Nonlinearities and Switch-Like Behavior in Gene Expression: From Genetics to Biochemistry and Back.

Many developmental and physiological outcomes depend on transcriptional thresholds or switches. The transcriptional output, ultimately measured as mature RNA, is determined by a series of molecular steps that can introduce thresholds, bottlenecks, or amplification mechanisms. Their combination can convert quantitative changes in regulator abundance into more-than-proportional changes in mature RNA production. Such nonlinear, S-shaped/sigmoidal input-output relationships may account, at least in part, for the haploinsufficiency (HI) of transcription factors (TFs) and chromatin regulators. Here, we link genetic evidence for dosage sensitivity to biochemical nonlinearity in the context of transcription regulation. Without aiming to be exhaustive, we discuss several sources of sigmoidicity, such as classical cooperative DNA binding and transcriptional synergy. We also analyze how chromatin factors, three-dimensional nuclear organization, and transcriptional condensates can generate thresholds. In addition, we explore the roles of epigenetic hysteresis and molecular memory. Finally, at the systems level, we consider how nonlinearities can emerge from regulatory networks that convert graded changes in regulator abundance into stable alternative states. Overall, this paper supports the view that HI of TFs and chromatin regulators does not rely on a single mechanism. Rather, HI is a phenotypic consequence of the disruption of diverse molecular and network architectures that translate dosage perturbations into discrete biochemical and phenotypic outcomes.