Altered PLP1 splicing causes hypomyelination of early myelinating structures.

OBJECTIVE: The objective of this study was to investigate the genetic etiology of the X-linked disorder "Hypomyelination of Early Myelinating Structures" (HEMS). METHODS: We included 16 patients from 10 families diagnosed with HEMS by brain MRI criteria. Exome sequencing was used to search for causal mutations. In silico analysis of effects of the mutations on splicing and RNA folding was performed. In vitro gene splicing was examined in RNA from patients' fibroblasts and an immortalized immature oligodendrocyte cell line after transfection with mutant minigene splicing constructs. RESULTS: All patients had unusual hemizygous mutations of PLP1 located in exon 3B (one deletion, one missense and two silent), which is spliced out in isoform DM20, or in intron 3 (five mutations). The deletion led to truncation of PLP1, but not DM20. Four mutations were predicted to affect PLP1/DM20 alternative splicing by creating exonic splicing silencer motifs or new splice donor sites or by affecting the local RNA structure of the PLP1 splice donor site. Four deep intronic mutations were predicted to destabilize a long-distance interaction structure in the secondary PLP1 RNA fragment involved in regulating PLP1/DM20 alternative splicing. Splicing studies in fibroblasts and transfected cells confirmed a decreased PLP1/DM20 ratio. INTERPRETATION: Brain structures that normally myelinate early are poorly myelinated in HEMS, while they are the best myelinated structures in Pelizaeus-Merzbacher disease, also caused by PLP1 alterations. Our data extend the phenotypic spectrum of PLP1-related disorders indicating that normal PLP1/DM20 alternative splicing is essential for early myelination and support the need to include intron 3 in diagnostic sequencing.

Novel hypomyelinating leukoencephalopathy affecting early myelinating structures.

OBJECTIVE: To describe 4 children with a novel hypomyelinating leukoencephalopathy, defined by a distinct pattern of magnetic resonance imaging (MRI) abnormalities. DESIGN: In our ongoing study on leukoencephalopathies of unknown origin, MRIs of patients are rated in a standardized manner. Patients are grouped according to their MRI abnormalities. The clinical and laboratory data are retrospectively reviewed. SUBJECTS: The MRIs of approximately 3000 patients with a leukoencephalopathy of unknown origin were initially evaluated. Four unrelated patients (all male, aged 1.8-7.4 years) displayed similar MRI alterations. RESULTS: Patients displayed mild T2 hyperintensity of the medulla oblongata, caudal part of the pons, hilus of the dentate nucleus, peridentate white matter, subcortical cerebellar white matter, optic radiation, and frontoparietal periventricular white matter. The posterior limb of the internal capsule showed alternating T2 hyperintense-hypointense-hyperintense stripes in 3 patients. The T1-weighted images showed hyperintensity, isointensity, or mild hypointensity of T2 hyperintense structures. The thalamus had a neonatal appearance with a mildly hyperintense signal except for a darker lateral part. Clinically, patients presented with nystagmus between ages 6 and 20 months. Over time, cerebellar ataxia and mild spasticity developed. All achieved unsupported walking. Cognition and language were normal. Known causes of hypomyelination were excluded. CONCLUSIONS: The patients share a striking pattern of MRI abnormalities and have a similar clinical picture, suggesting that they have the same disorder. The hypomyelination in this disorder specifically occurs in structures that normally myelinate early. We hypothesize that the disease is caused by a defect in a gene involved in early myelination.