Comparative Transcriptomics Reveals Inflammatory and Epigenetic Programs that Actively Orchestrate Pineal Brain Sand Calcification
Background: The pineal gland secretes melatonin but paradoxically calcifies more than any other intracranial structure, forming hydroxyapatite "brain-sand" (corpora arenacea) that correlates with reduced melatonin output, sleep disruption and heightened neuro-degenerative risk. Whether this mineralization is a passive dystrophic event or an active, bone-like process remains unclear. Methods: Analyzed RNA-seq datasets from pineal glands of six vertebrate species-calcifiers Homo sapiens, Rattus norvegicus and Capra hircus, versus non-calcifiers Mus musculus, Gallus gallus and Danio rerio. Species-specific transcripts were mapped to human orthologues, merged, and filtered. Phylogenetically informed differential expression testing used Brownian-motion and Pagel's lamda phylogenetic generalized least-squares models, calibrated on a TimeTree divergence phylogeny. Genes significant in both models (|logtwoFC| > 1; FDR < 0.05; lamda < 0.7) were assigned to functional pathways and visualized by PCA, heat-mapping and volcano plots. Results: Calcifying species segregated cleanly from non-calcifiers on the first two principal components, reflecting a shared 103-gene "calcifier module". Top up-regulated transcripts included developmental morphogens (GLI4, IQCE, NOTCH4), epigenetic regulators (SETD1A, ZNF274, ATF7IP), inflammatory mediators (CSF2RB), and quality-control factors (GABARAPL2, RHOT2). Every leading candidate exhibited minimal phylogenetic signal (lamda to 0), indicating that differential expression tracks the calcified phenotype rather than shared ancestry. Conversely, only three genes (RMI2, RASL11B, GPR18) formed a non-calcifier module, suggesting potential protective roles that are down-regulated during mineralization. Conclusions: Pineal calcification is not a passive by-product of aging but a regulated, lineage- restricted program that redeploys Hedgehog, Notch and chromatin-remodeling pathways classically required for skeletal ossification. The ten-gene core signature identified offers a molecular foothold for mechanistic dissection and therapeutic targeting aimed at preserving pineal function and circadian health. Significance This is the first phylogenetically controlled transcriptomic survey to link pineal "brain-sand" formation to specific developmental and inflammatory gene networks, revealing convergent evolution of calcification programs across divergent mammalian lineages.