by Sneha Shah, PhD

In 1991, the gene responsible for the Fragile X syndrome, FMR1, was first identified. Since then, important advances have been made in understanding the genetic inheritance of this gene, its regulation and the potential roles of its protein product, FMRP.

Fragile X research has greatly benefited from animal models that have a deletion of the Fmr1 gene: Fmr1 knockout (KO) animals. Mouse models of Fragile X have been extremely useful in guiding research efforts, but they may not recapitulate the heterogeneity of symptoms and severity that are manifest in humans.

But mice may not recapitulate the range of symptoms and severity seen in humans. Several therapies based on mouse models have been developed, which have not been as effective as hoped when treatment is applied to human patients. Additional approaches are needed.

The advent of human cell cultures of Fragile X syndrome patients is promising for basic research, drug discovery and pre-clinical validation. Human pluripotent stem cells (iPS cells) derived from Fragile X patients contain the CGG triplet repeat expansion that cause Fragile X syndrome in humans. Thus they can be used to identify features of human Fragile X syndrome that can be recapitulated in-vitro. These cells have been used to study many aspects of the syndrome, such as epigenetic regulation of FMR1 gene silencing, defects in gene expression, neuronal differentiation and synaptic plasticity.

In our studies, we aim to harness the power of patient-derived stem cells to generate excitatory neurons that would mimic the molecular profile of neurons in Fragile X syndrome patients. This provides us with an excellent system that can give us a meaningful snapshot of changes that would occur in humans.

Opposite Results in Mouse and Human Cells

Interestingly, when we compared our profiles obtained from the patient-derived neurons to that of our Fmr1 KO mouse models, we found that most genes show similar RNA expression changes across species. But a subset of genes showed a substantial increase in mRNA expression, opposite to the decrease seen in the mouse data. Among these are a subset of important FMRP RNA targets that mediate synaptic function, which show opposite effects in mouse and human Fragile X models. The differences in molecular patterns between our mouse and human data suggest the need for revisiting the sole reliance of animal models for drug discovery. Also, our studies reveal new targets that could be explored with relation to Fragile X syndrome.

By examining gene expression in the most rigorous and detailed way, we hope to have a complete analysis of human Fragile X patient derived neurons to identify potential therapeutic candidates and serve as a model for preclinical trials.