New screening approach reveals new regulators of microcephaly

EDITOR’S SELECTION IN GENETICS & GENOMICS

The paper
C. Esk et al., “Screening of Human Tissue Identifies a Regulator of ER Secretion as a Determinant of Brain Size.” science370, 935-41, 2020.

ÖRganoids can be an invaluable tool for studying human diseases. However, they are often difficult to work with – especially when it comes to evaluating multiple candidate genes underlying a particular condition.

Over eight years of work, Jürgen Knoblich and colleagues from the Institute for Molecular Biotechnology in Vienna have found a way to circumvent this problem. Their approach combines brain organoids with two other technologies – CRISPR-Cas9 to turn off specific genes and DNA barcodes to track individual cells and their offspring.

The researchers recently tested their approach, called CRISPR-LIneage Tracing at Cellular Disintegration in Heterogeneous Tissue (CRISPR-LIGHT), in a screen for genes associated with microcephaly, a condition where a baby’s head is smaller than is expected. The team found 13 genes with obvious roles in organ growth – genes that would show up in a 2-D screen of cells, as well as the team’s 3-D organoid system, Knoblich says – as well as another 12 that did not affect phenotype until later development of the organoid. The fact that these 12 genes didn’t have any visible effects until the cells began to differentiate and the organoid took the form of a small brain suggests that these genes have tissue-specific functions, Knoblich says.

Of these 12, the researchers focused on IER3IP1, a gene that produces a protein that is involved in packaging other proteins for extracellular export. “Nobody would have ever thought that [gene] had something to do with microcephaly, ”says Knoblich. The team showed that IER3IP1-Mutated organoids are unusually small because their neural progenitor cells differentiate into neurons before the organoids properly expand. The IER3IP1 peptide appears to block premature differentiation by promoting the secretion of extracellular matrix proteins that support tissue integrity and neuronal progenitor proliferation.

“It’s a really exciting piece of work that is eagerly awaited for those of us working on brain organoids and brain disorders,” said Yun Li, a developmental neuroscientist at Toronto Hospital for Sick Children who was not involved in the work worked with Knoblich’s group a few years ago. The characterization of the team from IER3IP1The function was “Very well done,” adds Li. “It would be really great to know how you are following a lot of the other hits in the future and how your team and others are driving this system forward.”

According to Knoblich, the team now plans to incorporate single-cell sequencing technologies into its approach and investigate other disorders. “In principle, this can be applied to any disease, any cell type in the brain. . . every organoid model, ”he says. “It’s very versatile.”



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