The Genetics of Cortical Folding Disorders: From Mechanisms to Diagnosis

Abstract

Gyration is a feature of many mammalian neocortices; though the developmental mechanisms that produce cortical gyri are poorly understood. Defects in gyrification can lead to polymicrogyria, a congenital cortical malformation characterized by abnormal gyration and cortical disorganization. To understand the genetic underpinnings of polymicrogyria and identify factors important for cortical gyration, we conducted genetic studies including both panel and exome sequencing on 276 families with individuals affected with polymicrogyria. Sequencing yielded genetic explanations for the affected individuals in 32.6% of cases, with mutations found genes previously associated (e.g., GPR56, TUBB2A) with polymicrogyria, as well as novel associations (e.g., TMEM161B, PANX1), highlighting the value of sequencing for specific diagnosis of structural brain disease. The cohort identified several novel polymicrogyria risk genes and provides a landscape of genetic causes of polymicrogyria that demonstrated shared mechanisms of pathophysiology for future study: developmental channelopathies, ciliopathies, disorders of microtubule regulation, genetic vascular dysfunction, and dysregulation of metabolism. Of the novel polymicrogyria genes identified, we focused further on TMEM161B, a previously undescribed gene in a novel superfamily with no clear function predicted. We show that TMEM161B is an 8-transmembrane domain protein that is highly conserved in eukaryotes, and although ubiquitously expressed at low levels, TMEM161B expression is enriched in the developing central nervous system. A Tmem161b knock-out mouse model demonstrated holoprosencephaly and microcephaly among other defects, which led to the hypothesis that TMEM161B may be involved in regulation of Sonic Hedgehog (Shh) signaling. To confirm this, we demonstrated TMEM161B is necessary for normal Shh signaling in vitro. Follow up experiments confirmed that TMEM161B KO mouse embryos demonstrated disrupted primary cilia in the developing cortex and point towards future experiments that will characterize the cellular roles of TMEM161B. Together, our findings demonstrate TMEM161B promotes normal ciliary structure and Sonic Hedgehog signaling in the developing central nervous system.