Researchers at MIT’s Picower Institute for Learning and Memory have discovered that different mutations in the gene MECP2 cause various abnormalities in Rett syndrome, challenging the notion that the disorder arises from a general gene dysfunction. The study found that each mutation needed a unique treatment approach to correct the specific issues they caused. “Individual mutations matter,” stated Mriganka Sur, senior author of the study published in Nature Communications.<\/p>\n
The study utilized advanced 3D human brain tissue cultures, known as “organoids” or “minibrains,” derived from patient-donated skin or blood cells, to model the specific effects of each mutation. Lead author Tatsuya Osaki highlighted that these organoids provided insights into mutation-specific consequences that previous studies did not reveal, particularly regarding how different mutations affected cell types and their interactions.<\/p>\n
There are over 800 mutations in MECP2 linked to Rett syndrome, but eight mutations account for a significant portion of cases. Sur and Osaki focused on the R306C mutation, which represents 7-8% of cases, and the rarer, more severe V247X mutation. In organoids cultured for three months, each mutation resulted in some common and some distinct effects compared to non-mutated controls.<\/p>\n
Using \u201cthree-photon\u201d microscopes, the researchers observed that V247X organoids displayed structural differences, such as larger size and varied layer thickness, whereas R306C organoids closely resembled their controls. Both mutations led to less-developed axon projections in neurons. The team also noted reduced spiking activity and synchronicity between neurons in both mutations compared to controls.<\/p>\n
Further examination revealed divergence in neural network properties. The R306C mutation decreased “small-world propensity” (SWP), while V247X increased it, indicating altered information processing network development. Collaborating with Charles Nelson’s team at Boston Children’s Hospital, they found similar SWP alterations in EEG readings from children with different Rett mutations.<\/p>\n
By labeling neurons with different colors, the researchers saw significant connectivity differences in V247X organoids. Each mutation caused specific structural, activity, and connectivity changes in organoids. The team investigated gene expression differences, finding that R306C organoids overexpressed the gene HDAC2, while V247X organoids had reduced expression of GABA receptor genes and astrocyte cell function defects.<\/p>\n
Both mutations disrupted molecular pathways critical for neuron circuit connections. In response, researchers treated organoids with drugs: an HDAC2 inhibitor normalized activity in R306C organoids, while the GABA agonist baclofen restored SWP in V247X organoids. Osaki mentioned these drugs are well-studied in other diseases and may be repurposed for Rett syndrome.<\/p>\n
With the organoid platform established for analyzing mutation consequences and testing treatments, the team plans to study four more mutations, comparing them to a standardized control organoid. Additional contributors to the paper include Chloe Delepine, Yuma Osako, Devorah Kranz, April Levin, and Michela Fagiolini. The research was supported by the National Institutes of Health, a MURI grant, The Freedom Together Foundation, and the Simons Foundation.<\/p>\n