We describe a pair of monozygotic twins with an attenuated form of mucopolysaccharidosis type I (MPS-I). At age 24, they both developed cervical myelopathy as a cardinal manifestation. They each also had mild valve abnormalities and both inguinal and umbilical hernia, however, other characteristic features of MPS-I were absent or very mild. Magnetic resonance imaging revealed the cervical cord compressed by pachymeningeal hypertrophy. Surgery with dural plasty and laminoplasty resulted in decompression of the cervical cord with clinical improvement, revealing marked thickening of the dura mater. Both patients showed a marked decrease of alpha-L-iduronidase (IDUA) activity with c.252insC (p.P55fsX62; known) and c.1209C>A (p.T374N; novel) mutations of the IDUA gene (IDUA). Patients with MPS-I have been reported to present with various clinical phenotypes and severities even if they have identical mutations of IDUA. The quite similar, unique phenotype in monozygotic twins suggests that not only IDUA mutation but also other genetic factors than IDUA markedly influence the clinical manifestations of MPS-I.
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