A new tissue screening assay for human cerebral organoids identified 25 additional candidate genes for microcephaly, nearly doubling the number of currently known genes linked to the rare neurological condition. Some of these newly identified genes were associated with pathways previously unconnected to the disease, suggesting that there are yet several unexplored processes that control the growth of the human brain.

Currently, 27 genes are implicated with microcephaly - characterized by abnormal and reduced brain size - and clinical genetic sequencing data has suggested 100 additional genes may also drive the disease. However, these genes are not fully characterized because of the challenge of finding appropriate model systems for microcephaly.

Human cerebral organoids can recapitulate the disease better than cell cultures or mouse models, but to screen for microcephaly genes in three-dimensional tissue structures remains difficult. Now, the authors developed a human brain tissue loss-of-function assay by combining CRISPR/Cas9 genetic screening with a cell barcoding technique that traces cell lineages. The loss-of-function assay identifies candidate genes that could be investigated for their impact on cerebral organoids after individual gene deletion.

Using the new "CRISPR-LICHT" technology, the researchers revealed 25 genes in a human dorsal forebrain organoid associated with both known and previously unknown microcephaly-driving pathways. Among these pathways, the researchers found that regulation of extracellular matrix protein secretion by IER3IP1, a protein in the endoplasmic reticulum, played an important role in tissue integrity and brain size.

The authors say this technique could potentially be applied to other organoids, allowing rapid testing of candidate genes for a range of human diseases.

https://science.sciencemag.org/content/early/2020/10/28/science.abb5390