Genetic analysis of inherited leukodystrophies: genotype-phenotype correlations in the CSF1R gene

Abstract

Importance: The leukodystrophies comprise a clinically and genetically heterogeneous group of progressive hereditary neurological disorders mainly affecting the myelin in the central nervous system. Their onset is variable from childhood to adulthood and presentation can be with a variety of clinical features that include mainly for adult-onset cases cognitive decline, seizures, parkinsonism, muscle weakness, neuropathy, spastic paraplegia, personality/behavioral problems, and dystonia. Recently, Rademakers and colleagues identified mutations in the CSF1R gene as the cause of hereditary diffuse leukoencephalopathy with spheroids (HDLS), offering the possibility for an in-life diagnosis. The detection of mutations in this gene in cases diagnosed with different clinical entities further demonstrated the difficulties in the clinical diagnosis of HDLS.

Objective: To better understand the genetic role of mutations in this gene, we sequenced a large cohort of adult-onset leukodystrophy cases.

Design: Whole-exome sequencing and follow up-screening by Sanger sequencing.

Setting: Collaborative study between the Institute of Neurology, University College London and the Inserm, Paris, France.

Participants: A total of 114 probands, mostly European patients, with a diagnosis of adult-onset leukodystrophy or atypical cases that could fit within a picture of leukodystrophy. These included 3 extended families within the spectrum of leukodystrophy phenotype.

Interventions: Whole-exome sequencing in a family and Sanger sequencing of CSF1R.

Main outcomes and measures: Mutations in CSF1R.

Results: We identified 12 probands with mutations in CSF1R. The clinical diagnoses given to these patients included dementia with spastic paraplegia, corticobasal degeneration syndrome, and stroke disorders. Our study shows that CSF1R mutations are responsible for a significant proportion of clinically and pathologically proven HDLS.

Conclusions and relevance: These results give an indication of the frequency of CSF1R mutations in a European leukodystrophy series and expand the phenotypic spectrum of disorders that should be screened for this gene.

Figure 1. Pedigrees of Families F375/174, F662, and F232

Sex is masked for confidentiality purposes. Affected individuals are represented with black, filled diamonds; unaffected family members, white diamonds. Exome sequencing was performed in individuals 001, 006, and 007 of family F375/174. *Individuals tested for the respective mutation found in CSF1R. †Individual in whom brain biopsy was performed.

Figure 2. Mutations Identified in CSF1R Associated With Hereditary Diffuse Leukoencephalopathy With Spheroids

A, The CSF1R gene structure. B, Exons 12 to 22, which code the protein kinase domain of the protein, with all the mutations identified. The bottom of part B shows mutations described by Rademakers et al, and the top of part B shows mutations described in the present study. Red arrows represent novel mutations described herein. Black arrows represent previously reported mutations.

Figure 3. Axonal Spheroids Stained With an Antibody to Phosphorylated Neurofilaments in Case 4 With Hereditary Diffuse Leukoencephalopathy With Spheroids

Where the CSF1R p.R777W mutation was identified (A). Scattered macrophages (B), often with pigment-laden cytoplasm (C), were also characteristic. For comparison, axonal spheroids from the spinal cord of a case with PLA2G6 mutation (D) and the external globus pallidus from a case with PANK2 mutation (E) are also given. The bar in part A represents 45 μm for parts A, B, and D; 22.5 μm in part E; and 16 μm in part C. A, D, and E, RT97; B, C68; and C, Luxol fast blue, cresyl violet.

Figure 4. Brain Magnetic Resonance Imaging of Patients With CSF1R Mutations From Families F375/174 and F662

A and B, Axial T2-weighted imaging for patient F375/174-008 (disease duration of 2 years) showing moderate frontal atrophy and discrete periventricular hyperintensity (arrows). C and D, Axial fluid-attenuated inversion recovery (C) and coronal T2-weighted (D) imaging for patient F662-008 (disease duration of 3 years) showing severe white matter hyperintensity and callosal atrophy. E, Sagittal T1-weighted imaging for patient F662-008.