Fruit Flies in the Diagnosis of Childhood Genetic Disorders

When I tell people that I conduct biomedical research with fruit flies, I very often get requests to remove these pests from people's kitchens during the summer. As much as I would love to help out, these ‘wildtype’ fruit flies, are of little interest to me. I am interested in flies that are a bit different. These would be flies that have mutations in genes related to neurological disease in humans.

All living things on Earth are deeply connected in our genetics, we are all cousins and our cousinship, meaning our most recent common ancestor with the fruit fly, dates back almost 800 million years. Remarkably, we still share 60% of our genes with these insects. However, about 85% of genes that are involved in human disease are also found in the fly genome. This tells us that genes important in human disease play fundamental roles in the cell and are common processes across most of the tree of life.

Although their size is small, fruit flies have a central nervous system with brains that have over 200,000 neurons. They also share with us chemical messengers called neurotransmitters, like glutamate, acetylcholine, dopamine, serotonin, and others. Compared to the 86 billion neurons of the adult human brain, the difference may seem drastic, but flies are capable of complex behaviours. In fact, one of the pioneering fly geneticists, Canada’s own Dr. David Suzuki, conducted research on flies with interesting neurological behaviours. He found a mutant fly that would become immobile or display seizure-like behaviour at higher temperatures. It was only years later that the sister gene in humans, called DNM1, was connected to an epileptic encephalopathy in children. Surprisingly, traditional anti-seizure medications added to the fly food can decrease seizure activity in flies with mutations in genes that cause seizures.

But why are fruit flies a common model organism in science?

Science has recognized a set of model-organisms that have helped drive our understanding of biology and medicine. They each have their pros and cons and it is important to understand what research questions you can and cannot answer with each model. While this list certainly includes others, the most studied include yeast, nematodes, fruit flies, zebrafish, and mice. Flies are in the middle of this list in multiple ways including: evolutionary distance, complexity, time to reproduce, and cost to maintain. However, well beyond their counterparts in other species, fruit flies have two other very powerful advantages. Firstly, we have mastered the fly genome and can easily manipulate it. What this means is, we can deplete or put excess of any fly gene in almost any tissue type we desire. We can also conduct these experiments at any time in the flies’ life cycle we want. For example, we can deplete a gene from only dopamine neurons, only in the adult fly but not in embryos or the larval/pupal stages. Most of the flies used in these studies are available commercially at public stock centers around the world. When a fly is not available, the fly community is ready, willing and well-known for sharing resources. The second advantage fruit flies have is that they are easily discernible. This means when we cross parent flies together, we do not need to do a DNA test (PCR) to figure out which flies in the next generation are carrying the mutations. We have visual markers on the fly like eye colour, curly wings, stubbly hair that identifies who’s who in the children after a genetic cross.

Discoveries in fruit flies have garnered six Nobel prizes in physiology or medicine. Often, key cellular pathways have been first discovered in flies. Indeed, Stephanie Mohr’s book ‘First in Fly’ outlines and celebrates many of the important biological processes that were first found using these little creatures.

But how do fruit flies help patient diagnosis?

We live in the post-genomic era where genome sequencing has gotten fast enough and cheap enough to help the worldwide medical genetics community identify variants in genes that cause disease. However, the more we sequence individuals, the more we know that everyone carries variants in genes that are benign and do not cause disease. What can we do when we find variants that haven’t been seen before, even if they are in a gene where other variants in the same gene are known to cause human disease? That is where functional studies come in. Whether in cells or a living system, functional studies test whether the patient variant actually affects the function of the protein for which that gene is the blueprint.  Fruit flies can step up to this challenge, resulting in evidence to support a diagnosis. We use the fly as a ‘living test tube’ to assess the effects of expressing the human gene with the typical sequence or with the patient’s variant.

The most notable examples of flies aiding in diagnosis have been through the US-based, Undiagnosed Diseases Network (UDN), funded by the National Institutes of Health. The UDN fly lab (Fly Core) is an arm of the model organism screening center and have contributed to diagnosis and the discovery of over 35 new human disease genes over the last 6 years. One example of this is how we have used flies to discover the variants in the gene called IRF2BPL as the cause of a childhood onset motor and seizure disorder.

Fruit flies beyond diagnosis – a role in therapeutics ?

With the Latin name, Drosophila melanogaster, the fruit fly has helped humans uncover biology, aide in diagnosis, and has also helped in drug testing. In rare disease and drug repurposing, flies can be used to quickly test drugs that have already passed human safety and be repurposed for rare and severe diseases. Many compounds can be screened in these animals faster than mice and offer a living organism beyond cells in a Petri dish.

Drug testing in flies has led to medically actionable changes in treatment for patients. This has worked for CACNA1A-related disorders and Infantile neuroaxonal dystrophy caused by variants in PLA2G6. We are currently screening approved drugs for IRF2BPL-related disorders.

Fruit Flies & Neurodevelopment and Rehabilitation Research in Child Health

My lab has expertise in using the fruit fly to assess genetic variants related to neurodevelopmental conditions like autism spectrum disorder and seizure disorders. We will continue to conduct these studies and dig deeper into the molecular and cellular pathways that are important in neurodevelopment. Lastly, we will test approved drugs in our fly models that may lead to eventual clinical implementation helping treat a variety of neurodevelopmental conditions.

References:

Bellen HJ, Wangler MF, Yamamoto S. The fruit fly at the interface of diagnosis and pathogenic mechanisms of rare and common human diseases. Human molecular genetics. 2019 Oct 15;28(R2):R207-14.

Lin, G., Lee, P.T., Chen, K., Mao, D., Tan, K.L., Zuo, Z., Lin, W.W., Wang, L. and Bellen, H.J. (2018) Phospholipase PLA2G6, a parkinsonism-associated gene, affects Vps26 and Vps35, retromer function, and ceramide kevels, similar to alpha-Synuclein gain. Cell Metab., 28, 605–618.

Luo, X., Rosenfeld, J.A., Yamamoto, S., Harel, T., Zuo, Z., Hall, M., Wierenga, K.J., Pastore, M.T., Bartholomew, D., Delgado, M.R. et al.  (2017) Clinically severe CACNA1A alleles affect synaptic function and neurodegeneration differentially. PLoS Genet., 13, e1006905.

Marcogliese PC, Shashi V, Spillmann RC, Stong N, Rosenfeld JA, Koenig MK, Martínez-Agosto JA, Herzog M, Chen AH, Dickson PI, Lin HJ. IRF2BPL is associated with neurological phenotypes. The American Journal of Human Genetics. 2018 Aug 2;103(2):245-60.

Marcogliese PC, Dutta D, Ray SS, Dang ND, Zuo Z, Wang Y, Lu D, Fazal F, Ravenscroft TA, Chung H, Kanca O. Loss of IRF2BPL impairs neuronal maintenance through excess Wnt signaling. Science advances. 2022 Jan 19;8(3):eabl5613.

Mohr SE. First in fly: Drosophila research and Biological Discovery. Cambridge, MA: Harvard University Press; 2018.

About the author

Paul Marcogliese, PhD conducted his doctoral studies at the University of Ottawa where he studied models of Parkinson’s disease in mice, fruit flies, and cell culture. He conducted his postdoctoral work at Baylor College of Medicine in Houston, Texas. There he learned sophisticated fly genetics and was part of the Undiagnosed Diseases Network using flies to help diagnose rare pediatric neurological diseases. He recently started his lab in the Department of Biochemistry & Medical Genetics at the University of Manitoba where he continues to use both fly and mouse models to investigate the molecular and cellular mechanisms underlying neurological disease.