POLR3H

POLR3H (RPC8/C25) is an essential subunit of RNA Polymerase III, the enzyme responsible for synthesizing tRNAs and other small RNAs, and is required for normal tRNA synthesis in vivo (PMID:8065349). Within Pol III it is present in submolar ratios and dissociates readily from the purified enzyme, and although a conditional mutant of the yeast ortholog C25 specifically reduces tRNA synthesis relative to 5.8S rRNA, the subunit is dispensable for transcription in certain in vitro assays (PMID:8065349). POLR3H is indispensable for development: complete loss of function causes early embryonic lethality in mice, whereas a homozygous missense mutation (p.Asp50Gly) is compatible with viability but impairs reproductive development, producing delayed puberty, reduced fertility, and fewer primary ovarian follicles in association with decreased ovarian Foxo3a expression (PMID:30830215). In humans, this same POLR3H mutation causes primary ovarian insufficiency (PMID:30830215).

1. 1994 High

   Established that POLR3H corresponds to a conserved, dissociable subunit of RNA Polymerase III essential for viability and for normal tRNA synthesis, defining its core enzymatic context.

   Evidence Yeast gene identification by sequence homology to RPB7, biochemical dissociation from purified Pol III, and temperature-sensitive conditional mutant analysis scoring tRNA versus 5.8S rRNA synthesis

   PMID:8065349

   Open questions at the time

   • Why the subunit is dispensable in certain in vitro transcription assays yet required in vivo is unresolved
   • The structural basis for its submolar stoichiometry and easy dissociation is not defined
   • Its sequence relationship to RPB7 implies but does not demonstrate a specific molecular role within Pol III
2. 2019 High

   Connected POLR3H function to mammalian development and human disease, showing the subunit is essential for embryogenesis and that hypomorphic missense alleles selectively disrupt reproductive development.

   Evidence Whole-exome sequencing in human families plus CRISPR/Cas9 mouse null (lethal) and knock-in point-mutation lines, with phenotyping of puberty, fertility, follicle counts, and Foxo3a expression

   PMID:30830215

   Open questions at the time

   • The mechanism linking POLR3H/Pol III activity to reduced Foxo3a expression in the ovary is not established
   • Whether the p.Asp50Gly allele alters Pol III assembly, stability, or transcriptional output is undefined
   • Tissue-specific basis for the ovarian/reproductive vulnerability is unknown

• How POLR3H contributes mechanistically to Pol III transcription and how its partial loss translates into the Foxo3a-associated folliculogenesis defect remain open.
• No structural model of POLR3H within the human Pol III complex in the corpus
• No direct link between specific Pol III transcript changes and the reproductive phenotype
• No biochemical characterization of how the p.Asp50Gly substitution perturbs subunit function