Novel Mutations in OCTN2 Identified in Newborn Screening Exhibit Reduced Function and Lead to Carnitine Transporter Deficiency

Abstract

Newborn screening programs have revolutionized the diagnosis and treatment of genetic disorders. A compelling example is found in SLC22A5 encoding OCTN2, a bi‐directional plasma membrane transporter, which acts in the influx carnitine and efflux of drugs. Newborns with low plasma carnitine levels and biallelic SLC22A5 variants are diagnosed with Carnitine Transporter Deficiency (CTD), a potentially fatal disorder that can be treated with L‐carnitine supplementation. Here, we sought to validate the role of novel OCTN2 variants in CTD via functional characterization of mutant transporters in vitro.In a recent study (PMID 31364285), three novel OCTN2 missense variants were identified in newborn screening. Following site‐directed mutagenesis and transient transfection in HEK293T cells, we performed isotopic uptake assays using 14C‐carnitine. Sodium phenylbutyrate, a chemical chaperone previously shown to increase function of other OCTN2 variants, was used to treat transfected cells. Welch’s two‐sample t‐test was used to determine significance.Two novel missense variants, p.Asp165Glu and p.Gly473Val, exhibited substantial reduction of carnitine transport, retaining less than 5% of wildtype transporter function (p = 9.63*10−6 and 1.02*10−5, respectively). In contrast, only moderate reduction was observed for p.Met433Thr, which retained 61% wildtype function (p = 0.021). Sodium phenylbutyrate showed trends (which were not significant) toward increasing the function of all three variants, suggesting that intracellular errors of protein folding, processing, or trafficking may contribute to their reduced function.Our studies validate the role of three novel variants in CTD, none of which have been associated previously with the disorder. The studies are consistent with the idea that even partial loss‐of‐function alleles may be disease‐causing for CTD. Finally, we suggest that sodium phenylbutyrate represents a potential modulator of OCTN2 function, with further studies warranted to confirm effects on function and membrane localization of OCTN2 variants.Support or Funding InformationThis work is supported by the Chan Zuckerberg Biohub.