Somatic Blood Mosaicism and Psychiatric Disease: Current Evidence and Causal Limits

Somatic blood mosaicism is an established feature of ageing and a heterogeneous marker of haematologic cancer and inflammatory and vascular risk, but its psychiatric evidence base remains small. This structured critical narrative review evaluates mutation-defined clonal haematopoiesis, clonal haematopoiesis of indeterminate potential (CHIP), autosomal mosaic chromosomal alterations (mCAs), and sex-chromosome loss while separating probable developmental mosaicism from adult-acquired blood clonality. Direct human evidence comprises seven methodologically heterogeneous studies. Psychiatric CHIP findings rely mainly on one prospective UK Biobank cohort of 360,451 participants, in which CHIP was associated with any incident psychiatric disorder (hazard ratio 1.15), particularly major depressive and substance-use disorders. Separate studies report excess mosaic losses in schizophrenia and bipolar disorder and autosomal or X-chromosome mosaicism in suicide. Clone-size gradients and lesion-specific findings add biological structure, but do not provide independent replication. Most mechanistic support comes from cardiovascular, stroke and neurodegenerative research: mutant myeloid cells can alter inflammasome signalling, endothelial function, thrombosis and tissue macrophage behaviour. These pathways make indirect brain effects plausible without establishing a psychiatric mechanism. Blood detection also cannot date a mosaic event or demonstrate the same clone in neural tissue. Taken together, the available evidence supports somatic blood mosaicism as a potentially informative marker of psychiatric heterogeneity and shared inflammatory, vascular and ageing-related biology, but does not establish a causal blood-clone-to-brain pathway. Routine psychiatric screening for CHIP or mCAs is therefore not currently justified. The immediate research priority is independent replication in longitudinal cohorts with repeated sequencing, detailed exposure and treatment histories, and direct testing of inflammatory, vascular and neuroimaging intermediates capable of distinguishing cause, consequence and shared risk.