What rare mutations can teach us about brain development and disease
In patients with unknown disorders, novel techniques such as next-generation sequencing can be used to identify unexplored genetic mutations. This way, mutations in the gene CHMP1A were found to be involved in a specific neurological disorder. A recent study published in Cell Reports by researchers from the Harvard Medical School and the Swammerdam Institute for Life Sciences at the University of Amsterdam, further examined the function of CHMP1A in brain development.
In previous research, it was found that loss of function mutations in the gene CHMP1A are a cause of a developmental brain disorder in three separate families (Mochida et al, 2012). Parts of the brain, such as the cerebellum and pons are severely reduced in size. Also the cerebral cortex is smaller, which is called microcephaly. A study led by professor Christopher Walsh, at Harvard Medical School, in collaboration with the lab of Frank Jacobs, at the University of Amsterdam, showed further proof that CHMP1A is required for normal development of the brain.
Insights in microcephaly
In microcephaly, a medical condition in which the brain does not develop properly, resulting in a smaller than normal head, the phenotype is often quite similar: the neocortex remains too small, because the amount of brain cells (neurons) is reduced in this structure. There are some commonly observed features on the molecular level as well. A disbalance in cell processes that regulate cell division and differentiation, often are at the root of the disorder. The underlying mechanisms of this disbalance can be quite different per individual though. For instance, microcephaly can be caused by a defect in proteins that orchestrate cell division directly (like proteins essential for mitosis, a process which separates a dividing cell into two cells) or by a defect in a protein that is involved in maintaining stem cell identity.
Mutations in some genes occur more frequently, and subsequently much is known about the function of those genes in microcephaly. Rare mutations, like in the CHMP1A gene, have not been studied in detail or have not even been identified yet. By examining the effect of various mutations related to microcephaly, fundamental insights in organisation of the human brain are gained. In this study it became clear how CHMP1A works and this information could be used in related new research. Also, the relatives of affected individuals often are in the dark for the cause of a rare neurological disorder, for which in part answers have been found to.
At the Swammerdam Institute for Life Sciences, Frank Jacobs and Gerrald Lodewijk investigated the function of CHMP1A using human brain organoids, often termed ‘mini-brains'. These organoids are used as a model for the developing human brain. They are derived from human embryonic stem cells, which are capable to form any cell type in the human body. Using defined cell culture conditions, the organoids can be pushed towards creating cerebral cortex brain structures – the region primarily affected in microcephaly.
With CRISPR/Cas9, a gene editing tool, researchers at the Harvard Medical School introduced a CHMP1A loss of function mutation in stem cells, similar as found in patients with the neurological disorder. Jacobs and Lodewijk used these mutated cells to grow organoids and compare their growth and development to organoids derived from control stem cells. The analysis indicated that the mutant organoids were differentiating too fast: they generated neurons earlier than the control organoids, while the amount of stem cells seemed decreased.
Sonic Hedgehog protein
In experiments from the collaborating researchers, it became apparent that CHMP1A is necessary for secretion of the protein Sonic Hedgehog. From other studies it is known that it is important for self-renewal and proliferation of neuronal stem cells. This knowledge was applied to the organoid culture, by adding a compound that mimics the effect of sonic hedgehog protein. This leads to activation of the sonic hedgehog signalling cascade, and its downstream effects.
As this is known to boost neuronal stem cell self-renewal, it was interesting to assess if there were any differences in response to the compound between the control and mutant mini-brains. While both groups showed similar levels of sonic hedgehog signalling activity after treatment, only in the control organoids the expression of many genes involved in cell division went up. In the CHMP1A mutant organoids this effect was much weaker, again pointing at a problem with maintenance of the neuronal stem cells. In the end, this means that CHMP1A is required for normal development of the brain.
Coulter et al., ‘The ESCRT-III Protein CHMP1A Mediates Secretion of Sonic Hedgehog on a Distinctive Subtype of Extracellular Vesicles’ in Cell Reports, 24 July 2018, Doi: 10.1016/j.celrep.2018.06.100