Concordant epigenetic and discordant clinical PHP1B/iPPSD3 manifestations in two monozygotic adolescent twins

BACKGROUND: Imprinting disorders are caused by epigenetic alterations leading to abnormal gene expression from imprinted loci. Pseudohypoparathyroidism type 1B is characterized by methylation defects at the GNAS locus, affecting hormone responsiveness. Monozygotic twins discordant for imprinting disorders provide a unique opportunity to investigate the timing, maintenance, and tissue-specific distribution of epimutations and their relation to clinical expression. Most previous studies focus on infants, where fetal blood chimerism may influence epigenetic profiles, but data in adolescents remain scarce. RESULTS: We studied adolescent monozygotic twin sisters presenting clinical and biochemical discordance for pseudohypoparathyroidism type 1B. Both twins exhibited an identical, partial methylation defect at the GNAS locus in peripheral blood, consistent with a postzygotic epigenetic alteration occurring before embryonic splitting. Exclusion of known genetic causes, including uniparental disomy and structural variants, and absence of multilocus imprinting disturbances or maternal-effect gene mutations, were confirmed through comprehensive genomic and methylation analyses. The affected twin showed involvement of methylation abnormalities across all three germ layers derived tissues, whereas the unaffected twin presented very mild or absent defects in some tissues, possibly explaining the phenotypic divergence. The persistent concordance of partial methylation defects in blood despite phenotypic discordance suggests stable early epimutations maintained in hematopoietic progenitors, while tissue-specific mosaicism and differential methylation maintenance across organs critical to disease expression contribute to clinical differences. Advanced long-read sequencing demonstrated sensitivity for detecting partial and mosaic methylation patterns, rather than just complete alterations. CONCLUSIONS: This study underscores the complexity of epigenetic mosaicism in imprinting disorders, highlighting that early postzygotic partial epimutations may yield identical blood methylation profiles but divergent tissue distributions, leading to discordant clinical phenotypes in monozygotic twins. The adolescent age of our subjects challenges the fetal blood chimerism explanation for methylation concordance and emphasizes the importance of multi-tissue evaluation and sensitive molecular assays. These findings have important implications for diagnosis, monitoring, and genetic counseling in imprinting disorders, advocating for integrated approaches to assess epigenetic heterogeneity and phenotypic variability.