Abstract
Down Syndrome (DS) is caused by triplication of human chromosome 21 (HSA21), resulting in
intellectual disability, motor dysfunction, and early onset Alzheimer’s Disease (AD). The
mechanistic connection between triplicated genes in DS and the all-encompassing imbalance in
gene expression underlying the abnormalities seen in DS brain cells has not been established. It
is also unknown how this global transcriptomic disbalance leads to intellectual disability and early
development of AD-related pathology in DS, nor what genes play a key role in these processes.
In our preliminary studies, we generated oligocortical spheroids (OLS) derived from DS
isogenic induced pluripotent stem cell (iPSCs) lines. Trisomic OLS recapitulate DS and AD-
related cellular phenotypes, including reduced cortical volume, aberrant myelination and
pathological depositions of amyloid beta and hyper-phosphorylated tau. Through our single-cell
RNA sequencing (scRNA-seq) analysis of euploid and trisomic OLS, we identified a cluster of X-
linked genes containing brain expressed X-linked/transcription elongation factor A (SII)
(BEX/TCEAL) family members, that is downregulated in trisomy. At the same time, trisomic cells
showed a significantly increased expression of non-coding X-inactive specific transcript (XIST),
responsible for the X-chromosome inactivation. Importantly, the dysregulation of BEX/TCEAL is
linked to psychiatric disorders, intellectual deficits, neurodegeneration, and AD. Noticeably, other
recent studies, including those performed in DS cell lines and human postmortem brains, showed
downregulated expression of different genes in this cluster accompanied by enhanced XIST
levels. However, the aberrant expression of these transcription factors and their potential
connection to brain pathology in trisomy has been neglected. Herein, we hypothesize that
downregulation of BEX/TCEAL is causatively related to abnormal brain development and
AD pathology in DS. In our studies, we will determine the contribution of X-chromosome-linked
genes’ repression to DS-related pathology by assessing whether reducing XIST or restoring
BEX/TCEAL gene expression can mitigate DS-associated cellular phenotypes. We will
specifically focus on the effect of BEX2 and TCEAL7 downregulation in DS since their expression
was most consistently downregulated across multiple developmental periods in DS human
transcriptomic study, further supporting our preliminary data. We will also define the genome-wide
downstream targets of BEX2 and TCEAL7 and their significance in trisomy. Our translational,
human-directed study will ultimately test the causative link between the dysregulation of X-
chromosome-linked genes’, and transcriptional and phenotypical alterations in trisomy.