Autism spectrum disorder (ASD) is one of the most common neurodevelopmental disorders in the United States, with 1 in 54 children in the country currently estimated to be affected. It can co-occur with other neurodevelopmental disorders (NDDs) such as attention deficit hyperactivity disorder (ADHD) and intellectual disability. NDDs present a significant challenge for patients, their families, and society, while research suggests that NDDs affect people of all countries, ethnicities, and socioeconomic backgrounds.
Clinical and genetic heterogeneity makes it quite challenging to develop a holistic understanding of the causes of NDDs. For instance, individuals with ASD exhibit a broad spectrum of symptoms, ranging from minor social deficits to severe behavioral impairment and cognitive disability. Moreover, ASD is a genetically complex condition. Sequencing of human subjects indicates hundreds of genes that contribute to risk. What further adds to this complexity is that ASD risk is often polygenic, with a single case involving mutations in multiple genes or mutations in the non-protein-coding segments of the genome. Such non-protein-coding mutations can increase risk by altering the regulation of gene expression.
Using Human Cellular Models to Understand NDDs
Over the past few years, interest in studying the biology of the developing human brain and its dysregulation to cause NDDs has gained traction. Among the researchers in this area, Kristen Kroll has emerged as a prominent figure. She is an American developmental and stem cell biologist and Professor of Developmental Biology at Washington University School of Medicine. Her ongoing work involves studying transcriptional and epigenetic regulation of human brain development and its disruption to cause neurodevelopmental disorders.
Kroll grew up in Wisconsin, where she graduated from Wilmot High School in 1984. She then obtained a bachelor’s degree in Molecular and Cell Biology from Northwestern University, supported by the Alice G. Hough Scholarship. As an undergraduate, she was inducted into Northwestern’s Phi Eta Sigma and Phi Beta Kappa honorary societies and conducted research in Drosophila genetics in the laboratory of Dr. Robert Holmgren. After completing her bachelor’s degree, Kroll acquired a Ph.D. in Biochemistry, Molecular, and Cell Biology from the University of California, Berkeley, working in the laboratory of Dr. John Gerhart. In her thesis research, she developed a highly effective and cost-efficient method for producing transgenic frogs. Xenopus laevis, commonly referred to as the African Clawed frog, has long been a favorite of embryologists. Its eggs are one millimeter in diameter, very large by the standards of experimental embryologists. Thousands of eggs are easily accessible from one adult, and these quickly develop into swimming tadpoles in a petri dish. The accessibility and ease of manipulating Xenopus embryos make them an important research organism, which has been used to identify many fundamental aspects of vertebrate embryogenesis. However, Xenopus could not be genetically manipulated, greatly limiting the kinds of research for which this model could be used.
When Kristen Kroll began to conduct her thesis research on Xenopus laevis, she harked back to the pioneering work of Dr. John Gurdon from the 1950s. This work used nuclear transplantation into Xenopus eggs to demonstrate that somatic cell (e.g. skin or intestinal cell) nuclei were genetically equivalent and could program the egg to develop into a normal adult frog. In using this approach to derive transgenic Xenopus, she was joined by Enrique Amaya, a colleague at UC Berkeley. Together, they developed an approach for making transgenic embryos that involved combining sperm nuclei, DNA, and interphase cell extract in a tube. They added a small amount of an enzyme to introduce breaks in the nuclei’s chromosomes, enabling a DNA construct to be stably inserted into the genome. They then transplanted these treated nuclei into freshly harvested frog eggs. Using this approach, they could transplant treated nuclei into ~500 eggs in an hour, with ~20% successfully developing into embryos carrying the new gene. This approach proved highly useful to researchers, being used for Xenopus research related to both gene regulation and gene function, particularly during later stages of embryonic development when embryos could not previously be genetically manipulated. This approach was also extensively used to derive new lines of transgenic Xenopus, many of which continue to be used by the research community.
Following her graduate work, Kroll conducted her post-doctoral research in Marc Kirschner’s lab, funded by a fellowship from the Damon Runyon-Walter Winchell Cancer Research Fund. She subsequently began a faculty position at Washington University School of Medicine, where her research has focused on studying molecular mechanisms controlling nervous system development. During her postdoctoral work, she cloned a novel gene that regulates neurodevelopment, Geminin. Her laboratory subsequently demonstrated that Geminin regulates several aspects of neural development and early embryogenesis by interacting with chromatin-modifying complexes. Research in Kroll’s laboratory over the years has also focused on other aspects of neural development, including characterizing roles of bHLH transcription factors in controlling neurogenesis, defining roles for other aspects of epigenetic regulation in early brain development, and, more recently, studying how disruption of transcriptional and epigenetic regulation contributes to NDDs, including ASD.
Throughout her academic career, Kristen Kroll has served the broader scientific community. She previously served as Co-Director, Instructor, or Lecturer for the Cold Spring Harbor Laboratory Course “Cell and Developmental Biology of Xenopus,” training senior postdoctoral fellows and new faculty in the use of this model organism. Kroll has been a member of multiple National Institutes of Health study sections for grant review, performing both ad hoc service on multiple NIH study sections and also serving as a permanent member of the Development-2 study section. She presently leads the Cellular Models Program for Washington University’s Intellectual and Developmental Disabilities Research Center (IDDRC) and co-leads the Cross-IDDRC Human Stem Cell Models Group in collaborative efforts to build and share human cellular models of IDDs. She also coordinates human cell and organoid-based modeling under the Precision Medicine Integrated Experimental Resources (PreMIER) platform at Washington University, a platform for screening disease-associated gene variants for precision medicine. This leadership enhances the use of patient-derived stem cell models in NDD research.
A passion for studying the biology of the developing human brain continues to drive work in Kristen Kroll’s laboratory. Her current work uses induced pluripotent stem cells derived from ASD patients to understand different genetic contributors to ASD etiology, including roles for inherited, polygenic contributors, gene copy number variants, mutations in genes encoding chromatin-modifying proteins, and mutations in the non-protein-coding genome. This work builds upon the work she has done to study developmental mechanisms throughout her career and uses new insights into the mechanisms that drive development and patterning of the fetal brain to understand how disrupting these processes contributes to disease.