Functional and molecular studies in primary carnitine deficiency.
Level 4 - case-series / case-control
Retrospective diagnostic case series and in vitro functional validation study without a prospective randomized comparator.
PubMed 28841266 · doi:10.1002/humu.23315
What was done
Carnitine transport was measured in cultured skin fibroblasts from 358 subjects referred for suspected primary carnitine deficiency. In 95 of 140 subjects who had carnitine transport reduced to 20% or less of normal, all 10 exons and flanking regions of the SLC22A5 gene were sequenced. In addition, 92 missense variants (from this cohort and previous reports) were expressed in Chinese hamster ovary (CHO) cells to directly quantify transport activity, and results were compared against PolyPhen-2 and SIFT in silico prediction tools.
What was found
Fibroblast carnitine transport was 20% or less of normal in 140 of 358 referred individuals (39.1%). Sequencing of the coding and flanking regions in 95 transport-deficient subjects identified causative SLC22A5 variants in 84% of alleles, leaving 16% of alleles without an identifiable coding variant. Expression assays in CHO cells showed that 73 of 92 missense variants (79.3%) impaired carnitine transport. In silico prediction algorithms (PolyPhen-2 and SIFT) correctly predicted functional effects in approximately 80% of cases.
Why it matters
Coding-region sequencing misses approximately one out of six causative alleles in biochemically confirmed primary carnitine deficiency, and computational prediction tools have a 20% error rate. Direct functional carnitine transport assays in fibroblasts remain essential to confirm or exclude a diagnosis when molecular testing is inconclusive.
Limits
Sequencing was restricted to 10 exons and immediate flanking regions, leaving deep intronic or non-coding regulatory variants uncharacterized in the 16% unexplained alleles. Only 95 of the 140 transport-deficient subjects underwent genetic sequencing, and clinical outcomes or phenotypes were not reported in the abstract.
Cited by
- supports Primary or secondary carnitine deficiency manifests clinically with elevated triglycerides and hypoketonemia due to impaired fatty acid metabolism.