Intersection of diverse neuronal genomes and neuropsychiatric disease: The Brain Somatic Mosaicism Network

Abstract

Neuropsychiatric disorders have a complex genetic architecture. Human genetic population-based studies have identified numerous heritable sequence and structural genomic variants associated with susceptibility to neuropsychiatric disease. However, these germline variants do not fully account for disease risk. During brain development, progenitor cells undergo billions of cell divisions to generate the ~80 billion neurons in the brain. The failure to accurately repair DNA damage arising during replication, transcription, and cellular metabolism amid this dramatic cellular expansion can lead to somatic mutations. Somatic mutations that alter subsets of neuronal transcriptomes and proteomes can, in turn, affect cell proliferation and survival and lead to neurodevelopmental disorders. The long life span of individual neurons and the direct relationship between neural circuits and behavior suggest that somatic mutations in small populations of neurons can significantly affect individual neurodevelopment. The Brain Somatic Mosaicism Network has been founded to study somatic mosaicism both in neurotypical human brains and in the context of complex neuropsychiatric disorders.

Fig. 1. An overview of approaches employed by the BSMN

The general approach of the BSMN is to identify mosaic variants in primary human brain tissue from large cohorts of neurotypical individuals and neuropsychiatric disease patients. The methods include bulk sequencing of tissues or sorted neurons (top), sequencing of single cells after whole-genome amplification (middle), or clonal expansion from single cells followed by bulk sequencing (bottom). Each method offers a trade-off between sensitivity and specificity.

Fig. 2. An example of brain somatic mosaicism that leads to a focal overgrowth condition

(A) Axial brain magnetic resonance imaging (MRI) of focal overgrowth of one hemisphere (arrows) from a 2-month-old child with intractable epilepsy and intellectual disability. MRI showed poor differentiation between the gray and white matter with dysplasia of the cortical gyri and sulci (arrows). (B) Brain mapping using high-resolution MRI or functional imaging such as positron emission tomography (PET), together with electrocorticography to fine-map specific epileptic foci, is followed by surgical resection of diseased brain tissue. (C) Histological analysis with hematoxylin/eosin showing characteristic balloon cells (arrows) consisting of large nuclei, distinct nucleoli, and glassy eosinophilic cytoplasm. (D) Immunostained section for phospho-S6 (green), as evidence of increased mTOR pathway activation. Arrows highlight large dysplastic cell showing strongest immunosignal. Scale bar, 50 μm. Bulk tissue sequencing showed somatic activating mutation in the MTOR gene c.6644C>T leading to p.S2215F in 15% of brain cells from the diseased hemisphere. After surgery, the patient showed clinical improvement.

Fig. 3. A potential strategy to determine functional consequences of mosaic variants

In utero electroporation (IUE) transfects a subpopulation of cortical neurons within a local area and will be combined with genome editing to generate mosaic mouse models for functional analysis. For example, a red fluorescent construct (CAG-TdTom) is shown labeling a transfected subset of neurons, shown in the context of a coronal brain section in which nuclei are stained blue with 4′,6-diamidino-2-phenylindole (DAPI). Scale bar, 500 μm.