CAUSES & HEREDITY
Cause of MPS Diseases
All diseases supported by The Candian MPS Society are classified as Lysosomal Storage Diseases. These can be described simply as a genetic error of metabolism. They are caused by the absence of an enzyme that is needed to break down a substance in the body. Without the enzyme, there is a buildup, or "storage," of these substances. Over time, individuals affected by MPS experience a loss of function, or deterioration, in one or several crucial areas of the body. This may result in mental and physical disability as well as a drastically shortened lifespan.
Inheritance: Autosomal Recessive
All of the Mucopolysaccharidoses, except MPS II (Hunter syndrome), have an autosomal recessive mode of inheritance. The disease only develops when a child inherits a "double dose" of the abnormal gene - one from each parent. The parents' risk of having further affected children is 1 in 4, or 25% in each pregnancy. You will see below how the odds in each pregnancy are 1 in 4 that the child will be affected. The unaffected children have a 2 in 3 chance of being carriers like their parents, and a 1 in 3 chance of being a normal non-carrier.
Inheritance: X-Linked Recessive
MPS II (Hunter syndrome) has an X-linked recessive mode of inheritance. The defective gene is situated on the X (female sex) chromosome and the disease only manifests itself in individuals with only one X chromosome - which limits the syndrome to the male sex. It normally does not occur in girls who have two normal X chromosomes, although there have been rare cases reported of girls affected with MPS II (Hunter syndrome). It must be stressed that the possibility of a female being affected by MPS II is very unlikely. MPS II is transmitted by a carrier mother to her sons. The risk of a son being born affected by MPS II is 50% in each pregnancy, or 1 in 2. The risk of a daughter being a born a carrier like her mother is also 50%, or 1 in 2.