Expanding the Clinical and Mutational Spectrum of the -Related Hypomyelination of Early Myelinated Structures (HEMS).

The gene, located on chromosome Xq22, encodes the proteolipid protein 1 and its isoform DM20. Mutations in cause a spectrum of white matter disorders of variable severity. Here we report on four additional HEMS patients from three families harboring three novel mutations in exon 3B detected by targeted next-generation sequencing. Patients experienced psychomotor delay or nystagmus in the first year of age and then developed ataxic-spastic or ataxic syndrome, compatible with a phenotype of intermediate severity in the spectrum of -related disorders. Regression occurred at the beginning of the third decade of the eldest patient. Extrapyramidal involvement was rarely observed. Brain MRI confirmed the involvement of structures that physiologically myelinate early, although the pattern of abnormalities may differ depending on the age at which the study is performed. These new cases contribute to expanding the phenotypic and genotypic spectrum of HEMS. Additional studies, especially enriched by systematic functional evaluations and long-term follow-up, are welcome to better delineate the natural history of this rare hypomyelinating leukodystrophy.