Molecular mechanisms of mitochondrial dysfunction upon the CMT-disease-associated mutations in the mitochondrial ThDP-regulated OADH and PDK3, are considered below in view of the therapeutic potential of vitamin B1.
3.2.1. Isoenzyme 3 of Kinase of the ThDP-Dependent Pyruvate Dehydrogenase
Mutation in the
PDK3 gene causes the substitution R158H in PDK3, representing the genetic cause of an X-linked dominant form of axonal CMT disease (CMTX6) (OMIM 300905) [
29,
30]. Males are more severely affected by this mutation than females. The symptoms onset occurrs within the first 13 years, when the muscle weakness and atrophy, predominantly in lower limbs, and sensory deficits are detected. Some of the affected men have moderate sensorineural hearing loss and perturbed auditory brainstem response.
The substitution R158H leads to a hyper-active PDK3 exhibiting an increased affinity to PDC, compared to the wild-type enzyme. As a result, higher levels of phosphorylation and inactivation of cellular PDC are observed in patients carrying this mutation [
30]. Metabolic characteristics of fibroblasts and iPSC-derived motor neurons of the patients reveal failures in energy production and mitochondrial function.
In vivo models where the R158H-substituted PDK3 ortholog, known as PDHK2 in
C. elegans, is knocked-in, or the wild-type and mutated PDK3 are overexpressed in the GABA-ergic motor neurons, recapitulate deficits in mitochondrial function and synaptic neurotransmission, observed in the cellular studies [
13]. Besides, the models reveal such characteristics of CMT disease as the locomotion defects and signs of progressive neurodegeneration.
ThDP is known to inhibit kinases of pyruvate dehydrogenase [
24,
25,
26]. Hence, patients with CMT disease induced by hyperactive PDK3, may benefit from administration of the PDK3 inhibitor ThDP, that is well-known in medicine and pharmacology as cocarboxylase. Pharmacological lipophilic forms of thiamine (vitamin B1), such as sulbutiamin (enerion) and benfotiamin, are also widely available. Remarkably, ThDP inhibition of PDK3 would increase the activity of pyruvate dehydrogenase by decreasing the enzyme phosphorylation level. Simultaneously, as a coenzyme of pyruvate dehydrogenase, ThDP may activate PDC-catalyzed oxidation of glycolytic product pyruvate by an independent mechanism, such as the saturation of the pyruvate dehydrogenase with its coenzyme. As a result, the thiamine administration may counteract the hyperactivity of the R158H-substituted PDK3 by inhibiting PDK3 and activating PDC, potentially relieving the CMT disease caused by this mutation.
3.2.2. Molecular Mechanisms of CMT Disease Caused by Mutations in the DHTKD1-Encoded ThDP-Dependent 2-Oxoadipate Dehydrogenase
Axonal type of CMT disease type 2Q (OMIM 615025) is described upon heterozygous loss-of-function mutation of the
DHTKD1 gene encoding for the mitochondrial ThDP-dependent OADH [
31]. The mutation c.1455T>G in exon 8 of the
DHTKD1 gene causes preterm termination of the transcription at Tyr485 codon and a 2-fold decrease in the OADH mRNA. The resulting autosomal dominant form of CMT disease is characterized by muscle atrophy, predominant weakness of the lower limbs, decreased or absent deep tendon reflex, and mild to moderate sensory impairment. The symptom onset is detected from 13 to 25 years.
Mice models of homozygous knockout of the
DHTKD1 gene mimic CMT disease phenotype, demonstrating anatomic and functional features of peripheral neuropathy with characteristic perturbations of the motor and sensory functions, axonal degeneration and muscle atrophy. The phenotype is accompanied by serious metabolic abnormalities, with significant increases in the urine levels of 2-oxoadipate and its transamination product 2-aminoadipate [
32].
Pathogenic homozygous
DHTKD1 mutations are associated with strongly increased levels of the OADH substrate 2-oxoadipate and its derivatives (2-aminoadipate, 2-hydroxyadipate), causing severe neurological manifestations, delayed development, chronic diseases of respiratory pathways and early muscle atrophy [
33,
34,
35]. Nevertheless, in some mutants clinical manifestations are absent even when biochemical parameters are perturbed [
34,
35].
Pathogenicity of heterozygous mutations in the
DHTKD1 gene varies. The mutations may be associated with neuropathies, such as CMT disease and similar states, with autoimmune disease, such as eosinophilic esophagitis, or with epilepsy, but often the same mutations may be present in the asymptomatic persons [
31,
36,
37,
38]. Most of the nucleotide substitutions in the
DHTKD1 gene are asympthomatic [
39]. Thus, understanding molecular basis of pathophysiological impact of the
DHTKD1-encoded OADH is not straightforward, with the impact potentially involving interactions with other pathways which are considered below.
Current level of characterization of the structure-function relationship in the OADH molecule allows one to predict pathogenicity of the amino acids substitutions in the enzyme active site and protein-protein interfaces. Structures of recombinant OADH [
39,
40] and homology modelling of the enzyme complexes with its ligands [
41,
42] reveal the impact of OADH mutations in the active site and dimeric interface of the enzyme on the enzyme-catalyzed reaction. Besides, some information on the protein residues involved in heterologous interactions upon formation of the OADH multienzyme complex, where OADH catalyzes oxidative decarboxylation of 2-oxoadipate with generation of glutarylCoA and NADH [
43,
44,
45], enables predictions of functional impairments due to mutations affecting the complex formation. Nevertheless, several considerations are important to note, regarding the varied pathophysiological impact of the same DHTKD1 mutations in different carriers of a mutation. First, the catalytic function of OADH is redundant and could well be substituted by an ubiquitously expressed OADH isoenzyme, 2-oxoglutarate dehydrogenase (OGDH).
In vitro, OGDH catalyzes oxidative decarboxylation of 2-oxoadipate at a rate up to 50% of that of oxidative decarboxylation of 2-oxoglutarate, with OGDH expression usually exceeding that of OADH [
46]. Second, in animal tissues OADH undergoes post-translational modifications strongly affecting the enzyme structure, compared to that of the characterized recombinant enzyme, that may correspond to the existence of a yet unidentified enzyme function [
47]. Third, biochemical effects of the OADH mutations, i.e. increases in the 2-oxo/aminoadipate levels, are not necessarily accompanied by clinical symptoms [
34]. These considerations suggest that the (patho)physiological impact of OADH relies on the OADH-specific interplay with other proteins/pathways rather than solely OADH catalytic function.
The assumption is supported by the data on the most characterized associations of OADH with insulin resistance, obesity and type II diabetes. The data not only reveal a significance of genetic background for the OADH biological impact, but also suggest perturbed glucose metabolism as molecular mechanism underlying the OADH-associated neurological disorders. Indeed, increased 2-oxoadipate and/or 2-aminoadipate levels in the mice knockouts of
DHTKD1 [
32,
48,
49] cause insulin resistance phenotype [
32]. Liver expression of
DHTKD1 is the major regulator of the level of 2-aminoadipate, the transamination sibling of the OADH substrate 2-oxoglutarate, in serum [
50]. Dependent on external (diet) and genetic (different alleles) factors, the
DHTKD1 gene expression also correlates with the levels of glucose, cholesterol and the diabetic status [
50]. In the blood plasma of humans and mice, glucose levels are negatively correlated with those of 2-aminoadipate, while the levels of 2-aminoadipate and insulin correlate positively [
51]. When β-cells of pancreas are treated with 2-aminoadipate, more insulin is secreted, corresponding to increased insulin secretion upon the accumulation of 2-aminoadipate in pancreas, as observed in mice on a high-fat diet [
51]. At the same time, the mice knockouts of
Csf2 gene (controlling the function of granulocytes and macrophages) are obese without diabetic symptoms. Remarkably, these mice exhibit increased expression of
DHTKD1 and decreased 2-aminoadipate level, compared to the wild-type mice [
52].
DHTKD1 expression is also increased by a natural compound mangiferin preventing obesity [
53]. Vitamin B1 (thiamine), which is a precursor of the OADH coenzyme ThDP, upregulates OADH activity in the rat cerebellum [
54].
Thus, relatively low expression of
DHTKD1 may be an early indicator of obesity and type II diabetes, while increased expression of
DHTKD1 is associated with clinical improvement of such states [
55]. Pharmacological regulation (e.g., mangiferin, vitamin B1) or genetic factors (exemplified by knockout of Csf2 gene) may increase
DHTKD expression. Compensating for functional impairments in the OADH mutants, this conditional increase in the
DHTKD expression may contribute to heterogeneity of the mutant
DHTKD1 phenotypes. The available data thus imply that pharmacological up-regulation of the OADH expression and/or activity by administration of vitamin B1 may be of therapeutic value.
If phenotypic manifestations of the
DHTKD1 mutations, particularly in CMT disease, depend on the function of other genes, what could be these other genes? To answer this question, a closer look on metabolic pathways potentially affected by OADH function, is required (
Figure 2).
Participating in the production of glutaryl-CoA for protein glutarylation, OADH is involved with metabolic regulation by post-translational modifications through the acylation of protein lysine residues. Recent discovery of glutarylation of the pyruvate dehydrogenase complex, a key player in the oxidative glucose metabolism, provides a mechanistic link between the OADH function and oxidative glucose metabolism [
17], characterized in the association studies discussed above. Depending on pathophysiological state, a short-term inhibition of OADH in the rat brain may cause a long-term increase in the enzyme expression, associated with increased glutarylation of the brain proteins and elevated expression of deglutarylase sirtuin 5 [
17].
Furthermore, existence in mammalian tissues of an N-terminus-truncated isoform of OADH, lacking the protein part involved into the formation of the multienzyme complex, but comprising the active site [
47], agrees with our earlier suggestion [
43], that OADH may also have some functions outside of the multienzyme complex. An example of such functions is glyoxylate detoxication that involves non-oxidative decarboxylation of 2-oxoacids by their ThDP-dependent dehydrogenases.
Finally, functioning in the tryptophan degradation pathway, OADH is linked to
de novo biosynthesis of NAD+ and its signaling derivatives from tryptophan (
Figure 2). The interplay is supported by perturbed nicotinamide (vitamin B3) homeostasis upon the OADH inhibition [
56]. Perturbed NAD+ metabolism is also shown in independent animal and cellular models of OADH impairment [
32,
57], further exposing the role of NAD+ in CMT disease arising upon the
DHTKD1 mutations. In fact, mutations and knockouts of the
DHTKD1 gene decrease NAD pool and energy metabolism [
32,
57]. Combined with the data on perturbed pool of vitamin B3 in the OADH-inhibited cells [
56] and neurological manifestations of impaired tryptophan catabolism [
58], the available data argue for significance of the NAD+-dependent pathways in the pathophysiological contribution of OADH function. Hence, administration of vitamin B3, especially in the form of its derivative nicotinamide riboside which helps overcoming the NAD+ biosynthetic hurdles [
59,
60], may be promising to alleviate the neurological symptoms in CMT disease caused by the
DHTKD1 mutations. The underlying molecular mechanisms are considered in more details below.