We present a male patient born at 38-wk gestation with rhizomelic shortening of extremities, hepatomegaly, ventriculomegaly, heart failure, severely depressed left ventricular function, biventricular hypertrophy, and biatrial enlargement. Additional physical findings included anteriorly displaced anus, vertebral anomalies, and brachydactyly. The patient\'s cardiac malformations led to persistent hypotension, sinus tachycardia, and multiorgan failure in the absence of arrhythmias. Rapid whole-exome sequencing was ordered on day of life (DOL) 8. The patient\'s family elected to withdraw supportive care, and he passed away that evening. Whole-exome sequencing returned posthumously and identified a variant in NAA10, E100K. The genotype-phenotype was closest to Ogden syndrome or amino-terminal acetyltransferase deficiency. Typical features of this rare X-linked syndrome include progeroid appearance, failure to thrive, developmental delays, hypotonia, and cardiac arrhythmias. Other family members were tested and the patient\'s mother, who has a history of mild intellectual disability, as well as a daughter born later, were identified as carriers. All carriers showed no cardiac findings. The carrier sister has manifested developmental delay and cortical atrophy. Protein modeling, evolution, dynamics, population variant assessments, and immunoprecipitation depict the deleterious nature of the variant on the interactions of NAA10 with NAA15 These findings had subsequent implications for posthumous diagnosis of the index patient, for female carriers, and regarding family planning. We highlight how these rapid genetic tests and variant characterization can potentially lead to informed decision-making between health-care providers and family members of patients with critical or lethal conditions when treatment options are limited.

N(α)-acetylation is a common protein modification catalyzed by different N-terminal acetyltransferases (NATs). Their essential role in the biogenesis and degradation of proteins is becoming increasingly evident. The NAT hNaa50p preferentially modifies peptides starting with methionine followed by a hydrophobic amino acid. hNaa50p also possesses N(ε)-autoacetylation activity. So far, no eukaryotic NAT has been mechanistically investigated. In this study, we used NMR spectroscopy, bisubstrate kinetic assays, and product inhibition experiments to demonstrate that hNaa50p utilizes an ordered Bi Bi reaction of the Theorell-Chance type. The NMR results, both the substrate binding study and the dynamic data, further indicate that the binding of acetyl-CoA induces a conformational change that is required for the peptide to bind to the active site. In support of an ordered Bi Bi reaction mechanism, addition of peptide in the absence of acetyl-CoA did not alter the structure of the protein. This model is further strengthened by the NMR results using a catalytically inactive hNaa50p mutant.