The paradigm of somatic mosaicism in complex diseases

The multifactorial etiology of complex diseases involves the interplay of polygenic/oligogenic susceptibility loci and environmental factors. Complex diseases are characterized by pronounced phenotypic variability, genetic heterogeneity, pleiotropy of genetic variants, variable penetrance, and expressivity. Advances in population-wide genomic sequencing have expanded our understanding of the genetic architecture of complex diseases significantly; however, germline variants explain only a fraction of the observed phenotypic variability. The introduction of deep DNA sequencing and single-cell multi-omics analysis has revealed an additional, previously underestimated category of risk factors: somatic mutations that continuously accumulate in cells throughout an individual's lifespan, giving rise to genetic mosaicism. This review considers the role of somatic mutations in the pathogenesis of complex diseases in the context of their combined contribution with germline determinants to the formation of disease phenotypes. Particular attention is paid to clonal hematopoiesis of indeterminate potential as the best-studied model of somatic mosaicism associated with age-related conditions. It is demonstrated that the interaction of somatic and germline variants occurs within specific tissue contexts through mechanisms of clonal selection, stochastic clonal drift, and epigenetic dysregulation, causing organ dysfunction. Within the concept of somatic mosaicism, this work suggests a selection mechanism alternative to classical oncogenesis. This pathway, defined as "passive clonal dominance", describes selection through the persistence of clones that gain advantage not via proliferation but through resistance to apoptosis under chronic stress, which may be particularly relevant to post-mitotic tissues such as the heart muscle and nervous tissue. From a practical standpoint, the somatic variants discussed herein are of interest as candidate biomarkers for predictive diagnostics and risk stratification of complex diseases, pending clinical validation. Moreover, they point to pathophysiological pathways that may reveal targets for therapies aimed at modulating clonal composition and preventing disease progression. The paper also discusses prospects for studying somatic mosaicism in the context of complex diseases, including the potential of dynamic lineage tracing technologies for experimental verification of the proposed clonal selection mechanisms, as well as the need to integrate somatic and germline genetic variant data into unified models for individual disease risk assessment.