Plain language summary of a study looking at whether genetic testing can help doctors diagnose the severity of mucopolysaccharidosis type I (MPS I)

What is MPS I & what is this study about? Mucopolysaccharidosis type I (MPS I) is a rare genetic condition, resulting from disease-causing changes in the IDUA gene called pathogenic variants. There are two different types of variants: Null variants – cells cannot make the alpha-L-iduronidase enzyme Missense variants – cells make a partly functional or severely reduced amount of the fully functional enzyme The alpha-L-iduronidase enzyme breaks down large sugar molecules called glycosaminoglycans (GAGs). In people with MPS I, the enzyme is unable to break down GAGs and they build up in cells. Based on the severity of the condition, doctors categorise MPS I into two types, called phenotypes: Attenuated MPS I Severe MPS I Treatment options differ depending on MPS I phenotype. There are currently no tests that accurately determine MPS I phenotype. Using information from the MPS I Registry, researchers wanted to find out whether knowing the specific combination of the 2 disease-causing gene variants (genotype) can help predict whether a person has an attenuated or severe phenotype. What were the results? In people with attenuated MPS I: 19 in 20 had genotypes where at least one of the gene copies was a missense variant. The cell makes a partly functional or a severely reduced amount of the fully functional enzyme, and is unable to completely break down GAG No-one had 2 null variants 2 in 5 had unique genotypes (their genotype was only seen in one person) In people with severe MPS I: 2 in 3 had genotypes where both copies of the gene contained null variants. The cell is not able to make any enzyme 1 in 5 had unique genotypes What do the results mean? This study showed that genotype can predict phenotype in some people with MPS I. For example, if a person has inherited two copies of the genes containing null variants, they will have severe MPS I. However, due to the large number of unique gene variants found in this study, doctors cannot predict the severity of MPS I from just the genotype for some people.