Anyone who’s passed basic biology knows that we get one copy of a gene from our mother, a second from our father. But few people realize that not all of these genes end up being treated equally. Imprinted genes are expressed from only the maternal or paternal allele, rather than both. And, when this process goes wrong, it can actually lead to diseases. Now, researchers have identified a possible way to treat imprinting errors. In the brain, Ube3a is an imprinted gene; only the maternal allele is expressed, even if it is mutated and the paternal allele is normal. This is the case in Angelman syndrome, a severe neurodevelopmental disorder caused by mutation or deletion of the maternal allele of Ube3a. Ube3a is imprinted only in the brain, though; in other tissues, the paternal allele is expressed along with the maternal one.
This led Benjamin Philpot and his colleagues at UNC Chapel Hill to wonder: wouldn’t it be great if we could get the normal, paternal version of Ube3a to work in the brain — to unsilence it? Maybe this could help kids with Angelman syndrome.
To find a drug that might allow the paternal copy of Ube3a to be expressed, they first made mice in which only the paternal copy of the gene was linked to the gene for yellow fluorescent protein. They then isolated cortical neurons from these mice and exposed the neurons to a variety of chemicals. If any of these chemicals caused the cells to glow yellow, that meant that they allowed expression of the paternal UBE-3A-yellow fluorescent protein hybrid.
The scientists screened 2,306 different compounds, four times each. Most of the compounds have already been approved for use in humans, so if the researchers found anything promising, clinical trials would be expedited. They concentrated on agents known to be active in the central nervous system and those that are known to interfere in epigenetic regulation (like the methylation often used in imprinting). Unfortunately, none of these activated the paternal UBE-3A-yellow fluorescent protein hybrid.
But there was one compound that unsilenced the gene: topotecan, a drug that is part of a class called topoisomerase inhibitors. Topoisomerases are enzymes that alleviate the stress on a DNA double helix that occurs when the two strands are pulled apart, as they are when a gene is expressed.
Once topotecan was identified, the researchers went on to show that other topoisomerase-inhibiting drugs, both those structurally similar to topotecan and those with different structures, could unsilence paternal Ube3a. They then injected topotecan into mouse brains to demonstrate that it could work in vivo, and not just in tissue culture dishes. They found that paternal Ube3a expression persisted in spinal cord neurons for up to 12 weeks after drug treatment; this long lasting effect is significant because genetic imprinting is thought to be established during specific points in embryonic development and then maintained for life.
Paternal Ube3a is normally silenced by what’s called an antisense transcript—a piece of RNA that covers up the gene to prevent its expression. (This antisense transcript is not made from the maternal chromosome.) Topotecan worked on the paternal chromosome, dampening antisense transcription there.
Inherited neurological disabilities have been extremely difficult to treat. In Angelman’s syndrome, the brain architecture seems normal at birth, so it is possible that the restoration of normal gene expression could correct some of the pathologies. Topotecan is approved for use in people with cancer, and it has been shown to be well tolerated in children. Hopefully, it could be therapeutically valuable for those with Angelman’s syndrome; it has definitely been valuable in showing how a dormant but functional gene can be reactivated.