The researchers suspect that the newly identified gene, Cyfip2, determines how mammals respond to cocaine, although it is too soon to tell what the indications are for humans or for addiction, said Dr. Joseph Takahashi, chair of neuroscience and a Howard Hughes Medical Institute investigator at UT Southwestern and the senior author of the study.
The findings, reported in Science, evolved from examining the genetic differences between two substrains of the standard C57BL/6 mouse strain: a “J” strain from the Jackson Laboratory (C57BL/6J) and an “N” strain from the National Institutes of Health (C57BL/6N). Researchers compared the two strains of mice and used their differential responses to cocaine to identify the causative gene.
“We found that the ‘N’ strain has accumulated mutations over time, one of which has a very strong effect on cocaine response,” Dr. Takahashi said. “We propose that CYFIP2 – the protein produced by the Cyfip2 gene – is a key regulator of cocaine response in mammals.”
The Takahashi laboratory has identified about 100 genetic differences that affect protein sequences between the two mouse strains, meaning that there are many genetic differences whose effects are not yet known, he added.
“We identified this gene by first using a forward genetics strategy to search for differences in traits between the two mouse strains. We found a difference in cocaine response between them, with the C57BL/6N strain showing a reduced behavioral response,” Dr. Takahashi said. “We then carried out genetic mapping and whole genome sequencing, which allowed us to pinpoint the Cyfip2 gene as the causative one in a rapid and unambiguous way.”
The C57BL/6J “J” mouse is the gold-standard strain for most research involving the mouse. For example, the reference sequence for the mouse genome, as well as most behavioral and physiological experiments, are based on the “J” strain. However, the International Knockout Mouse Consortium will be shifting emphasis to the “N” strain since they have created 17,000 embryonic stem cell lines with gene mutations that originate from the “N” strain. Thus, identifying genetic differences between these two mouse strains is important, Dr. Takahashi said.
“Although mouse geneticists pay close attention to the specific strains of mice that they use, it has not been generally appreciated that sublines of the same strain of mouse might differ so profoundly. Thus, a ‘C57BL/6’ mouse might appear to be the same, but in fact there are many, many sublines of this laboratory mouse, and it is important to know which exact one you are using. Since the knockout mouse project has produced so many mutations (17,000) derived from the ‘N’ strain, it will be even more important to keep in mind that not all C57BL/6 mice are the same.”
The study was supported by the National Institute on Drug Abuse, by the National Institutes of Health and by the Howard Hughes Medical Institute.
Other UT Southwestern authors include Dr. Vivek Kumar, Instructor; Kyungin Kim, Research Associate; Chryshanthi Joseph, Research Associate; Dr. Saïd Kourrich, Assistant Professor of Psychiatry; and Dr. Hung Chung Huang, Computational Biologist II. Other researchers included Seung-Hee Yoo, former instructor of neuroscience at UT Southwestern and now an assistant professor of biochemistry and molecular biology at UT Health Science Center, Houston; Martha Vitaterna from Northwestern University; Gary Churchill from The Jackson Laboratory; Fernando Pardo-Manuel de Villena from the University of North Carolina at Chapel Hill; and Antonello Bonci from the Intramural Research Program of the National Institute of Drug Abuse / National Institutes of Health.