Department of Molecular Medicine

University of Texas Health Science Center at San Antonio

San Antonio, Texas

HARRY E. BOVAY, JR. RESEARCH PROJECT

Novel human Oguichi disease mechanisms

Research Interests

Oguchi disease is an autosomal recessive disorder disabling the arrestin (Arr1)2 and the rhodopsin kinase (Grk1) genes which are required for timely deactivation of visual pigments in rod and cone photoreceptors. Reported first by Dr. Chuta Oguchi in 1907, affected individuals are night blind with a diagnostic shiny metallic fundus appearance that turns normal following prolonged dark adaptation, aka the Mizuo-Nakamura phenomenon. Dr. Chen’s laboratory modeled this disease in the 90s in the Grk1-/- mice and reported a previously unknown light-dependent rod degeneration phenotype5, which was later confirmed and found in human patients.

This project is built on Dr. Chen’s experience with the Grk1 gene5 and the not fully understood rhodopsin kinase (GRK1) it encodes to systematically investigate thirteen disease-causing missense mutations found in this gene with the hope to gain insights on its activation mechanism, as well as settling a long-standing mystery concerning the mechanism of reproducibility of rod’s single photon response (SPR).

The purpose of this project is to figure out how the V380D missense mutation in human Grk1 gene causes the Oguchi disease. Contrary to contemporary thoughts, and when modeled in mice, the V380D mutation causes low GRK1 enzyme level and reduced mRNA quantity. Studying this and 11 other known disease-causing missense mutations can unravel novel Oguchi disease mechanisms, as well as GRK1’s roles in photoreceptor survival and the still-enigmatic reproducibility of rod’s single photon responses.

Plans for 2026

Dr. Chen will finish characterizing the four knock-in mouse lines, and prepare for funding application to larger sources to further generate and characterize the remaining missense mutations.  His lab will also seek pharmacological intervention to enhance GRK1 stability and observe its functional consequences.

Specific Aims:
Aim-1. Investigate mRNA processing and stability as a novel disease mechanism for Oguchi disease.
Aim-2. Characterize the L157P knock-in mice to investigate how mutation in teh GRK1RH domain contributes to disease.

Progress in 2025

Dr. Chen successfully expanded the founder mice bearing K219R, L463P, and G199R mutations, and confirmed germline transmission of these mutations. The lab also obtained new founder mice bearing the proposed L157P mutation. Preliminary characterization of these mutant mice revealed a similar phenotype to that of V380D with much reduced GRK1 protein level. Surprisingly, the GRK1 level in K219R – kinase dead mutant, is also reduced, suggesting that GRK1 depends on its kinase activity to maintain its cellular stability.

Lab personnel participated in the 2025 ARVO meeting with poster presentations.  They found catalytic activity in mutant V380D GRK1 in vitro.  Additionally, the lab generated and characterized a transgenic mouse line expressing mutant V380D GRK1 under the control of the rhodopsin promoter. The transgenic V380D mutant GRK1 is stable and confer light-dependent rhodopsin phosphorylation in vivo.  Taken together, the most plausible explanation of the data generated to date is that both protein stability and mRNA processing are affected by disease-causing grk1 missense mutations.

Progress in 2024

Dr. Chen presented a well-received talk on the discrepancy of the observed V380D mouse phenotypes with the consensus of Oguchi disease mechanism (lack of catalytic activity) at the 2024 ARVO annual meeting (Chen CK et al. Invest. Ophthalmol. Vis. Sci. 2024; 65(7):6199). His team also published a review to advocate for the investigation of all Grk1 missense mutations for further insights into rod’s single photon responses (Margo TE, Chen FS, Chen YJ, et al. (2024) Grk1 Missense Mutations in Type II Oguchi Disease: A Literature Review. Ann Biomed Res 5: 128).

Experimentally, Dr. Chen generated founders for the “kinase dead” K219R mutant mice as proposed, as well as for L463P and G199R mice to expand theirour model collection. The previously proposed in vitro studies were completed, and lab members found catalytic activity in mutant V380D GRK1. Excitingly, his team also found that mRNA level in the V380D mutant mice was reduced to < 10% of the control level, suggesting that mRNA processing/stability might be a novel disease mechanism in addition to the previously proposed protein instability.

At the outset of 2024, Dr. Chen hypothesized that some Grk1 missense mutations hamper the activation of GRK1 and reduce its ability to compete with transducin resulting in enlarged and varied rod SPR. His plans for 2024 included establishing that there are more disease-causing mechanisms than the one and only contemporary view of catalytic incapability for Oguchi disease. The information to be obtained here will significantly impact the genetically heterogeneous retinitis pigmentosa (RP) field where the rhodopsin gene, GRK1’s sole substrate, harbors the most disease-causing mutations. There is no cure yet for rhodopsin-associated RP due to insufficient basic information concerning how it is deactivated and when not, how prolonged rhodopsin signaling kills rod cells. The proposed study will thus provide the missing piece of information toward a full understanding of GRK1’s activation process, an indispensable step toward developing new therapeutic modalities to treat rhodopsin-related and other hereditary human blinding diseases.