About MPS I and MPS II

Classification and aetiology

Mucopolysaccharidosis type I (MPS I) and mucopolysaccharidosis type II (MPS II) are part of a subgroup of lysosomal storage disorders (LSDs) called the mucopolysaccharidoses (MPSs). The MPSs are defined by impaired degradation of different glycosaminoglycans (GAGs; previously called mucopolysaccharides) as a result of absent or deficient activity of a specific lysosomal enzyme. The enzyme deficiencies causing MPS I and MPS II are, respectively, α-L-iduronidase (IDUA) and iduronate-2-sulphatase (IDS). These enzyme deficiencies arise from mutations in the genes encoding α-L-iduronidase and iduronate-2-sulphatase.1

Enzyme deficiencies in MPS I and MPS II2

Disorder Deficient enzyme  GAG accumulated in lysosomes
MPS I α-L-iduronidase  Dermatan sulphate, heparan sulphate
MPS II Iduronate-2-sulphatase  Dermatan sulphate, heparan sulphate

Glycosaminoglycans are a class of very diverse molecules with a range of functions in cell signalling and cell adhesion.3 Although different enzyme deficiencies underlie MPS I and MPS II, both lead to abnormal degradation of the same specific GAGs, dermatan sulphate and heparan sulphate.2 These partially degraded GAGs accumulate in lysosomes and cause progressive cellular damage, ultimately leading to organ failure and premature death.2 Multiple organ systems are affected in both disorders, reflecting the widespread distribution of GAGs throughout the body.


Both MPS I and MPS II are pan-ethnic.4 MPS I is more common than MPS II, with an estimated incidence of 0.69-1.66 per 100,000 live births. The estimated incidence of MPS II is 0.30-0.71 per 100,000 live births.2

Males and females are equally affected by MPS I, which is inherited in an autosomal recessive fashion (see Genetics). MPS II is an X-linked recessive disorder and generally affects males, but there are rare cases in females.

Clinical Impact of the Diseases

Clinical Impact of the Diseases

The enzyme deficiencies that define MPS I and MPS II each produce a continuum of clinical phenotypes ranging from severe to attenuated. The two disorders share many signs and symptoms and can be difficult to differentiate clinically.



MPS I and MPS II are inherited disorders caused by mutation of genes encoding α-L-iduronidase (IDUA) and iduronate-2-sulphatase (IDS), respectively.



The first clinical descriptions of MPS I and MPS II recognising the whole phenotype as a disease date back almost a century, preceding the first classification of the MPSs and the discovery of the lysosome by more than three decades.


  1. 1.Neufeld EF, Muenzer J. The Mucopolysaccharidoses In: Valle D, Beaudet AL, Vogelstein B, Kinzler KW, Antonarakis SE, Ballabio A, et al., editors. The Online Metabolic and Molecular Bases of Inherited Disease. New York, NY: McGraw-Hill; 2014.
  2. 2.Muenzer J. (2011) Overview of the mucopolysaccharidoses. Rheumatology (Oxford) 50 Suppl 5: v4-12.
  1. 3.Clarke LA. (2008) The mucopolysaccharidoses: a success of molecular medicine. Expert Rev Mol Med 10: e1.
  2. 4.Martin R, Beck M, Eng C, et al. (2008) Recognition and diagnosis of mucopolysaccharidosis II (Hunter syndrome). Pediatrics 121(2): e377-386.