Identification and characterization of early human photoreceptor states and cell-state-specific retinoblastoma-related features.

Abstract

Human cone photoreceptors differ from rods and serve as the retinoblastoma cell-of-origin, yet the developmental basis for their distinct behaviors is poorly understood. Here, we used deep full-length single-cell RNA-sequencing (scRNA-seq) to distinguish post-mitotic cone and rod developmental states and identify cone-specific features related to retinoblastomagenesis. The analyses revealed nascent, immediately post-mitotic cone and rod precursors characterized by higher THRB or NRL regulon activities, immature and maturing cone and rod precursors with concurrent cone- and rod-related gene and regulon expression, and distinct early and late cone and rod maturation states distinguished by maturation-associated declines in RAX regulon activity. Cell-state-specific gene expression features inferred from full-length scRNA-seq were consistent with past 3' scRNA-seq analyses. Beyond the cell state characterizations, full-length scRNA-seq revealed that both L/M cone and rod precursors co-expressed <i>NRL</i> and <i>THRB</i> RNAs yet differentially expressed functionally antagonistic <i>NRL</i> isoforms and prematurely terminated <i>THRB</i> transcripts. Moreover, early L/M cone precursors sequentially expressed several lncRNAs along with <i>MYCN</i>, which composed the seventh most L/M-cone-specific regulon, and <i>SYK</i>, which was implicated in the cone precursors' proliferative response to <i>RB1</i> loss. These findings reveal previously unresolved photoreceptor precursor states and suggest a role for early cone-precursor-intrinsic <i>SYK</i> expression in retinoblastoma initiation.