Enzymatic testing sensitivity, variability and practical diagnostic algorithm for pyruvate dehydrogenase complex (PDC) deficiency

Abstract

Pyruvate dehydrogenase complex (PDC) deficiency is a major cause of primary lactic acidemia in children. Prompt and correct diagnosis of PDC deficiency and differentiating between specific vs generalized, or secondary deficiencies has important implications for clinical management and therapeutic interventions. Both genetic and enzymatic testing approaches are being used in the diagnosis of PDC deficiency. However, the diagnostic efficacy of such testing approaches for individuals affected with PDC deficiency has not been systematically investigated in this disorder. We sought to evaluate the diagnostic sensitivity and variability of the various PDC enzyme assays in females and males at the Center for Inherited Disorders of Energy Metabolism (CIDEM). CIDEM data were filtered by lactic acidosis and functional PDC deficiency in at least one cell/tissue type (blood lymphocytes, cultured fibroblasts or skeletal muscle) identifying 186 subjects (51% male and 49% female), about half were genetically resolved with 78% of those determined to have a pathogenic PDHA1 mutation. Assaying PDC in cultured fibroblasts in cases where the underlying genetic etiology is PDHA1, was highly sensitive irrespective of gender; 97% (95% confidence interval [CI]: 90%-100%) and 91% (95% CI: 82%-100%) in females and males, respectively. In contrast to the fibroblast-based testing, the lymphocyte- and muscle-based testing were not sensitive (36% [95% CI: 11%-61%, p=0.0003] and 58% [95% CI: 30%-86%, p=0.014], respectively) for identifying known PDC deficient females with pathogenic PDHA1 mutations. In males with a known PDHA1 mutation, the sensitivity of the various cell/tissue assays (75% lymphocyte, 91% fibroblast and 88% muscle) were not statistically different, and the discordance frequency due to the specific cell/tissue used for assaying PDC was 0.15±0.11. Based on this data, a practical diagnostic algorithm is proposed accounting for current molecular approaches, enzyme testing sensitivity, and variability due to gender, cell/tissue type used for testing, and successive repeat testing.

Keywords: Biochemical testing; Diagnostic algorithm; PDHA1; Pyruvate dehydrogenase complex deficiency; Sensitivity; Variability.

Copyright © 2017 Elsevier Inc. All rights reserved.

Figures

Fig. 1

PDC assay sensitivity for blood lymphocytes, cultured fibroblasts and skeletal muscles. A and B, proportion of subjects with PDC activity deficiency in at least one cell/tissue sample and lactic acidosis (All-CIDEM group). C and D, proportion of subjects with a pathogenic PDHA1 mutation and functional PDC deficiency in at least one cell/tissue type (PDHA1-Only group). 95% Confidence intervals are indicated along with statistical significance (p <0.05) when applicable.

Fig. 2

Recommended algorithm for the diagnosis of PDC deficiency. “Enzymatic testing” is an all-inclusive descriptive term for various enzyme-based testing options noted below (after the initial pursuit of PDC testing in a specific cell/tissue). Enzymatic (functional) testing, when molecular testing is negative or a variant of uncertain significance (VUS) in a specific gene is identified, may include assaying PDC (in a different cell/tissue type than the initial testing), KDC, pyruvate carboxylase (PC), phosphoenolpyruvate carboxykinase (PEPCK), propionyl-CoA carboxylase (PCC), succinyl-CoA ligase (SUCL), branched-chain α-ketoacid dehydrogenase complex (BCKDC), electron complex chain complexes (ETC), and/or oxidative phosphorylation (OxPhos) activities. Please refer to Discussion for when to suspect PDC deficiency and what tests to pursue in a patient with lactic acidosis and hyperalaninemia in order to differentiate between primary and secondary PDC deficiencies. For brevity, left side colored blocks “A” (pink), “B” (blue) and “C” (green) represent steps (algorithmic options) that are repeated (as smaller pink, blue and green blanks) within the algorithm. “Positive TGP” implies identifying a pair of known pathogenic mutations, VUSs or a combination of both in a known autosomal PDC-associated gene; for the X-linked PDHA1 gene, either a single known pathogenic mutation or VUS is expected. Abbreviations used are as follows: TAT, turn-around-time; TGP, targeted gene panel; and WES/WGS, whole exome/genome sequencing.