Allelic strengths of encephalopathy-associated UBA5 variants correlate between in vivo and in vitro assays.

Although rare diseases only impact a small fraction of the population, they still affect hundreds of millions of people around the world. Many of these conditions are caused by variations in inherited genetic material, which nowadays can be readily detected using advanced sequencing technologies. However, establishing a connection between these genetic changes and the disease they cause often requires further in-depth study. One such rare inherited disorder is developmental and epileptic encephalopathy type 44 (DEE44), which is caused by genetic variations within the gene for UBA5 (short for ubiquitin-like modifier activating enzyme 5). For DEE44 to occur, both copies of the gene for UBA5, known as alleles, must contain one or more detrimental variation. Although all these variations prevent UBA5 from working correctly, the level of disruption they cause, known as allelic strength, varies between them. However, it remained unclear whether the severity of the DEE44 disease directly corresponds with the allelic strength of these variants. To answer this question, Pan et al. tested how different genetic variants found in patients with DEE44 affected the behavior and health of fruit flies. These results were then compared against in vitro biochemical assays testing how alleles containing these variants impacted the function of UBA5. When the fly gene for the enzyme was replaced with the human gene containing variations associated with DEE44, flies exhibited changes in their survival rates, developmental progress, lifespan, and neurological well-being. However, not all of the variants caused ill effects. Using this information, the patient variants were classified into three categories based on the severity of their effect: mild, intermediate, and severe. Biochemical assays supported this classification and revealed that the variants that caused more severe symptoms tended to inhibit the activity of UBA5 more significantly. Pan et al. further analyzed the nature of the variants in the patients and showed that most patients typically carried one mild and one strong variant, although some individuals did have two intermediate variants. Notably, no patients carried two severe variants. This indicates that DEE44 is the result of UBA5 only partially losing its ability to work correctly. The study by Pan et al. provides a framework for assessing the impact of genetic variants associated with DEE44, aiding the diagnosis and treatment of the disorder. However, further research involving more patients, more detailed clinical data, and testing other newly identified DEE44-causing variants is needed to solidify the correlation between allelic strength and disease severity.