The protein G9a participates in silencing maternal copies of PWS genes. With expertise in G9a targeting, Dr. Al-Sady aims to define and specifically block the mechanisms bringing G9a to PWS genes without off-target effects.
On the molecular level, Prader-Willi Syndrome (PWS) is characterized by the inactivity of a cluster of genes on chromosome 15 in the patient. This is because the chromosome inherited from the father is missing the DNA sequence encoding for those genes. Normally, the mother’s chromosome should be able to provide this missing information, at least partially. Unusually, this region of chromosome 15 is always silent on the maternal copy. This means that in PWS there is no detectable activity for those genes, even though the information is present on the maternal chromosome. One approach towards a PWS cure is to find ways to activate the genes from this silenced maternal chromosome.
A key protein called G9a “writes” gene-silencing information and is implicated in this effect. It is thought that reversing the action of G9a can lead to restoring activity of the affected genes. One proposed avenue to do so is to simply fully inactivate G9a’s “writing” activity. However, G9a has major roles in normal cognitive development, behavior, and when reduced early in development, causes phenotypes similar to PWS. Therefore, we believe that eliminating the function of the writer in the whole organism is not a useful approach, instead we want to block its ability to silence specifically the chromosomal portion involved in PWS. If we can do this, we may have an avenue for therapy that is both effective, but also safe, preserving what G9a normally does in brain development.
My lab is uniquely positioned to address this problem as we have biochemically dissected how G9a operates on the chromosome, and how it forms complexes with itself, and a “sister” writer protein called GLP. We know that this pair of writers is recruited to specific chromosomal sites via intermediary factors. A recent breakthrough in PWS biology showed that a small gene seems to recruit this writer to the maternal chromosomal locus switched off in PWS patients. We will apply our expertise to understand how exactly G9a and or GLP, are recruited. It is our aim to interrupt this recruitment, relieving silencing and allowing expression of genes on the silent maternal chromosome. This will restore the genes missing in PWS patients, without risking devasting mis regulation in normal brain development and behavior. Alternatively, if we find that a specific G9a-GLP pair is involved in PWS, we aim to interrupt that pairing rather than block the function of the whole family of writers. This is significant as the field has lacked biochemical insight into the inner working of G9a and GLP and the tools to precisely measure which writer pair is involved in a given process.
We expect over the time of this grant to understand what surfaces are critical to attract G9a and or GLP to the part of the chromosome silenced in PWS. Over the next 2-5 years we will validate whether these surfaces are sufficient to restore normal gene expression in PWS derived cells. In the same timeframe, we will work our colleagues at UCSF to develop small molecules that can block G9a and or GLP’s recruitment to the affected part of chromosome 15. In 5-10 years, we will be testing pilot molecules in animal models.