CoQ₁₀ deficiencies and MNGIE: Two Treatable Mitochondrial Disorders

Abstract

Background

Although causative mutations have been identified for numerous mitochondrial disorders, few disease-modifying treatments are available. Two examples of treatable mitochondrial disorders are coenzyme Q₁₀ (CoQ₁₀ or ubiquinone) deficiency and mitochondrial neurogastrointestinal encephalomyopathy (MNGIE).

Scope of Review

Here, we describe clinical and molecular features of CoQ₁₀ deficiencies and MNGIE and explain how understanding their pathomechanisms have led to rationale therapies. Primary CoQ₁₀ deficiencies, due to mutations in genes required for ubiquinone biosynthesis, and secondary deficiencies, caused by genetic defects not directly related to CoQ₁₀ biosynthesis, often improve with CoQ₁₀ supplementation. In vitro and in vivo studies of CoQ₁₀ deficiencies have revealed biochemical alterations that may account for phenotypic differences among patients and variable responses to therapy. In contrast to the heterogeneous CoQ₁₀ deficiencies, MNGIE is a single autosomal recessive disease due to mutations in the TYMP gene encoding thymidine phosphorylase (TP). In MNGIE, loss of TP activity causes toxic accumulations of the nucleosides thymidine and deoxyuridine that are incorporated by the mitochondrial pyrimidine salvage pathway and cause deoxynucleoside triphosphate pool imbalances, which, in turn cause mtDNA instability. Allogeneic hematopoetic stem cell transplantation to restore TP activity and eliminate toxic metabolites is a promising therapy for MNGIE.

Conclusions

CoQ₁₀ deficiencies and MNGIE demonstrate the feasibility of treating specific mitochondrial disorders through replacement of deficient metabolites or via elimination of excessive toxic molecules.

General Significance

Studies of CoQ₁₀ deficiencies and MNGIE illustrate how understanding the pathogenic mechanisms of mitochondrial diseases can lead to meaningful therapies.

Coenzyme Q₁₀

An essential component of the mitochondrial respiratory chain, coenzyme Q₁₀ (CoQ₁₀) shuttles electrons from complexes I and II and from electron transferring flavoprotein dehydrogenase (ETF-DH) to complex III (Figure 1)[1]. In addition, CoQ₁₀ is a potent antioxidant, and is a cofactor of dihydro-orotate dehydrogenase a critical enzyme for de novo pyrimidine biosynthesis.

Figure 1.

Coenzyme Q₁₀ biosynthetic pathway and electron transport role in the mitochondrial respiratory chain. Red arrows indicate coenzyme Q₁₀ biosynthetic pathway.

A lipophillic molecule, CoQ₁₀ is composed of a redox-active benzoquinone and a hydrocarbon tail comprised of 10 isoprenyl units. The reduced form is ubiquinone while the oxidized form is ubiquinol. CoQ₁₀ is synthesized within mitochondria through a complex pathway that is incompletely characterized in humans (Figure 1) [2]. The benzoquinone ring is derived from the amino acids phenylalanine and tyrosine while the decaprenyl side-chain is generated from acetyl-CoA via the mevalonate pathway. After condensation of para-hydroxybenzoate with the decaprenyl tail, the ring undergoes decarboxylation, hydroxylation, and methylation modifications to produce CoQ₁₀.

Screening for CoQ₁₀ Deficiency

The gold standard test for diagnosing CoQ₁₀ deficiency is high performance liquid chromatography (HPLC) measurement of ubiquinone in a skeletal muscle biopsy [3]. CoQ₁₀ levels reduced more than 2 standard deviations below control mean values are considered deficient [4]. Decreased activities of CoQ₁₀ dependent enzymes (e.g. NADH-cytochrome c reductase [complexes I+III] or succinate cytochrome c reductase [complex II+III]) strongly support the diagnosis of CoQ₁₀ deficiency; however, cases of mild CoQ₁₀ deficiency have shown normal activities of complexes I+III, II+III, or both. Plasma CoQ₁₀ level is dependent on concentration of lipoproteins, which act as carriers of CoQ₁₀ in the circulation and on dietary intake; therefore, plasma concentrations of CoQ₁₀ are not reliable for the diagnosis of CoQ₁₀ deficiency. Measurements of CoQ₁₀ level in blood mononuclear cells (MNCs)has detected deficiency in a small number of patients; however, correlations with muscle CoQ₁₀ measurements is a larger group of patients will be necessary to assess clinical utility of MNC ubiquinone levels. Cultured lymphoblastoid cell lines and primary fibroblasts have revealed CoQ₁₀ deficiency in most, but not all patients with ubiquinone deficiency in muscle [5–9]. Primary CoQ₁₀ deficiencies (due to defects of ubiquinone biosynthesis) cannot be distinguished from secondary deficiencies based on CoQ₁₀ levels.

Primary CoQ₁₀ Deficiencies

The first patients with CoQ₁₀ deficiency were reported in 1989 by Ogasahara and colleagues who described a pair of sisters, ages 12 and 14 years-old, with a mitochondrial disorder characterized by encephalopathy (mental retardation and seizures) and myopathy evident as elevated serum creatine kinase, and recurrent myoglobinuria [5]. Muscle biopsies showed ragged-red fibers, reduced biochemical activities of complexes I+III and II+III, and marked CoQ₁₀ deficiencies. Both patients improved markedly with CoQ₁₀ supplementation. Although 3 additional patients with similar encephalopathies and CoQ₁₀ deficiency have been reported [5, 10–12], causative molecular genetic defect has only been identified in one patient who has mutations in the ADCK3 (CABC1) gene, which encodes a kinase, which is thought to modulate ubiquinone biosynthesis [7, 10, 13].

An infantile multisystemic presentation of CoQ₁₀ deficiency has been associated with mutations in four genes required for ubiquinone biosynthesis. The syndrome typically manifests as a combination of encephalopathy and kidney disease that is usually a steroid-resistant nephrotic syndrome. This condition was originally reported by Rötig and colleagues, who described 3 siblings who initially manifested neurological signs including nystagmus, optic atrophy, sensorineural hearing loss, ataxia, dystonia, and weakness with progressive nephropathy that was fatal in one and treated with renal transplant in the other [14]. The two living siblings improved with CoQ₁₀ supplementation. In 2006, we described the first defect of CoQ₁₀ biosynthesis in two siblings who shared a homozygous missense mutation in the COQ2 gene, which encodes para-hydroxybenzoate-polyprenyl transferase [9]. The older sibling had steroid-resistant nephrotic syndrome that required renal transplantation and then a severe encephalopathy [15]. After a muscle biopsy at age 33 months revealed ubiquinone deficiency, treatment with high-dose CoQ₁₀ led to neurological improvements. The younger sister, at age 12 months, developed nephrotic syndrome, which improved with CoQ₁₀ supplementation [16, 17]. COQ2 mutations have been reported in four additional patients; a pair of siblings with fatal neonatal multisystemic disease, including nephrotic disease [18] and two other unrelated patients had early-onset glomerulopathy; one had only steroid-resistant nephrotic syndome that improved with CoQ₁₀ therapy while the other progressed to end-stage renal disease and epileptic encephalopathy leading to death at age 6 months [16].

Mutations in PDSS1 and PDSS2, genes encoding the two subunits of polyprenyl diphosphate synthase that produces the decaprenyl tail of CoQ₁₀, have been also been identified as causes of the infantile-onset form of CoQ₁₀ deficiency. In an infant who presented with neonatal hypotonia and subsequently developed nephrotic syndrome and Leigh syndrome, we identified compound heterozygous mutations in PDSS2 [8]. Despite CoQ₁₀ supplementation from age 3 months, the disease progressed to death at 8 months. In a consanguineous family, two siblings had CoQ₁₀ deficiency due to a homozygous PDSS1 mutation manifesting as a complex multisystemic disease with early onset deafness, encephaloneuropathy, obesity, livedo reticularis and cardiac valvulopathy [18].

Duncan and colleagues reported mutations in COQ9 in a newborn with generalized limb hypertonia, reduced truncal tone, lactic acidosis, renal tubulopathy and cardiomyopathy [6]. Despite CoQ₁₀ supplementation, he developed seizures and dystonia and died at age 2 years. In 2011, mutations in COQ6 have been identified in 11 patients from 5 different kindreds, with a phenotype similar to that observed in patients with mutations in COQ2 or PDSS2 [19]. All had nephrotic syndrome with onset in the first years of life, associated with sensorineural hearing loss in most cases, and some also had CNS involvement with seizures or ataxia.

In 2001, Musumeci and colleagues described six patients with the most common phenotype associated with CoQ₁₀ deficiency – juvenile-onset cerebellar ataxia and atrophy variably associated with peripheral neuropathy, seizures, mental retardation, muscle weakness, and hypogonadism [20]. In a follow-up study of 135 patients with genetically undefined cerebellar ataxia, 13 (~10%) had CoQ₁₀ deficiency. ADCK3 mutations have been reported in 12 patients (8 families) with juvenile-onset cerebellar ataxia and atrophy making this gene the most frequent cause of primary CoQ₁₀ deficiency [7, 13, 21]. CoQ₁₀ supplementation has been associated with mild subjective improvement in patients with ADCK3 mutations.

Secondary CoQ₁₀ deficiencies

Of 6 patients with cerebellar ataxia and CoQ₁₀ deficiency reported by Musumeci and colleagues, three had APTX gene mutations known to cause ataxia oculomotor apraxia 1 (AOA1). APTX encodes aprataxin, a protein involved in single- and double-strand DNA break repair and localized to nuclei and mitochondria [22]. Low levels of CoQ₁₀ were subsequently found in muscle and fibroblasts of some patients with typical AOA1 [23]. However in other AOA1 patients, CoQ₁₀ levels may be normal or even elevated. Clear genotype-phenotype correlations have yet to be established, and the mechanism of CoQ₁₀ deficiency in this disorder remains unknown.

Mutations in the ETFDH gene (encoding the electron transfer flavoprotein dehydrogenase) were identified in patients who had been initially reported as having the isolated myopathy phenotype of CoQ₁₀ deficiency [24]. Biochemical investigations showed increased CK and lactate levels, with low carnitine and a characteristic acylcarnitine profile on tandem mass spectrometry suggestive of multiple acyl-CoA dehydrogenase deficiency (MADD) [25]. Muscle biopsies exhibited mild to severe vacuolar changes, increased lipid content, ragged-red fibres, focal or diffuse SDH deficiency, COX-negative fibres, combined complex I/II+III deficiency, and decreased level of CoQ₁₀ in muscle (50% of normal). The mechanism of CoQ₁₀ deficiency in MADD remains unknown and it does not appear to be a universal feature of this disorder, since some patients have been demonstrated to have normal CoQ₁₀ levels [26, 27].

Secondary CoQ₁₀ deficiency has also been reported in a number of patients with primary mtDNA mutations. For example, levels of CoQ₁₀ in muscle from 25 patients with mitochondrial encephalopathies, mostly due to mtDNA mutations, were significantly lower than controls [28] while a multicenter study of 76 patients with heterogeneous mitochondrial diseases detected CoQ₁₀ deficiency in 28 (37%) of which nine had pathogenic mtDNA mutations [29]. Cultured skin fibroblasts from patients with MELAS had secondary CoQ₁₀ deficiency that triggered autophagy [30]. However the signficance of CoQ₁₀ deficiency in these patients remain unclear.

Cell Culture Models of CoQ₁₀ Deficiency

In vitro models have revealed insights into the pathogenesis of CoQ₁₀ deficiency. Initial studies in cultured fibroblasts from two siblings with infantile-onset CoQ₁₀ deficiency showed mild respiratory chain defects, but did not detect elevated superoxide anions, lipid peroxidation, or apoptosis-mediated cell death [31]. Lopez-Martin and colleagues showed that COQ2 mutant fibroblasts require uridine to sustain growth and proposed that deficiency of CoQ₁₀ impaired de novo pyrimidine biosynthesis because of the dependence of dihydro-orotate dehydrogenase on ubiquinone [32]. In the same two cell lines plus two other cells lines from patients with genetically undefined CoQ₁₀ deficiency, Rodriguez-Hernandez and colleagues noted increased autophagy of mitochondria [33].

We initially observed that PDSS2 mutant fibroblasts have 12% residual CoQ₁₀ and markedly reduced ATP synthesis, but do not show increased reactive oxygen species (ROS) or oxidative stress, while COQ2 mutant fibroblasts with 30% CoQ₁₀ content have mild defects of ATP synthesis and significantly increased ROS production as well as oxidation of lipids and proteins and cell death [34]. We have extended the studies to other patient cell lines with variable degrees of CoQ₁₀ deficiency due to different mutations in COQ2, ADCK3, and COQ9, and have observed similar correlations between levels of CoQ₁₀ and cellular phenotype: 10–15% or >60% residual CoQ₁₀ levels are not associated with significant ROS production whereas 30–50% residual CoQ₁₀ content is associated with maximal ROS production and cell death [35]. Thus, in vitro, severe deficiency of CoQ₁₀ causes bioenergetic defects, while moderate deficiency produces ROS, oxidative damage, and cell death.

Our in vitro therapeutic studies of patients’ fibroblasts with mutations in COQ9, COQ2 and PDSS2 have revealed that the prolonged pharmacokinetics (>24 hours) of CoQ₁₀ required to reach the mitochondrial respiratory chain critically affects restoration of energy status of human CoQ₁₀ deficient cells [36]. Additionally, we demonstrated that short tail ubiquinone analogues cannot substitute CoQ₁₀ in the mitochondrial respiratory chain of human CoQ₁₀ deficient fibroblasts, thus revealing the importance of the decaprenyl tail most likely through interactions with complex I[37, 38]. In contrast, oxidative stress and cell death were attenuated by the administration of lipophilic (idebenone and CoQ₂) and hydrophilic (vitamin C) antioxidants.

Mouse Model of CoQ Deficiency

In the early 1970s, a spontaneous mutation (kd) in mouse colony was noted to cause autosomal recessive kidney disease [39]. Homozygous kd/kd mice appear healthy for at least the first 8 weeks of life, but histological examination of the kidneys at about 12 weeks reveals a mononuclear cell infiltrate and tubular dilation. Over time the entire kidney is involved and the mice die of renal failure [40]. The mutation responsible for the kd/kd phenotype was identified as a V117M amino acid substitution in Pdss2 – murine homolog of human PDSS2 [41, 42]. Although young homozygous Pdss2^kd/kd mice have a normal content of CoQ₉ (the predominant ubiquinone in rodents) and CoQ₁₀ in kidney, they fail to elevate levels of CoQ₉ and CoQ₁₀ in kidneys as wild-type mice do at about age 40 days. The onset of proteinuria and kidney disease ensues several weeks after the normal increase in CoQ₉ and CoQ₁₀ fails to occur [42]. Conditional knockout mice showed that proteinuria and kidney disease could be recapitulated when Pdss2 was deleted in podocytes (Podocin/cre,Pdss2^loxP/loxP) but not when targeted to renal tubular epithelium, monocytes or hepatocytes [41].

Treatment of CoQ10 Deficiencies

Supplementation with oral CoQ₁₀ (10–30 mg/kg/day in children and 1,200–3,000 mg/day in adults) has been effective in patients with COQ2 mutations, especially for the neurological and renal manifestations of this disorder. In contrast, poor responses to CoQ₁₀ supplementation have been observed in patients with PDSS2 and COQ9 mutations.

Response to treatment is also variable in patients with secondary CoQ₁₀ deficiency syndromes. In patients with myopathy due to ETFDH mutations, riboflavin supplementation was generally followed by clinical improvement and normalisation of CK and lactate levels with further positive effects in few patients additional supplementation with CoQ₁₀ [24]. In three siblings with cerebellar ataxia due to a homozygous stop codon mutation in APTX and secondary CoQ₁₀ deficiency in muscle, high-dose CoQ₁₀ supplementation (up to 3,000 mg daily) was associated with improved ambulation in all and resolution of seizures in the affected sister [43].

The reasons for the disparate responses to CoQ₁₀ are not known. A likely contributing factor is the poor bioavailability of CoQ₁₀; less than 5% of oral CoQ₁₀ reaches plasma in humans and rodent studies have demonstrated low uptake of CoQ by tissues with little or no detectable uptake by brain except in aged rats [1, 44–46]. Thus, the blood-brain barrier appears to impair central nervous system uptake of CoQ₁₀. Hence, tissue involvement may influence response to CoQ₁₀ supplementation as illustrated by the ubiquinone-responsive CoQ₁₀ deficient patients with COQ2 mutations who manifested nephrotic syndrome and in one case had stroke-like episodes suggesting involvement of vascular structures, which is likely to be amenable to CoQ₁₀ supplementation [9, 15–17]. In contrast brain tissue was clearly affected in the CoQ₁₀-refractory patients with Leigh syndrome due to PDSS2 mutations and refractory seizures caused by COQ9 deficiency [6, 8]. Furthermore, because CoQ is highly lipophilic, exogenously administered CoQ will be integrated into plasma and other cellular membranes before reaching the inner mitochondrial membrane. This notion is supported by cell culture studies [36]. Because of the poor bioavailability and delayed mitochondrial uptake of ubiquinone, early rather than late supplementation is likely to successfully treat CoQ₁₀ deficiency.

Mitochondrial Neurogastrointestinal Encephalomyopathy (MNGIE)

Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE; OMIM 603041) is a rare autosomal recessive disease caused by mutations in the TYMP gene encoding thymidine phosphorylase (TP; E.C. 2.4.2.4). Seventeen years ago, we described MNGIE as a clinically distinct disorder characterized by extraocular muscle weakness causing ptosis and ophthalmoplegia, peripheral neuropathy, gastrointestinal dysmotility causing severe cachexia, leukoencephalopathy, and mitochondrial abnormalities including multiple deletions of mitochondrial DNA (mtDNA) in skeletal muscle[47]. Because of the prominent gastrointestinal dysfunction and cachexia, patients are often misdiagnosed with anorexia nervosa, inflammatory bowel disease, or celiac disease[48]. The disease is relentlessly progressive and fatal with an average age-at-onset of 19-years-old and an average age-at-death of 37-years-old[48–50]. We mapped the disease locus to chromosome 22q13.32-tel, identified the causative gene[51, 52], characterized the molecular pathogenesis[53–59], and generated a mouse model[60].

TYMP mutations cause severe TP deficiency

Since our 1999 report describing TYMP mutations as the cause of MNGIE[52], we have received more than 1000 blood samples to screen for TP deficiency. To date, we have identified 102 MNGIE patients from 79 families with a spectrum of 54 TYMP mutations. In addition, 36 molecularly confirmed patients have been reported by others[59, 61–77]. The 138 MNGIE patients are of diverse ethnic origins and males and females are equally affected.

To confirm the pathogenicity of the TP gene mutations, we used a fixed-time spectrophotometric assay to measure TP activity in buffy coat based on techniques developed in the 1950s[52, 53, 58, 78–80]. Our TP assay measures thymine produced by cell extracts incubated at 37 °C for one hour in the presence of excess thymidine[53]. The assay is highly reliable based on measurement of day-to-day imprecision of frozen buffy coat aliquots from a single blood collection of a healthy individual (n=20; range 638–889 nmol/h/mg-prot; coefficient of variation=10%)[53]. All typical MNGIE patients showed less than 10% TP activity (10±15 nmol/h/mg-prot; mean±standard deviation) relative to the mean TP activity in controls (634±217)[53], thus, the assay is highly sensitive in identifying patients. In asymptomatic TYMP mutation carriers, TP activity was approximately 35% (222±89) of control mean, which is consistent with the homodimeric composition of TP. We also identified three patients with late-onset MNGIE (age-at-onset ~40 years) and milder TP deficiency (~15% residual activity) than typical patients[54]. Thus, there is a relationship between TP activity and clinical phenotypes: <10% of normal activity causes typical MNGIE, 10–20% residual activity produces less-severe late-onset MNGIE, and >30% activity does not cause overt disease.

TP deficiency causes accumulations of Thd and dUrd in plasma and tissues

To assess the consequences of loss of TP function, we measured thymidine and deoxyuridine in plasma and autopsy tissues using high-performance liquid chromatography (HPLC). Thymidine peaks were identified by retention time and confirmed by disappearance of the peak with TP treatment. Normal controls had no detectable thymidine in plasma (<0.05 μM, n=20) while all patients had markedly elevated thymidine (Thd) levels (8.6±3.4 μM, mean±standard deviation; normal controls <0.05), and deoxyuridine (dUrd) (14.2±4.4 μM, normal controls <0.05)[53, 55, 58]. The late-onset MNGIE patients had less dramatic increased in Thd (1.03±0.45) and dUrd (2.9±1.9) Interestingly, asymptomatic relatives with heterozygous TYMP mutations and reduced TP activity had no detectable thymidine in plasma (n=14). This finding has therapeutic implications because it indicates that 35% residual TP activity is sufficient to eliminate circulating thymidine.

Post-mortem analyses of tissues from five MNGIE patients (ages at death 29–39 years old), showed accumulation of dThd (range=1.9–80 pmol/mg-tissue) and dUrd (range 3–48 pmol/mg-tissue) whereas control tissues had no detectable nucleosides[81]. The highest concentrations of dThd and dUrd were detected in affected tissues peripheral nerve, small intestine, occipital white matter, and liver while kidney and heart, two unaffected organs and skeletal muscle, a less severely affected tissue, had lower levels of nucleosides.

Increased levels of Thd and dUrd cause deoxynucleotide pool imbalances and mtDNA instability

We hypothesized that elevated levels of Thd and dUrd nucleosides in MNGIE patients cause deoxynucleotide triphosphate (dNTP) pool imbalances, which, in turn, cause mtDNA instability (Figure 2). This notion is supported by in vitro studies. Dr. Christopher Mathew’s lab demonstrated that Hela cells exposed to 50 μM Thd for four hours had unbalanced mitochondrial dNTP pools with increased dTTP (260%) and dGTP (220%) and decreased dATP (78%) and dCTP (43%)[82]. After eight months in 50 μM Thd, the Hela cells developed multiple deletions of mtDNA by PCR. Similarly, Dr. Vera Bianchi’s group demonstrated that cultured quiescent fibroblasts, maintained in 10–40 μM thymidine, had ~3.5-fold increases in cytosolic dTTP and ~3.7-fold increases in mitochondrial dTTP and ~50% depletion of mtDNA without deletions or point mutations[83, 84].

Figure 2.

Molecular pathomechanism of MNGIE. Loss of thymidine phosphorylase (TPase) activity causes toxic accumulations of thymidine and deoxyuridine nucleosides in plasma and tissues and dNTP pool imbalances, which, in turn impair mtDNA replication.

We have observed multiple deletions, depletion, and site-specific somatic mtDNA point mutations in cultured fibroblasts, blood, and tissues of MNGIE patients[47, 56, 57]. This selective effect on mtDNA may be explained by two factors. First, mitochondrial nucleotide pools are probably more vulnerable to the toxic effects of excessive dThd than are nuclear nucleotide pools because mtDNA is more dependent on dThd salvage than is nuclear DNA, which relies upon the de novo synthesis of dThd particularly in post-mitotic cells[83, 85, 86]. Second, human mitochondria apparently lack an effective DNA mismatch repair system[87, 88].

Our studies of TP/UP double knockout (TP^−/−UP^−/−) mice have also supported the hypothesized pathomechanism in MNGIE[60]. As a consequence of TP deficiency, the mutant mice showed 4–65-fold elevated tissue levels of Thd and dUrd relative to in wild-type littermates. Brain of TP^−/−UP^−/− mice showed increased dTTP (+60%) and decreased dCTP (−40%) in mitochondria, ~50% depletion of mtDNA with ~30% reductions of biochemical activities of mitochondrial respiratory chain complexes I and IV, and multiple vacuoles in white matter and showed increased MRI T2-signal in cerebral white matter.

Defects of mtDNA cause mitochondrial pathology in MNGIE

In our MNGIE patients, we found clear evidence of mitochondrial dysfunction. Specifically, resting serum lactic acid levels were elevated in 12/20 patients (60%), while skeletal muscle biopies showed defects of mitochondrial respiratory chain enzymes in 7/14 individuals (50%), cytochrome c oxidase-deficient muscle fibers in 17/18 (94%), depletion of mtDNA in 12/15, and multiple mtDNA deletions in 9/18 samples tested by Southern blot analysis[49]. In contrast to muscle, other tissues do not have abnormal mtDNA bands by Southern blot analysis, but have detectable mtDNA deletions by polymerase chain reaction [57]. Furthermore, in autopsies of six MNGIE patients, we observed marked atrophy, mitochondrial proliferation, and mtDNA depletion in the external muscularis propria layer of the stomach and small intestine indicating a visceral myopathy[59, 89] due to lack of mtDNA.

Rationale for AHSCT for MNGIE

Our preliminary studies of MNGIE indicate that therapies to eliminate the toxic levels of thymidine and deoxyuridine from patients will be effective treatments for the disease[55, 58]. We have demonstrated that hemodialysis can transiently eliminate these nucleosides from blood, but was not effective as a long-term treatment in one patient[90]. To catabolize nucleosides, we have administered platelets (which contain abundant TP) to two patients[91]. In both individuals, plasma thymidine levels were reduced for 24–48 hours. The transient effects of platelet infusions suggest that permanent TP replacement therapy through AHSCT should be an effective treatment for MNGIE. While AHSCT will not restore total body TP levels to normal, our analyses of asymptomatic TYMP mutation carriers show that 30–50% of normal TP activity is sufficient to prevent MNGIE.

AHSCT has been successfully applied to other rare metabolic diseases caused by enzyme deficiencies[92, 93]. For example, over 200 bone marrow transplantations in patients with Hurler syndrome have resulted in resolution of the enlarged liver and spleen, improved cardiac function, reversibility of airway obstruction, and, in some cases, stabilization of neurological deterioration. In general, AHSCT in patients with lysosomal storage diseases leads to improvement of visceral organs with variable effects on neurological symptoms. Furthermore, AHSCT appears to be effective in purine nucleoside phosphorylase (PNP) deficiency, a disease that biochemically parallels MNGIE because loss-of-function mutations in the PNP gene lead to accumulations of purine nucleosides. The disease presents with immunological and neurological dysfunction and AHSCT can restore the immune system and may stabilize neurological functions[94, 95]. Based on positive results in other metabolic disorders, assessment of AHSCT therapy for MNGIE is merited.

AHSCT for MNGIE

As of 2010, of the 11 patients who underwent AHSCT, 5 were alive after successful transplant. All show normalization of blood TPase activity and reductions of plasma thymidine and deoxyuridine to barely-detectable levels; slight clinical improvements were noted after one year. A patient transplanted in 2005 exhibits markedly improved gastrointestinal function; she discontinued parenteral nutrition and gained 3 kilograms; her neuropathy has also improved as assessed clinically and by electrophysiological studies (Hirano, unpublished observation). A prospective clinical trial of AHSCT for MNGIE is being planned.

Conclusions

Because mitochondrial diseases are clinically and genetically heterogeneous disorders, no single therapeutic approach will address the diverse biochemical pathogenic mechanisms. CoQ₁₀ deficiency is an example of a mitochondrial syndrome caused by lack of a key metabolite, which can be successfully treated through supplementation. In contrast, MNGIE is a prototypical mitochondrial disease caused by toxic accumulations of metabolites, dThd and dUrd, which, if eliminated, can lead to clinical improvements. Although both disorders are treatable by normalizing paucity or excess of small molecules, not all CoQ₁₀ deficient patients have responded to ubiquinone supplementation and initial AHSCT for MNGIE have been successful in fewer than half of the patients. Thus, further studies will be required to optimize therapy for these readily treatable mitochondrial diseases.

Highlights.

• CoQ₁₀ deficiencies can be primary (biosynthetic defects) or secondary (other causes)

• CoQ₁₀ deficiencies often improve with high-dose CoQ₁₀ supplements

• MNGIE is caused by thymidine phosphorylase deficiency

• MNGIE can improve after allogeneic hematopoetic stem cell transplantation