Mucopolysaccharidosis type I due to maternal uniparental disomy of chromosome 4 with partial isodisomy of 4p16.3p15.2.

Mucopolysaccharidosis type I (MPS I) is a rare lysosomal storage disease caused by biallelic mutations in IDUA, the gene coding for the lysosomal enzyme alpha L-iduronidase. Clinically MPS I is a chronic progressive multisystem disease typically presenting with coarse facial features, skeletal deformities, joint contractures, and multi-organ involvement. Hurler syndrome (MPS IH) represents the severe end of the spectrum of mucopolysaccharidosis type I and is characterized by central nervous system involvement leading to childhood dementia. Here we report on a severe affected MPS IH patient who is homozygous for a splice site mutation (c.158 + 1G > A) in the IDUA gene. Further analyses revealed maternal uniparental disomy of chromosome 4 with partial isodisomy of the telomeric end of chromosome 4 (4.p16.3p15.2), representing an extraordinary mode of inheritance with a much lower re-occurrence risk for MPS I in the family.