Lysosomes are membrane-bound organelles which contain hydrolases that degrade unneeded proteins and infectious bacteria. When these substrates accumulate without degradation inside the lysosome, the cells die lysosomal storage diseases (a subset of enzymopathies ensue. Hurler Syndrome is a mucopolysaccharidose, a kind of disease involving accumulation of mucopolysaccharides (or glcyosaminoglycans) within lysosomes due to deficiencies in degradative enzymes. An autosomal recessive disease, Hurler Syndrome is caused by extreme impairment of the enzyme α-L-iduronidase, leading to facial dysmorphia, developmental retardation and death by ∼10 years. In Scheie Syndrome, also autosomal recessive, α-L-iduronidase has residual activity and patients sometimes have normal life spans and normal intelligence (despite facial dysmorphia).
Hunter Syndrome is a phenotypically similar mycopolysaccharidose, but is X-linked recessive and impairs iduronate sulfatase (not α-L-iduronidase). Hurler Syndrome fibroblasts synthesize normal iduronate sulfatase, and Hunter Sydrome fibroblasts synthesize normal α-L-iduronidase. When grown together in culture, they uptake secreted proteins and thereby complement each other to produce non-diseased cells. Complementation analysis is the study of genetic complementation, where mutant gene products can complement each other to neutralize disease.