Human organoid model of PCH2a recapitulates brain region-specific pathology
Pontocerebellar hypoplasia type 2 a (PCH2a) is a rare, autosomal recessive pediatric disorder with limited treatment options. Its anatomical hallmark is the hypoplasia of the cerebellum and pons accompanied by progressive microcephaly. PCH2a results from a homozygous founder variant in TSEN54, which encodes a tRNA splicing endonuclease (TSEN) complex subunit. Despite the ubiquitous expression of the TSEN complex, the tissue-specific pathological mechanism of PCH2a remains unknown due to a lack of model system. In this study, we developed human models of PCH2a using brain region-specific organoids. We therefore obtained skin biopsies from three affected males with genetically confirmed PCH2a and derived induced pluripotent stem cells (iPSCs). Proliferation and cell death rates were not altered in PCH2a iPSCs. We subsequently differentiated cerebellar and neocortical organoids from control and PCH2a iPSCs. Mirroring clinical neuroimaging findings, PCH2a cerebellar organoids were reduced in size compared to controls starting early in differentiation. We observed milder growth deficits in neocortical PCH2a organoids. While PCH2a cerebellar organoids did not upregulate apoptosis, their stem cell zones showed altered proliferation kinetics, with increased proliferation at day 30 and reduced proliferation at day 50 compared to controls. In summary, we have generated a human model of PCH2a, which provides the foundation for deciphering brain region-specific disease mechanisms.