Causes & Heredity

How are MPS and 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.

All of the MPS syndromes, 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 affected children is 1 in 4 or 25% in each pregnancy. You will see below how the odds that each child will be affected are 1 in 4. The unaffected children have a 2 in 3 chance of being carriers like their parents and a 1 in 3 chance of being a 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 does not normally occur in females who have two normal X chromosomes, although there have been rare cases reported of girls affected with MPS II (Hunter syndrome). In all but these rarest of cases, MPS II is transmitted by a carrier mother to her male children. The risk of a male being born affected by MPS II is 50% in each pregnancy (1 in 2). The risk of a carrier mother giving birth to a carrier female is also 50% for each pregnancy (1 in 2).