De Novo GRID2 Variant as a Cause of Ataxia with Oculomotor Apraxia and Alpha-Fetoprotein Elevation

Bi-allelic pathogenic variants in GRID2 have been initially associated to an autosomal recessive form of spinocerebellar ataxia, namely SCAR18. Subsequently, few monoallelic cases have been described. Here we present a new subject harboring a novel de novo heterozygous GRID2 missense variant presenting with progressive ataxia together with cerebellar atrophy and, for the first time, alpha-fetoprotein (AFP) elevation. We retrospectively collected data of the patient followed at our clinic. Genetic analysis was performed through clinical exome sequencing with an in-house in-silico ataxia-related genes panel. Variant effect prediction was performed through in silico modeling. The patient had normal psychomotor development except for mild fine and gross motor impairment. In adolescence, he started presenting dysarthria and progressive ataxia. Blood tests showed significant AFP elevation. Brain MRI showed cerebellar atrophy mainly involving the vermis. The novel de novo heterozygous GRID2 (c.1954C>A; p.Leu652Ile) missense variant was disclosed. This variant is located within a highly conserved site with low tolerance to variation and it is predicted to cause protein structure destabilization. GRID2 expression appears to be influenced by other genes related with ataxia and AFP elevation, like ATM and APTX, suggesting a possible shared mechanism. This additional patient increases the scarce literature and genotypic spectrum of the GRID2-related ataxia and evidences a fairly homogeneous phenotype of ataxia with oculomotor abnormalities for the autosomal-dominant form. Alfa-fetoprotein elevation is a novel finding in this condition and this data must be confirmed in larger case-series to definitively state that GRID2-related ataxia can be included among ataxias with AFP increase.