Newly Discovered Regulatory Pathways in Fragile X

Paving the Way to Comprehensive Treatments

Recent advances in gene sequencing and molecular physiology have crossed paths in the lab of Leonard Kaczmarek, PhD at Yale, resulting in a novel therapeutic target for fragile X to treat hypersensitivity to sensory stimuli, including environmental sounds. Says Michael Tranfaglia, MD, Medical Director and Chief Scientific Officer of FRAXA, “We are excited about the potential of AUT00206 as a treatment for fragile X. With its novel mechanism of action, this compound has demonstrated its ability to rescue many disease-relevant phenotypes in animal models. AUT00206 has the potential to be a disease-modifying therapy for people with fragile X, and we are looking forward to clinical trials.”

Kaczmarek’s work builds on decades of research. Science is a long and winding road with unexpected turns. Experience leads you to believe you will see deer and rabbits along the way. One day you turn a corner to the pond where there was nothing but grass, rocks, and water yesterday. But this time there is a swan there, something extraordinary. Science is like that. Sort of. Only it relies on tangible results, accurate models, critical peer review, and delivers its impact through discussion, opening gateways to new discoveries.

Understanding How the Brain Understands: A Complex Network of Cells and Signals

The brain is an intricate, unusually well-regulated system, with many interconnected cells and structures. Complex networks of neurons process and transmit signals collected from sensory nerves throughout the body. Each neuron connects to many other neurons at focused areas called synapses. The neuron multitasks, extending transmitting presynapses, while accepting neurotransmitters, protein signaling molecules, at dendrite postsynapses. The finely controlled release of neurotransmitters is related to what neuroscientists call short-term plasticity (STP), and is thought to be essential to information processing, working memory and decision making.

Once a signal is collected and processed by the cell, the message is rapidly transmitted by electrochemical relays called action potentials along the axon, the wire-like extension radiating from the neuron cell body. Action potentials are electrical pulses propagated by tiny changes in the flow of chemical ions across the cell membrane, the boundary between the cell and its surroundings. Seizures are caused by excess or prolonged action potentials. The electrical potential of the neuron is created by sodium and potassium flux across the membrane, selectively driven by ion channels, proteins with pores that open or close depending on voltage, chemical gradients or the binding of molecules. The action potential ultimately signals the release of neurotransmitters at the presynaptic terminal, completing the cycle.

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Building blocks: Discovery of the Fragile X Gene

Fragile X syndrome (FXS) is caused by mutation of a gene on the X chromosome, FMR1, which leaves unable to produce its protein, FMRP. While FMR1 is in all cells of the body, and FMRP is found in many tissues, it is most heavily concentrated in the brain, which is why FXS impacts brain development and function so significantly.

Assembling Protein (or not): Fragile X Knockout Mice

Knockout mice are genetically modified, usually so that a single gene is silenced, in order to study the function of the protein it encodes. As early as 1994, knockout mice with FMR1 silenced became available to study the FMRP deficient brain. Initially, FMRP was recognized as an RNA binding protein. Since RNA provides a template for building proteins according to the DNA blueprint, FMRP binding blocks protein assembly. In fact, in mouse brains without FMRP, overall protein synthesis increases, and dendritic spines on the neuron become elongated and dense. These are major changes to the brain circuitry, a system that modifies connections required to think, learn and form memories.

Proteins Gone Wild and How to Tame Them

Neuroscientists looked even more carefully at proteins of the synapse, and found that metabotropic glutamate receptor 5 (mGluR5) becomes overactive in the absence of FMRP. FMRP appears to be part of a feedback loop that balances mGluR5 function. The ‘mGluR5 theory of Fragile X’ is a story of proteins gone wild, with mGluR5 triggering a rapid supply of excess proteins without the essential FMRP regulation, and is the basis of targeting drugs that inhibit mGluR5. The mGluR5 therapeutic approach was meticulously validated in cell culture and animal models, but did not perform well in clinical trials. It turns out there is more to the FMRP story, although by a different path.

Genomics, Proteomics, and Potassium Channels

It turns out that FMRP plays a significant role in the regulation of potassium channels. As gene sequencing technology has evolved and become less expensive, many more patients have undergone genetic analysis. At the same time proteomics, protein analysis, has advanced to accommodate high throughput experiments. A broader understanding for the role of potassium channels in FXS has come from studying spontaneous mutations in individual patients and from basic neuroscience, particularly the function of potassium channels in the pre-synaptic part of the neuron, where neurotransmitters are released.

In 2006, Silvain Briault’s group in France reported an autistic boy with a reduction in large potassium channel (BK) function caused by a genetic defect. Remarkably, they could restore function of remaining BK channel in his cells using a channel opener, an investigational drug for stroke. In 2008, a paper was published by Peter Vanderklish’s group at Scripps describing overall protein changes in the neuronal synapse of FMR1 knockout mice. They found a 50% reduction in a subunit of the very same BK channel. Subsequently, Briault, and Jacques Pichon, PhD have been able to experimentally rescue some sensory functions in fragile X mice using drugs called BK channel openers.

In 2010 a group at the Emory University School of Medicine published the results of a large sequencing study of nearly 1000 men with developmental disabilities, not FXS, looking for alternative genetic variants in the FMR1 gene. Several novel mutations in the FMR1 gene were found, which revealed a completely different purpose for the FMRP protein. One particularly interesting case in a patient with acute seizures was subsequently generated by Steve Warren and FRAXA Fellow Josh Suhl in knockout mice. While the man’s new FMRP protein could bind RNA, it could not bind BK channels!

FMRP and the Big Potassium Channel: Novel Pathway and a New Drug Target for FXS

Evidence was building in the case for FMRP controlling neuronal excitability, and experiment by experiment, the relationship between FMRP and potassium channels has become clearer. Vitaly Klyachko and Pan-Yue Deng of Washington University have shown that FMRP directly controls BK channels. In FXS and with the loss of FMRP, action potentials are prolonged resulting in increased release of neurotransmitters. Klyachko believes that there is a strong connection between the loss of FMRP and changes in neurotransmitter release, which triggers neural mechanisms that play a large part in working memory and decision making.

Therefore, there is a twofold strike against people with fragile X: they have less of the BK channel and less of the signal to intensify the activity of the residual BK channels as a way of toning down hyperexcitability. This neurological hyperexcitability has been linked to intellectual disabilities including FXS. Kaczmarek believes this could be a significant basis of behavioral symptoms associated with “sensory overload” and interfere with attention, learning, language development and social interactions and are likely due to changes in synaptic connections in the brain circuitry.

Looking forward, channel opener drugs could rescue some symptoms of FXS in humans. Through a connection to the UK-based Autifony Therapeutics, Kaczmarek has started tests in the fragile X mouse model using compounds which can selectively inhibit or activate subsets of potassium channels. And in July, 2017 the FDA granted AUT00206 an Orphan Drug Designation for the treatment of fragile X syndrome. Orphan drug status conveys incentives to encourage companies to develop drugs for rare diseases. Dr Charles Large, CEO of Autifony, said: “We believe that our promising findings in the preclinical model of fragile X support investigation of AUT00206 in clinical trials, which we plan to initiate as rapidly as possible in 2018.”

And so we turn another corner in the treatment of fragile X.

THE AUTHOR
J. Dora Levin, PhD

J. Dora Levin, PhD is a science writer who is passionate about explaining complex scientific ideas and making them accessible to non-scientists. When speaking about this writing opportunity she said “I am truly excited about volunteering for FRAXA, an organization that consciously streamlines research and clinical trials through strategic planning and creative resourcing.”