Department of Molecular Medicine

University of Texas Health Science Center at San Antonio

San Antonio, Texas

BASIC 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).

To figure out how the V380D missense mutation in human Grk1 gene causes the Oguchi disease. Contrary to contemporary thoughts, the V380D mutation confers low GRK1 enzyme level due speculatively to mutant protein instability when modeled in laboratory mice. Studying this and 11 other known disease-causing missense mutations thus bear great potential to unravel novel Oguchi disease mechanisms, as well as GRK1’s roles in photoreceptor survival and the unsolved remarkable reproducibility of rod’s single photon responses.

Plans for 2025

Dr. Chen will focus on expanding and characterizing the four knock-in mouse lines generated for this project. In addition, his laboratory will generate the mouse line with the diseasing-causing mutation situated in the N-terminal RH domain instead of the central kinase domain to cover all grounds. Ultimately, Dr. Chen believes that all known Grk1 missense mutations need to be modeled in mice to gain a front seat view on the roles of GRK1 in photoreceptor survival and function.

Specific Aims:
Aim-1. Investigate mRNA processing and stability as a novel disease mechanism for Oguchi disease.
Aim-2. Characterize three knock-in mouse lines: K219R, L463P, and G199R.
Aim-3. Generate the L157P knock-in line to investigate how mutations in the GRK1 RH domain contributes to disease.

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.