A novel de novo ATP2B1 variant causes autosomal dominant intellectual developmental disorder 66 by disrupting calcium homeostasis via impaired membrane trafficking

Heterozygous pathogenic variants in <i>ATP2B1</i> (encoding PMCA1) cause autosomal dominant intellectual developmental disorder 66 (MRD66; OMIM #619910). To date, only 12 pathogenic <i>de novo ATP2B1</i> variants have been reported in MRD66. This study aimed to identify the genetic etiology in a Chinese infant with a neurodevelopmental disorder characterized by early-onset seizures and global developmental delay (GDD) and functionally characterize a novel <i>ATP2B1</i> missense variant. Trio-based whole-exome sequencing revealed a heterozygous <i>de novo ATP2B1</i> variant (c.2140A>C, p.Thr714Pro) in the proband. The proband presented with infantile spasms, GDD (Gesell Developmental Quotient: 65-74), and severe growth restriction (height/weight <-2 SD). To investigate the variant's pathogenicity, the wild-type (WT) and mutant <i>ATP2B1</i> constructs, N-terminally tagged with mScarlet, were transfected into HEK293T cells. Confocal imaging demonstrated profound cytoplasmic mislocalization of the p.Thr714Pro mutant protein, contrasting sharply with the characteristic plasma membrane localization of WT ATP2B1. Measurement of intracellular Ca²⁺ levels using Fluo-4 AM showed a significant 2.07-fold increase in basal Ca²⁺ levels in cells expressing the mutant compared to WT. This finding expands the spectrum of ATP2B1 variants associated with MRD66 and confirms calcium dyshomeostasis as the core pathomechanism. This case of MRD66 demonstrates a very early onset of seizures, consistent with the recognized phenotypic variability and the critical role of PMCA1 in early neurodevelopment.