Causes & Heredity

How are MPS Related Diseases Inherited?

Lysosomal storage diseases are usually autosomal recessive disorders, inherited from healthy parents who have no idea they carry a common recessive gene. For carrier parents, there is a 1 in 4 chance with every pregnancy that the child will inherit one recessive gene from each parent and suffer from the carried disease. MPS II (Hunter Syndrome) and Fabry Disease are x-linked recessive disorders, meaning they are transmitted by carrier mothers to her sons. For carrier mothers, there is a 1 in 2 chance of an x-linked disorder occurring in the birth of a son.

Parents of an affected child have the option of prenatal testing to determine if their next child will be affected by the same disease. Parents are encouraged to seek genetic counselling as part of their family planning and to inquire about available carrier testing for their unaffected children.

The occurrence of MPS in the population is estimated to be 1 in 25,000 births.

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 Diagram

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.

Inheritance: X-Linked Recessive