Single cell RNA-seq of seizure-affected brain tissue with mosaic copy number gain of chromosome 1q

Somatic mosaicism is a known cause of neurological disorders, including developmental brain malformations and epilepsy. Brain mosaic copy number gain of chromosome 1q is associated with cortical malformations, early-onset epilepsy, and developmental delay. Pathogenic brain mosaicism is traditionally attributed to post-zygotic genetic alterations arising in a neural progenitor cell during fetal development. However, in our cohort, in at least five of six patients with brain mosaic copy number gain of chromosome 1q, the alteration occurred pre-conception. We observed a third non-constitutive, but parentally-derived, haplotype for chromosome 1q in patient brain tissue, demonstrating that the copy number alteration occurred in a gamete pre-conception. The altered cells had no representation in parental buccal or proband blood or buccal samples, but were prominently observed in proband brain tissue, suggesting the copy number gain was lost in most cell lineages during embryonic development. Single-nuclei genotyping coupled with gene expression profiling revealed a strong enrichment of the chromosome 1q gain in astrocytes, which correlated with the unusual finding of hyaline astrocytic inclusions in all six cases. We used surgically resected brain tissue from children with epilepsy to dissociate single nuclei (from frozen tissue) and prepare for sequencing using 10x Genomics single cell 3' v3.1. In most tissues, we previously identified a chr1q gain at mosaic levels; some tissues do not have the chr1q gain and are from adjacent brain sections.