Dissection of the Molecular Basis of Pleiotropy Between GnRH Neuronal Development and Cranial Suture Fusion

PROJECT ABSTRACT
Pleiotropy is defined as the phenomenon in which one gene affects multiple distinct phenotypic features.
Although unbiased genomic analyses have unmasked its increasing prevalence in Mendelian and complex traits,
the full extent of pleiotropy and its molecular basis in humans remains elusive. In this proposal, we will leverage
two seemingly distinct developmental genetic conditions, Idiopathic Hypogonadotropic Hypogonadism (IHH) and
Craniosynostosis (CS), as hallmark examples to study pleiotropy. IHH is a rare reproductive disorder caused by
defective embryonic migration and/or secretion of hypothalamic Gonadotropin-releasing hormone (GnRH)
neurons. CS is a birth defect impacting craniofacial development in which one or more of the sutures of the skull
fuse prematurely. Several molecular cascades (e.g., FGF, Wnt, BMP, hedgehog, ephrin, and retinoic acid
signaling) are known to overlap during GnRH neurogenesis and vertebrate cranial suture fusion. Yet, in humans,
shared genetic etiology between IHH and CS has not been widely reported, with only recent appreciation of a
partial genetic overlap. Loss-of-function (LOF) FGFR1 mutations cause IHH, while gain-of-function FGFR1
mutations result in CS, offering a plausible underpinning for this pleiotropic observation (opposing variant
directional effect). By contrast, LOF mutations in TCF12 (encoding a pro-neural transcription factor) have been
shown previously to cause coronal CS in humans, and recently, we identified LOF TCF12 mutations as a novel
cause of IHH. Notably, we observed phenotypic co-occurrence of IHH and CS in TCF12 mutation carriers,
providing compelling evidence for developmental pleiotropy between IHH and CS. Building on these
observations, and the still largely unexplained biologic basis of IHH/CS pleiotropy, the overarching goal of this
proposal is to unravel the ensemble of shared molecular pathways that coregulate GnRH and cranial suture
development. Our strong investigative team, with complementary expertise in reproductive endocrinology,
craniofacial biology, human genetics, animal models of disease, and human clinical investigation will juxtapose
human studies (Mendelian disease and population cohorts) with relevant zebrafish models in three distinct Aims:
(1) We will define the shared molecular pathways between GnRH neurogenesis and cranial suture fusion using
genomic studies in human Mendelian cohorts with validation in transgenic zebrafish; (2) We will utilize differential
transcriptomic atlases from human cranial suture and relevant zebrafish GnRH and cartilage cell types to
discover novel genes for IHH and CS; and (3) We will utilize population biobanks, IHH, and CS cohorts to define
the full extent of phenotypic pleiotropy relating to genes linked to both traits. Defining the underlying mechanisms
of pleiotropy for CS and IHH will serve as a blueprint to elucidate further the genetic architecture of human
diseases; chart precise genotype-phenotype maps in Mendelian conditions and in population settings; and inform
precision therapies for optimal clinical care.

Grant Number: 5R01DE031452-04
NIH Institute/Center: NIH
Principal Investigator: Ravikumar Balasubramanian