To systematically study somatic variants arising during development in the human brain across a spectrum of neurodegenerative disorders. In this study we developed a pipeline to identify somatic variants from exome sequencing data in 1461 diseased and control human brains. Eighty-eight percent of the DNA samples were extracted from the cerebellum. Identified somatic variants were validated by targeted amplicon sequencing and/or PyroMark® Q24. We observed somatic coding variants present in >10% of sampled cells in at least 1% of brains. The mutational signature of the detected variants showed a predominance of C>T variants most consistent with arising from DNA mismatch repair, occurred frequently in genes that are highly expressed within the central nervous system, and with a minimum somatic mutation rate of 4.25 × 10 per base pair per individual. These findings provide proof-of-principle that deleterious somatic variants can affect sizeable brain regions in at least 1% of the population, and thus have the potential to contribute to the pathogenesis of common neurodegenerative diseases.
Mutations in protein kinase Cgamma (PKCgamma) cause the neurodegenerative disease spinocerebellar ataxia type 14 (SCA14). In this study, expression of an extensive panel of known SCA14-associated PKCgamma mutations as fusion proteins in cell culture led to the consistent formation of cytoplasmic aggregates in response to purinoceptor stimulation. Aggregates co-stained with antibodies to phosphorylated PKCgamma and the early endosome marker EEA1 but failed to redistribute to the cell membrane under conditions of oxidative stress. These studies suggest that Purkinje cell damage in SCA14 may result from a reduction of PKCgamma activity due its aberrant sequestration in the early endosome compartment.
To discern presymptomatic changes in brain structure or function using advanced MRI in carriers of mutations predisposing to amyotrophic lateral sclerosis (ALS). T1-weighted, diffusion weighted and resting state functional MRI data were acquired at 3 T for 12 asymptomatic mutation carriers (psALS), 12 age-matched controls and affected patients with ALS. Cortical thickness analysis, voxel-based morphometry, volumetric and shape analyses of subcortical structures, tract-based spatial statistics of metrics derived from the diffusion tensor, and resting state functional connectivity (FC) analyses were performed. Grey matter cortical thickness and shape analysis revealed significant atrophy in patients with ALS (but not psALS) compared with controls in the right primary motor cortex and right caudate. Comparison of diffusion tensor metrics showed widespread fractional anisotropy and radial diffusivity differences in patients with ALS compared to controls and the psALS group, encompassing parts of the corpus callosum, corticospinal tracts and superior longitudinal fasciculus. While FC in the resting-state sensorimotor network was similar in psALS and controls, FC between the cerebellum and a network comprising the precuneus, cingulate & middle frontal lobe was significantly higher in psALS and affected ALS compared to controls. Rather than structural brain changes, increased FC may be among the earliest detectable brain abnormalities in asymptomatic carriers of ALS-causing gene mutations. With replication and significant refinement, this technique has potential in the future assessment of neuroprotective strategies.