Retained cone-responses in homozygous start codon variant in KIZ-associated retinitis pigmentosa

To report the clinical phenotype, imaging characteristics, and electrophysiologic findings of a 62-year-old patient with retinitis pigmentosa (RP) harboring a likely pathogenic homozygous KIZ (NM_018474.6) start codon variant (c.3G > A, p.Met1?), only previously reported in the compound heterozygous state.

The patient underwent clinical evaluation including full medical history, best-corrected visual acuity (BCVA), slit lamp exam, and dilated fundus examination (DFE), spectral-domain optical coherence tomography (SD-OCT), fundus autofluorescence (FAF), and full-field electroretinography (ffERG), following the ISCEV standard protocols. Genetic testing was performed using the Invitae inherited retinal disorders panel of 330 genes.

BCVA was 20/40 in both eyes. Fundus examination revealed mild optic disc pallor, arteriolar attenuation, peripheral pigment migration, and macular hyper-autofluorescence along the arcades. with macular sparing. Outside the arcades, there are hypo-autofluorescence spots corresponding to retinal pigement epithelium atrophy. There is marked peri-papillary atrophy. SD-OCT showed diffuse outer retinal thinning, ellipsoid zone constriction, mild cystoid macular edema, and epiretinal membrane. ffERG was consistent with a rod-cone dystrophy, with extinguished dark-adapted responses and severely attenuated 30 Hz flicker amplitudes. Genetic testing identified a heterozygous variant of uncertain significance in CTNNA1 (c.1486C > T, p.Arg496Cys) and a homozygous KIZ variant (c.3G > A, p.Met1?), previously observed only in compound heterozygosity. The patient was diagnosed with KIZ-associated RP and initiated on topical dorzolamide.

This case expands the clinical spectrum of KIZ-associated RP by describing the phenotype associated with a homozygous start codon variant. Despite the disruptive nature of the mutation, the patient exhibited a relatively mild rod-cone dystrophy with retained cone responses into the seventh decade. These findings support the inclusion of KIZ in diagnostic panels for autosomal recessive RP and contribute valuable genotype-phenotype correlation data for this rare ciliopathy.