De novo variants in UPF1 associated with intellectual disabilities: Human genetic and functional evidences using Drosophila model

Nonsense-mediated mRNA decay represents a biologic clearing system against aberrant mRNAs harboring nonsense and frameshift mutations and depends on three factors, UPF1, UPF2, and UPF3 (UPF3A, UPF3B). While germline pathogenic variants of UPF3B and UPF2 are known to be associated with neurodevelopmental disorders, germline variants in UPF1 have not been reported, until date, as being associated with any human disorders. Herein, we report two unrelated patients with de novo UPF1 variants. Patient 1 was a 5-year-old girl with intellectual disabilities, frontal bossing, hypertelorism, high frontal hairline, and thin upper lip. Patient 2 was a 2-year-old female child with intellectual disabilities and similar features. Trio exome analysis revealed a de novo heterozygous variant in UPF1 in both the patients (Patient 1: NM_002911.4): c.949_951del, p.(Asp317del); Patient 2: c.1984G>A, p.(Asp662Asn)). We conducted experiments using Drosophila models to evaluate the functional relevance of these UPF1 variants. Enforced expression of the wild-type Upf1 allele under the control of the pan-neuronal nSyb-GAL4 driver caused mortality, mostly at the pupal stage, but still yielded adult flies. By contrast, expression of the Asp294del (Asp317del in humans) variant caused embryonic or early larval lethality and that of the Asp643Asn (Asp662Asn in humans) caused third instar larval lethality; neither produced pupa nor adult fly. Thus, the developmental defects caused by the variants, especially Asp294del, were more severe than those caused by the wild-type allele. These observations suggest that both variants are deleterious mutations. In conclusion, germline variants in UPF1 are associated with intellectual disabilities in humans.