  infants have severe hypotonia and are also unable to sit  or  roll.  
  • The  age  of  onset  for  Type  2  SMA is  6–18  months;  infants  with  this  form  of  SMA are able to sit but unable to walk independently.
  • Type  3   is   usually   diagnosed   after  18  months  of age,  but  the  actual  age  of  onset  is  very  variable and    may    not    happen    until   late   childhood   or early adulthood. Individuals  with  this  type  of  SMA  have  a  normal  life span but may lose the ability to walk. 
  • Finally, Type 4, also known as adult-onset SMA is the least severe form of this disorder.   It  is  most often diagnosed in early adulthood, with individuals experiencing mild motor impairment.

Learn more about Spinal Muscular Atrophy and research at Roche.

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