GNAT2

Abstract

GNAT2 encodes guanine nucleotide-binding protein G(t) subunit alpha-2, which is a part of the transducin complex that amplifies and transmits the visual signal in cone photoreceptors. Autosomal recessive mutations in GNAT2 cause approximately 1–2% of achromatopsia [1, 2]. Patients with GNAT2-related achromatopsia exhibit nystagmus beginning in early infancy, photophobia, abnormal color vision, and poor visual acuity (VA) (usually worse than 20/200–20/400). Progressive deterioration of VA may be observed, while nystagmus may improve in some patients [3]. Color vision testing is usually abnormal, with rod monochromacy observed in many patients, although some patients may retain some color vision [3–5]. One study has suggested that the retention of color vision may be related to splice site mutations that sometimes result in functional protein product [5]. Fundoscopy usually reveals a normal retinal appearance, though some patients exhibit an abnormal foveal reflex or atrophy (Fig. 33.1a–d) [3]. Full-field ERG testing typically reveals varying degrees of abnormal cone function along with normal rod function, although scotopic a-wave amplitudes are reduced in some patients [3–5]. Microperimetry may show central scotomas; SD-OCT usually reveals well-maintained foveal architecture without hypoplasia but may exhibit a hyporeflective zone at the fovea (Fig. 33.1e–f) [6].