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, was 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, 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, surpassing traditional techniques. 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.

研究背景：印记疾病（Imprinting disorders）由表观遗传改变引发，导致印记基因座（imprinted loci）的基因表达异常。1B型假性甲状旁腺功能减退症（Pseudohypoparathyroidism type 1B）以GNAS基因座（GNAS locus）的甲基化缺陷为特征，会影响激素应答。表型不一致的同卵双生子（Monozygotic twins discordant for imprinting disorders）为研究表观突变（epimutations）的发生时机、维持机制与组织特异性分布，及其与临床表型的关联提供了独特研究模型。既往多数研究聚焦于婴儿群体，而胎儿血液嵌合（fetal blood chimerism）可能影响该群体的表观遗传谱（epigenetic profiles），但青少年群体的相关数据仍较为匮乏。

研究结果：本研究针对一对表现出1B型假性甲状旁腺功能减退症临床与生化表型不一致的青春期同卵双生姐妹展开分析。两名受试者在外周血中均存在一致的GNAS基因座部分甲基化缺陷，这与胚胎分裂前发生的合子后表观遗传改变（postzygotic epigenetic alteration）相符。通过全基因组与甲基化综合分析，已排除已知遗传诱因（包括单亲二体（uniparental disomy）与结构变异（structural variants）），且未发现多位点印记紊乱（multilocus imprinting disturbances）或母源效应基因突变（maternal-effect gene mutations）。受累双生子的三胚层衍生组织均存在甲基化异常，而未受累双生子的部分组织仅表现出极轻微或无甲基化缺陷，这解释了二者的表型分化。尽管临床表型不一致，但两名受试者的外周血部分甲基化缺陷仍保持一致，提示造血祖细胞（hematopoietic progenitors）中维持着稳定的早期表观突变；而疾病关键脏器的组织特异性嵌合与甲基化维持的差异性，则促成了临床表型差异。先进的长读长测序（long-read sequencing）可灵敏检测部分甲基化与嵌合甲基化模式，其性能优于传统检测技术。

研究结论：本研究凸显了印记疾病中表观遗传嵌合（epigenetic mosaicism）的复杂性，表明早期合子后部分表观突变可使外周血甲基化谱保持一致，但组织分布存在差异，最终导致同卵双生子出现临床表型不一致。本研究受试者的青春期年龄，驳斥了“甲基化一致性由胎儿血液嵌合导致”的解释，并强调了多组织评估与灵敏分子检测的重要性。上述发现对印记疾病的诊断、监测与遗传咨询（genetic counseling）具有重要指导意义，呼吁采用整合方法评估表观遗传异质性（epigenetic heterogeneity）与表型变异性。