From chromatin to proteostasis: multilevel gene regulation in Mendelian diseases.

Pathogenic variation can alter gene output at multiple levels, from chromatin accessibility and enhancer-promoter communication to RNA processing, translation and protein turnover. The diagnostic challenge is not to catalogue these layers, but to determine which regulatory mechanism is plausible for a given variant, phenotype, tissue and developmental context. This targeted narrative review synthesises representative Mendelian mechanisms across five linked levels: chromatin, transcription, RNA processing and stability, translation, and post-translational proteostasis. Three cross-cutting principles recur. First, distinct molecular lesions may converge on the same functional bottleneck: altered dosage, timing, localisation or disrupted protein homeostasis. Second, regulatory effects are often cell-type-, isoform-, developmental-stage- and state-specific, so assay and tissue selection are integral to interpretation. Third, a negative result in an accessible surrogate such as blood does not exclude a disease-relevant effect in brain, muscle, liver or stimulated immune cells. From these principles, the review derives a mechanism-directed diagnostic framework that links variant class to orthogonal assays, including DNA methylation profiling, chromatin and reporter assays, RNA sequencing, ribosome-level methods and protein-based studies. The framework is offered as an integrative clinical synthesis rather than a new model of gene regulation, to support variant interpretation, functional-test selection and translational prioritisation in unsolved Mendelian disease.