Trisomy 21 Perturbs Oligodendrocyte Developmental States and Oligo-Neuron Communication in Human Forebrain Assembloids

Intellectual disability is a hallmark of Down syndrome (DS), caused by the triplication of human chromosome 21. Abnormal neuronal connectivity and impaired myelination are consistently observed in DS, yet the underlying mechanisms remain poorly understood. Here, we used DS-derived isogenic induced pluripotent stem cell-derived forebrain assembloids (FAs) to investigate oligodendrocyte lineage cell (OLLC) development and neuron-glia interactions. Trisomic FAs exhibited an expansion of BCAS1+ pre-myelinating OLLCs, without a corresponding increase in myelin basic protein (MBP) expression. Transcriptomic analyses identified a euploid-enriched, trophic and maturation-supportive state (OLLC_2-TMS) and a trisomy-enriched synaptic interactions and differentiation state (OLLC_3-SID), lacking coordinated enrichment in myelination-promoting programs. Corresponding transcriptional signatures were identified in OLLCs from early postnatal human cortex. Increased excitatory neuronal contacts with pre-myelinating OLLCs accompanied ultrastructural myelin abnormalities in trisomic assembloids. Our findings suggest that trisomy redirects OLLC development toward a pre-myelinating state, uncoupling oligodendrocyte differentiation from productive myelination.