CSF1R-related leukoencephalopathy: A major player in primary microgliopathies

Abstract

Since the discovery of CSF1R gene mutations in families with hereditary diffuse leukoencephalopathy with spheroids in 2012, more than 70 different mutations have been identified around the world. Through the analyses of mutation carriers, CSF1R-related leukoencephalopathy has been distinctly characterized clinically, radiologically, and pathologically. Typically, patients present with frontotemporal dementia-like phenotype in their 40s-50s, accompanied by motor symptoms, including pyramidal and extrapyramidal signs. Women tend to develop the clinical symptoms at a younger age than men. On brain imaging, in addition to white matter abnormalities, thinning of the corpus callosum, diffusion-restricted lesions in the white matter, and brain calcifications are hallmarks. Primary axonopathy followed by demyelination was suggested by pathology. Haploinsufficiency of colony-stimulating factor-1 receptor (CSF1R) is evident in a patient with a frameshift mutation, facilitating the establishment of Csf1r haploinsufficient mouse model. These mice develop clinical, radiologic, and pathologic phenotypes consistent with those of human patients with CSF1R mutations. In vitro, perturbation of CSF1R signaling is shown in cultured cells expressing mutant CSF1R. However, the underlying mechanisms by which CSF1R mutations selectively lead to white matter degeneration remains to be elucidated. Given that CSF1R mainly expresses in microglia, CSF1R-related leukoencephalopathy is representative of primary microgliopathies, of which microglia have a pivotal and primary role in pathogenesis. In this review, we address the current knowledge of CSF1R-related leukoencephalopathy and discuss the putative pathophysiology, with a focus on microglia, as well as future research directions.

Figure 1. Changing nomenclature with a growing understanding of pathology and genetics

We currently have at least 3 different leukoencephalopathies from this adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP) axis: colony-stimulating factor-1 receptor (CSF1R)–related leukoencephalopathy, alanyl-transfer RNA synthetase 2 (AARS2)–related leukoencephalopathy, and CSF1R/AARS2-negative ALSP. It is controversial whether AARS2-related leukoencephalopathy can be classified under ALSP. HDLS = hereditary diffuse leukoencephalopathy with spheroids; POLD = pigmentary orthochromatic leukodystrophy.

Figure 2. World distribution of colony-stimulating factor-1 receptor (CSF1R) mutations and CSF1R gene/protein diagram with reported mutations

(A) World distribution of CSF1R mutations. The countries in which the mutation was reported are shown in green. This world map was created using mapchart (http://mapchart.net/). (B) Diagram of CSF1R gene and protein with reported mutations. All mutations are found within the tyrosine kinase domain (TKD) of CSF1R except for a nonsense mutation, p.Y540*, and 2 frameshift mutations, p.T567fs*44 and p.S688Efs*13, shown in red. These are located in the intervening sequence between transmembrane domain (TM) and juxtamembrane domain (JMD) and in the kinase insert domain (KID). ^aThese 2 variants, located outside the TKD, have been reported recently in Chinese patients. One (p.G17C) is in the signal peptide sequence of CSF1R and the other (p.F971Sfs*7) is at the C-terminal sequence. The former variant does not change the TKD, but might fail to express CSF1R on the cell surface if it influences the signal peptide function, resulting in loss of CSF1R function. The latter induces subtle changes in a couple of the last amino acids of CSF1R, but the TKD remains unchanged. This has been reported as a rare variant in an East Asian population (rs766047383; allele frequency is 0.0004641 based on The Genome Aggregation database, gnomad.broadinstitute.org/). Therefore, the pathogenicity of these variants remains to be determined. ^bThese 5 mutations were identified in a mixed cohort from the United Kingdom, Greece, and Ireland. Ig = immunoglobulin-like domain; UTR = untranslated region.

Figure 3. Brain MRI/CT findings of colony-stimulating factor-1 receptor (CSF1R)-related leukoencephalopathy

(A–D, F, H) A 44-year-old woman with CSF1R p.G589R. (E) A 27-year-old woman with CSF1R c.2442+5 G > A. (G) A 31-year-old woman with CSF1R p.A652P. All of these patients have been reported previously. (A, B), Bilateral diffuse white matter hyperintensity with pyramidal tract involvement (arrows in A), cortical atrophy, and enlarged lateral ventricles on fluid-attenuated inversion recovery MRI. (C) Longitudinal pyramidal tract involvement (arrows) on coronal T2-weighted image. (D) Thinning of the corpus callosum with hyperintensity on sagittal fluid-attenuated inversion recovery image. (E) Hyperintensity lesions in the subcortical white matter on diffusion-weighted image. (F) Small calcifications located bilaterally near the anterior horns of the lateral ventricles on brain CT image. (G) Calcifications in parietal subcortical white matter. (H) Stepping stone appearance of calcifications (arrows) in the frontal pericallosal region on sagittal CT image.

Figure 4. Pathologic findings of colony-stimulating factor-1 receptor (CSF1R)-related leukoencephalopathy

(A–H) A 78-year-old man with CSF1R p.M875T who was a member of a previously reported family (VA-27 in reference 2).^,e60 (doi.org/10.5061/dryad.498j63f) At 71 years of age, he developed cognitive impairment followed by personality and behavior change, depression, executive dysfunction, apraxia, parkinsonism, and pyramidal weakness. He died with 7 years disease duration. (A) Luxol fast blue stain shows severe myelinated fiber loss in the superior frontal and cingulate white matter, whereas the U-fibers are relatively spared. Note the thinning of the corpus callosum (arrow). (B) The axonal spheroids in the affected white matter are stained with amyloid precursor protein (APP). (C) Numerous axonal spheroids (arrows) are seen within the frontal white matter (hematoxylin & eosin). (D) CD68-immunopositive macrophages in the frontal white matter. (E, F) An axonal spheroid in the white matter depicted by phosphorylated neurofilament (SMI31) (E) and APP (F). (G) A bizarre astrocyte in the white matter (αB-crystallin). (H) A ballooned neuron in the superior frontal cortex (αB-crystallin). (I, J) A 55-year-old woman with autopsy-confirmed adult-onset leukodystrophy with neuroaxonal spheroids and pigmented glia, but genetic testing was not performed because DNA was unavailable. (I) Note the small calcified lesion (arrow) located in the pericallosal region. An arrowhead indicates the paper-like atrophy of the corpus callosum. (J) An enlarged image of the calcification. Bars in A, B, and I = 5 mm; C and D = 100 μm; E, F, G, and H = 50 μm; and J = 400 μm.

Figure 5. Microglia-oriented hypothesis for colony-stimulating factor-1 receptor (CSF1R)-related leukoencephalopathy

Based on the haploinsufficiency or loss of function of mutant CSF1R, it is thought that microglia possess a half amount of functional CSF1R in patients with CSF1R-related leukoencephalopathy. This may influence not only microglial physiologic function in adult brains, but also microglial development in fetal brains since embryonic microglial development and maturation are dependent on CSF1R. Therefore, the character of microglia in patients may be somewhat different from individuals with a full amount of functional CSF1R, even at the time of birth. In adult brains, aberrant microglia may have a pivotal role in the white matter degeneration, which is characterized by primary axonopathy (spheroid formation shown in brown) and following demyelination (shown as dot lines surrounding the neuronal axon). Ballooned neurons are frequently found in the overlying cortex. Given that patients develop clinical symptom in adult, a half amount of functional CSF1R is considered to be sufficient for surviving to adulthood; however, white matter degeneration and corpus callosum atrophy could precede symptom onset. Importantly, brain calcification can be observed at birth, implying that the calcifications are independent of white matter degeneration or clinical symptoms.