Lissencephaly is a neurodevelopmental disorder characterised by a loss of brain folds. In a new study, Jin-Wu Tsai and colleagues uncover a new variant of BAIAP2 that is associated with lissencephaly. To learn more about the story behind the paper, we caught up with first authors Meng-Han Tsai and Wan-Cian Lin, and corresponding author Jin-Wu Tsai, Professor at National Yang Ming Chiao Tung University, Taiwan.
From left to right: Jin-Wu Tsai, Wan-Cian Lin and Meng-Han Tsai.
Jin-Wu, can you give us your scientific biography and the questions your lab is trying to answer?
JWT: With backgrounds in physics and biology, I have been fascinated by the mechanobiology of cellular processes, particularly cell migration and division during development. In the past, we have pioneered the use of in utero electroporation and microscopy in live brain slices to investigate the cellular mechanisms of migration of neurons and neural stem cells during brain development. Recently, we invented an innovative genetic screen in live animals for the identification of genes involved in foetal brain development. In collaboration with Meng-Han, we further identified mutations in a centrosomal protein, CEP85L, and a dynein-recruiting protein, BICD2, from lissencephaly patients. We also elucidated the effects of a variety of genetic variants on the pathogenesis of lissencephaly and movement disorders. The current study represents our continued efforts to study the basic mechanism of neurodevelopmental disorders and neuronal migration.
Meng-Han and Wan-Cian, how did you come to work in the field and what drives your research today?
MHT: As a clinician working at Kaohsiung Chang Gung Memorial Hospital, I encountered patients with lissencephaly and their parents who desperately needed an answer to why their children suffered from this severe developmental brain disorder. Using the latest sequencing technology to find the genetic cause and unravelling the underlying mechanisms for these families drive my research career and have led me to my close collaboration with Prof. J.-W. Tsai at National Yang Ming Chiao Tung University.
WCL: I became fascinated by brain development during a speech delivered by Prof. Tsai, and decided to become part of the team after a laboratory visit. One of our research focuses is lissencephaly, which is a cortical malformations and is mostly attributed to genetic events. The patients with lissencephaly usually suffer from refractory seizures, multiple neurological deficits and severe developmental disorders. The desire to better understand brain development and lissencephaly, as well as the hope of contributing to a therapeutic strategy drive our team forward on our research journey.
Tell us about the background of the field that inspired your work
JWT: The development of the vertebrate central nervous system involves a series of intricate steps, starting with the induction of neuroepithelium from the ectoderm, leading to the formation of the neural tube and subsequent development of the brain and spinal cord. The telencephalon gives rise to the cerebral cortex in mammals. Neuronal progenitors undergo a complex process of division, migration and differentiation, which are crucial for proper cortical development. In lissencephaly, a severe form of brain malformation, disruptions occur during neuronal migration, leading to a smooth brain surface, thickened cortex and neurological deficits. Although several genes are already associated with lissencephaly, a significant number of cases remain genetically unexplained. Our study focuses on a de novo missense variant (p.Arg29Trp) in the BAIAP2 gene identified in a lissencephaly patient. BAIAP2, not previously linked to lissencephaly, plays a role in membrane dynamics and actin regulation. However, the functional roles of the BAIAP2 variant and its contribution to lissencephaly pathogenesis were yet to be explored at the time of discovery.
Can you give us the key results of the paper in a paragraph?
JWT: Our study identified a de novo missense variant (c.C85T, p.Arg29Trp) in the BAIAP2 gene in a 6-year-old lissencephaly patient exhibiting severe developmental delay, epilepsy and classical lissencephalic features. BAIAP2, not previously associated with lissencephaly, was prioritized due to its functional relevance to the cytoskeleton. Notably, most previously identified lissencephaly genes were associated with the microtubule cytoskeleton, while BAIAP2 is the first actin-binding protein implicated in the disorder. Expression analysis in mouse and human foetal brains revealed a gradient pattern of Baiap2/BAIAP2 from anterior-low to posterior-high in the developing cortex. Baiap2 knockdown in mouse embryos resulted in impaired neuronal migration, abnormal morphogenesis and altered neuronal fate. Additionally, the p.Arg29Trp variant, located in the N-terminal IMD (IRSp53 and MIM homology domain), failed to rescue migration delay, suggesting a potential loss-of-function effect. Despite maintaining dimerization ability, the variant exhibited reduced membrane localization, emphasizing the importance of BAIAP2 in cortical development and implicating the p.Arg29Trp variant in lissencephaly pathogenesis.
Meng-Han and Wan-Cian, when doing the research, did you have any particular result or eureka moment that has stuck with you?
MHT: Of course: when I read through the functions of candidate genes of the patient and BAIAP2 jumped out as the best candidate. You know you are probably on the money and then it was confirmed in the lab that the variant altered neuronal migration in mice. We know that we have an answer for the family of a lissencephaly patient.
WCL: As the research progressed, a eureka moment occurred when we observed the correlation between the patient's phenotype and the Baiap2 expression gradient in the developing mouse cerebral cortex. The Baiap2 expression gradient from anterior-low to posterior-high in the cortex aligned with the observed posterior-to-anterior gradient of cortical malformation in the lissencephaly patient. This connection strengthened the link between Baiap2 and lissencephaly, emphasizing its potential role in cortical development. Moreover, when we conducted experiments involving the loss of Baiap2 in animal models through knockdown approaches, we observed significant neuronal migration defects and morphological changes. The in vivo and in vitro studies provided compelling evidence that Baiap2 plays a crucial role in regulating neuronal migration, morphogenesis and differentiation during cortical development. This further supported the notion that the identified BAIAP2 variant could lead to functional consequences, contributing to the manifestation of lissencephaly in the affected individual.
A eureka moment occurred when we observed the correlation between the patient's phenotype and the Baiap2 expression gradient in the developing mouse cerebral cortex
The Baiap2 expression gradient, exhibiting an anterior-low to posterior-high pattern in the cortex, closely correlated with the observed posterior-to-anterior gradient of cortical malformation in the lissencephaly-affected individual.
And what about the flipside: any moments of frustration or despair?
MHT: I would say when I spent night after night digging through the exome data, but still could not identify any candidate gene or variant in unsolved cases, those were the most frustrating moments. You really hope that you could end the diagnostic odyssey for these patients. ‘We will keep her company as long as we can’, the parents of the patient said when they left the clinic. They were sad and received no answer as to why their first child suffered from this devastating brain developmental disorder. Sequencing for known lissencephaly genes disclosed nothing. A few months later, the exome data came back, and BAIAP2 was considered the best candidate but we were still unsure because it had not been associated with lissencephaly before. We needed functional experiments to corroborate, and J.-W.T.'s lab came into the picture. After months of hard work by the lab, we were able to be confident that the answer had been found. We finally have an answer for the family! It was not due to hypoxia or prolonged labour. It was due to a tiny change in the BAIAP2 gene.
You really hope that you could end the diagnostic odyssey for these patients
Why did you choose to submit this paper to Development?
JWT: We chose to submit our article to Development based on the novelty of our findings, specifically the identification of the BAIAP2 gene associated with lissencephaly. The distinctiveness of our discovery lies in BAIAP2 being the first actin-binding protein linked to lissencephaly, offering a fresh perspective on the molecular mechanisms underlying the disorder. Recognizing Development as a leading primary research journal in developmental biology, we believe the uniqueness and significance of our work align well with the standards of this prestigious journal. Our study's timeliness is underscored by the increasing interest in the genetics of neural development and human diseases. The comprehensive nature of our research, encompassing the identification of the BAIAP2 variant, functional consequences through in vivo and in vitro studies, and mechanistic insights, positions our paper as a fitting contribution to Development, which is known for publishing cutting-edge research articles across various aspects of plant and animal development. Finally, we also value the journal's strong commitment to a timely decision process, rapid publication and ethical publishing practices. These factors, combined with the journal's wide international readership, further supported our choice of submitting the paper to Development.
Meng-Han and Wan-Cian, what is next for you after this paper?
MHT: We are working on cracking the unsolved lissencephaly cases using new sequencing technologies, such as whole-genome sequencing or long-read sequencing techniques. We hope this could also lead to a better understanding of how the human brain develops.
Jin-Wu, where will this story take your lab next?
JWT: The successful identification of the association of BAIAP2 with lissencephaly and the comprehensive understanding of its role in neuronal migration and cortical development pave the way for several intriguing directions and future research avenues for the lab. Moving forward, the focus will be on elucidating the detailed molecular pathways through which BAIAP2 influences neuronal migration and morphogenesis, including downstream signalling cascades and interaction partners. Further investigations into the functional aspects of BAIAP2, especially its role in regulating actin and myosin activities, might be pursued to understand how the identified variant disrupts these functions and leads to observed defects. Exploring the involvement of BAIAP2 in other neurodevelopmental processes, such as synapse formation and neural circuitry development, could broaden the impact of the research. Additionally, identifying a novel gene associated with lissencephaly has direct implications for genetic counselling, and understanding the spectrum of BAIAP2-related phenotypes can contribute to more informed counselling for affected families. Overall, the continuous exploration of BAIAP2 is likely to contribute to the collective knowledge aimed at unravelling the complexities of brain development and associated disorders, with potential implications for translational research and therapeutic targets.
Finally, let's move outside the lab – what do you like to do in your spare time?
JWT: Well, going outdoors, reading a good book or watching movies are my ways to relax from busy clinical and research work.
M.-H.S.: Department of Neurology & Department of Medical Research, Kaohsiung Chang Gung Memorial Hospital, Kaohsiung 833, Taiwan and School of Medicine, Chang Gung University, Taoyuan 333, Taiwan.
W.-C.L.: Institute of Brain Science and Faculty of Medicine, College of Medicine, National Yang Ming Chiao Tung University, Taipei 112, Taiwan.
J.-W.T.: Institute of Brain Science, College of Medicine, National Yang Ming Chiao Tung University, Taipei 112, Taiwan; Advanced Therapeutics Research Center and Brain Research Center, National Yang Ming Chiao Tung University, Taipei 112, Taiwan; and Department of Biological Science and Technology, College of Biological Science and Technology, National Yang Ming Chiao Tung University, Hsinchu 300, Taiwan.
E-mail: [email protected]