Why this study matters

Achromatopsia is an inherited retinal condition in which cone photoreceptors—the cells responsible for color vision, sharp central vision, and vision in bright light—do not function normally. Research into its genetic causes and potential treatments depends heavily on animal models. For patients and families, the quality of these models matters: reliable preclinical studies are an essential step toward developing and testing future therapies.

A 2026 study in Investigative Ophthalmology & Visual Science found that many male albino Swiss CD-1 mice, a widely used outbred laboratory strain, had spontaneous retinal abnormalities resembling achromatopsia. The findings show why researchers must carefully assess baseline vision in individual mice before using them in studies of retinal disease or treatment.

Three distinct retinal phenotypes

The researchers first used full-field flash electroretinography (ffERG) in 205 male CD-1 mice. An ERG records the retina’s electrical response to flashes of light and can help distinguish activity from rods, which support low-light vision, and cones, which support daylight and color vision.

The mice separated into three groups:

  • Normal retinal function: 74 mice had expected ERG responses.
  • Cone dysfunction: 126 mice had absent cone responses, despite retaining other aspects of retinal function.
  • Severe retinal dysfunction: 5 mice had complete loss of measurable ERG responses.

This was a striking result. Cone dysfunction was not rare in this group of CD-1 mice; it was the most common phenotype identified. Additional visual evoked potential testing, which measures how visual signals reach the brain, supported the ERG findings. Mice with cone dysfunction lacked cone-driven visual responses, while mice in the severe group had no detectable retinal or visual-cortical responses.

A genetic link to Gnat2

Genetic testing confirmed mutations in Gnat2 in the affected groups. Gnat2 provides instructions for a protein involved in cone phototransduction, the process by which cones convert light into electrical signals. When this pathway is disrupted, cones may be unable to communicate light information effectively to the rest of the visual system.

This is directly relevant to achromatopsia research because defects in cone-specific genes are central to the condition. The study describes the CD-1 phenotype as achromatopsia-like rather than as a direct model of every feature of human achromatopsia. Nevertheless, the absent cone responses and Gnat2 association make these mice highly relevant to investigations of cone dysfunction.

Structure was preserved in some mice—but not all

The study also showed that similar-looking functional problems can be accompanied by very different structural changes.

Optical coherence tomography and OCT angiography allowed the team to examine retinal layers and blood-vessel networks. In the cone dysfunction group, overall retinal structure was retained. This suggests that cone-driven function could be profoundly impaired even when broad retinal anatomy appears relatively preserved.

In contrast, mice with severe retinal dysfunction had thinning of the outer retina, where photoreceptors are located, along with remodeling of the deep capillary plexus, a vascular network that helps support retinal tissue. Histologic analysis supported these imaging findings: cone dysfunction mice had isolated cone loss, whereas the severe group had combined loss of rods and cones.

These distinctions are important for treatment research. A retina with dysfunctional but structurally retained cone pathways may differ substantially from a retina in which photoreceptors have already been lost and the retinal blood supply has remodeled.

Glial activation and ganglion-cell loss

The investigators also found evidence of broader retinal stress. Both affected groups showed increased activation of glial cells. Microglia and macroglia help maintain retinal health, but their activation can signal inflammation or a response to injury and degeneration.

In addition, the number of Brn3a-positive retinal ganglion cells was significantly reduced in both the cone dysfunction and severe dysfunction groups compared with normal mice. Ganglion cells are the retina’s output neurons: they carry visual information toward the brain. Their reduction indicates that the consequences of spontaneous Gnat2-associated dysfunction may extend beyond cone photoreceptors.

Implications for therapy development

The study does not test a treatment, but it has clear implications for how treatments should be studied. Gene-based approaches for inherited cone disorders aim to restore or preserve function in affected photoreceptors. For such work, researchers need to know whether a mouse begins an experiment with normal retinal function, isolated cone dysfunction, or a more advanced disorder involving rods, blood vessels, glia, and ganglion cells.

Baseline ERG screening is therefore more than a technical detail. Without it, untreated comparison groups may contain mice with unrecognized cone dysfunction, and treatment results could be difficult to interpret. The findings also emphasize the value of combining functional tests with imaging, genetics, and tissue analysis rather than relying on a single measurement.

Looking ahead

This work strengthens the foundation for achromatopsia research by identifying hidden variability in a commonly used mouse strain. It also highlights an important principle for the field: successful translation depends on matching the right model to the right biological question. As researchers continue to investigate cone-directed treatments, rigorous phenotyping will help ensure that promising results reflect a therapy’s effects rather than unrecognized differences already present in the animals studied.