**Lack of Overt Retinal Degeneration in a K42E** *Dhdds* **Knock-In Mouse Model of RP59**

### **Sriganesh Ramachandra Rao 1,2,**†**, Steven J. Fliesler 1,2,**†**, Pravallika Kotla 3, Mai N. Nguyen 3 and Steven J. Pittler 3,\***


Received: 17 February 2020; Accepted: 4 April 2020; Published: 7 April 2020

**Abstract:** Dehydrodolichyl diphosphate synthase (DHDDS) is required for protein *<sup>N</sup>*-glycosylation in eukaryotic cells. A K42E point mutation in the DHDDS gene causes an autosomal recessive form of retinitis pigmentosa (RP59), which has been classified as a congenital disease of glycosylation (CDG). We generated K42E *Dhdds* knock-in mice as a potential model for RP59. Mice heterozygous for the *Dhdds* K42E mutation were generated using CRISPR/Cas9 technology and crossed to generate *Dhdds*K42E/K42E homozygous mice. Spectral domain-optical coherence tomography (SD-OCT) was performed to assess retinal structure, relative to age-matched wild type (WT) controls. Immunohistochemistry against glial fibrillary acidic protein (GFAP) and opsin (1D4 epitope) was performed on retinal frozen sections to monitor gliosis and opsin localization, respectively, while lectin cytochemistry, plus and minus PNGase-F treatment, was performed to assess protein glycosylation status. Retinas of *Dhdds*K42E/K42E mice exhibited grossly normal histological organization from 1 to 12 months of age. Anti-GFAP immunoreactivity was markedly increased in *Dhdds*K42E/K42E mice, relative to controls. However, opsin immunolocalization, ConA labeling and PNGase-F sensitivity were comparable in mutant and control retinas. Hence, retinas of *Dhdds*K42E/K42E mice exhibited no overt signs of degeneration, ye<sup>t</sup> were markedly gliotic, but without evidence of compromised protein *<sup>N</sup>*-glycosylation. These results challenge the notion of RP59 as a DHDDS loss-of-function CDG and highlight the need to investigate unexplored RP59 disease mechanisms.

**Keywords:** retinitis pigmentosa; knock-in mouse model; congenital disorder of glycosylation; retina
