Mitochondrial Hepatopathies: Advances in Genetics, Therapeutic Approaches, and Outcomes

Cellular mitochondria play important roles in the production of energy required by human cells, thermogenesis, calcium and iron homeostasis, innate immune responses, production of reactive oxygen species, and programmed cell death (apoptosis).¹ Approximately 1000 nuclear genes encoding mitochondrial proteins have been identified to date. A mutation in any of these genes has the potential to give rise to monogenic or primary mitochondrial disorders.² Mitochondrial dysfunction is also associated with many common human conditions, including cardiac disease,³ diabetes,⁴ cancer,⁵ epilepsy,⁶ obesity,⁷ and degenerative diseases such as Parkinson and Alzheimer's diseases.^8,9

A unique feature of mitochondria in mammalian cells is the presence of a separate genome, mitochondrial DNA (mtDNA), which is distinct from nuclear genes.¹ The respiratory chain peptide components are encoded by both nuclear and mtDNA genes.¹ Thirteen essential polypeptides are synthesized from the small (16.5-kb), circular, double-stranded mtDNA, and nuclear genes encode more than 70 respiratory chain subunits and an array of enzymes and cofactors required to maintain mtDNA.¹

Mitochondrial hepatopathies, disorders in which dysfunction of hepatocyte mitochondria plays a key role in the pathogenesis of liver injury or failure,^10-12 are divided into primary and secondary disorders.^10,11 Mitochondrial hepatopathies as a whole have been characterized only relatively recently,¹⁰ and the identification of more types is anticipated. Primary mitochondrial hepatopathies occur when a mitochondrial protein, transfer RNA, or ribosomal RNA is miscoded by a mutation in either a nuclear gene or an mtDNA gene. Secondary mitochondrial hepatopathies are conditions in which mitochondria are the targets of endogenous or exogenous toxins. Examples include Reye syndrome, copper and iron overload conditions, drugs (eg, salicylates, reverse-transcriptase inhibitors, antimycin A) and toxins (eg, ethanol, cyanide), cholestasis, nonalcoholic steatohepatitis, and α-1 antitrypsin deficiency. ^10,11

In primary mitochondrial hepatopahies, liver involvement is often part of multiorgan manifestations (Table I).^1,10,12 Much of our current knowledge about mitochondria has come from studying patients with respiratory chain disorders, which compose a growing number of individually rare syndromes, each presenting in a unique and often devastating way.¹²

Table I. Phenotypic classification of primary mitochondrial hepatopathies.

RC (electron transport) defects (OXPHOS)

Neonatal liver failure

Complex I deficiency

Complex IV deficiency (SCO1 mutations)

Complex III deficiency (BCS1L mutations)

Co-enzyme Q deficiency

Multiple complex deficiencies (transfer and elongation factor mutations)

mtDNA depletion syndrome (DUGOK, MPV17, POLG, SUCLG1, C10orf2/Twinkle mutations)

Later-onset liver dysfunction or failure

Alpers-Huttenlocher disease (POLG mutations)

Pearson's marrow pancreas syndrome (mtDNA deletion)

Mitochondrial neurogastrointestinal encephalopathy (TYMP mutations)

NNH (MPV17 mutations)

Fatty acid oxidation defects

Long-chain 3 hydroxyacyl-coenzyme A dehydrogenase

Carnitine palmitoyltransferase I and II deficiencies

Carnitine-acylcarnitinetranslocase deficiency

Urea cycle enzyme deficiencies

Electron transfer flavoprotein and electron transfer flavoprotein dehydrogenase deficiencies

Phosphoenol pyruvate carboxykinase (mitochondrial) deficiency; nonketotic hyperglycemia

Citrin deficiency; neonatal intrahepatic cholestasis caused by citrin deficiency (SLC25A13 mutations)

OXPHOS, oxidative phosphorylation; NNH, Navajo neurohepatopathy; RC, respiratory chain. Adapted with permission.¹⁰

Prevalence of Mitochondrial Hepatopathies

Epidemiologic studies from northern Europe and Australia have shown a population prevalence of all mitochondrial diseases ranging from 1.6 to 6.57/100 000.^13-17 Estimating the population prevalence of mitochondrial hepatopathies is difficult, however. In the Australian childhood mitochondrial population prevalence study, 3 of the 107 patients (2.8%) had jaundice.¹⁷ In a retrospective, single-center French study, 13 of the 57 patients (22.8%) with neonatal mitochondrial cytopathies had hepatic manifestations,¹⁸ consistent with previous studies indicating liver involvement in approximately 20% of patients with mitochondrial cytopathy.

Mitochondrial hepatopathies are important causes of acute liver failure (ALF) in childhood, particularly in infancy.^19-21 A retrospective French review of infants (aged ≤12 months) with ALF identified mitochondrial disease in 22.5% (17 of 80).¹⁹ In a British retrospective review, 8 of 39 infants aged ≤12 months with ALF (20.5%) had a mitochondrial disorder.²⁰ In a prospective pediatric ALF study in North America and the United Kingdom, 5.4% (8 of 148) of infants aged <90 days had mitochondrial disease identified as the cause of ALF.²¹

Clinical Features

A striking feature of mitochondrial disorders is their clinical heterogeneity, ranging from single-organ disease to multiorgan involvement (Table II).^1,22 Generally, the clinical presentations in mitochondrial hepatopathies can be divided into neonatal liver failure, later-onset disease (eg, Alpers-Huttenlocher disease), and variable later presentations, which may include hepatic enlargement with elevated liver enzyme concentrations, hepatic steatosis, liver failure, and cholestasis.²³ Initial clinical signs of mitochondrial liver dysfunction may be subtle and undetected.²³

Table II. Genotypic classification of primary mitochondrial hepatopathies and organ involvement.

Gene	Respiratory chain complex	Protein	Function	Hepatic histology	Other organs involved	Clinical features
Class 1A: mtDNA genes
Deletion33	Multiple (Pearson)			Steatosis, fibrosis	Kidney, heart, CNS, muscle	Sideroblasticanemia, variable thrombocytopenia and neutropenia, persistent diarrhea
MPV1734	I, III, IV	Mitochondrial inner membrane		Steatosis	CNS, muscle, gastrointestinal tract	Adult-onset multisystemic involvement: myopathy, ophthalmoplegia, severe constipation, parkinsonism
Class 1B: nuclear genes mtDNA depletion syndromes
DGUOK29	I, III, IV	dGK, mitochondrial nucleotide salvaging	dGK, with TK2, functions to maintain the supply of dNTPs for mtDNA synthesis	Steatosis, fibrosis	Kidneys, CNS, muscle	Nystagmus, hypotonia, renal Fanconi syndrome, acidosis
MPV1730	I, III, IV	Mitochondrial inner membrane	mtDNA maintenance and regulation of OXPHOS; absence or malfunction causes OXPHOS failure and mtDNA depletion	Steatosis, fibrosis	CNS, PNS	Hypotonia
SUCLG131	I, III, IV	α -subunit of succinate coenzyme A ligase	Catalyzes conversion of succinyl coenzyme A and ADP or GDP to succinate and ATP or GTP	Steatosis	Kidneys, CNS, muscle	Myopathy, sensorineural hearing loss, respiratory failure
POLG125	I, III, IV	mtDNA polymerase γ	Polymerase γ activity essential for mtDNA replication and repair	Steatosis, fibrosis	CNS, muscle	Liver failure preceded by neurologic symptoms, intractable seizures, ataxia, psychomotor regression
C10orf2/Twinkle32	I, III, IV	Hexameric DNA helicase	Together with single-stranded mtDNA binding protein and polymerase γ, plays a key role in mtDNA replication	Steatosis	CNS, muscle	Infantile-onset spinocerebellar ataxia, loss of skills
Nuclear assembly fact —respiratory complexes
BCS1L35	III (GRACILE)	Assembly protein of complex III (ubiquinol cytochrome c reductase)	Encodes an assembly factor/chaperone that incorporates the Rieske iron sulphur protein into complex III		CNS ±, muscle ±, kidneys	Fanconi type renal tubulopathy
SCO136	IV	Copper chaperone of complex IV (cytochrome c oxidase)	Transfers copper from Cox17p to cyctochrome c oxidase subunits I and II	Steatosis, fibrosis	Muscle
Nuclear transfer factor genes
TRMU37	I, III, IV	Transfer RNA-modifying enzyme	Enzyme required for the 2-thio modification of 5-taurinomethyl-2-thiouridine transfer RNA-lysine	Steatosis, fibrosis		Infantile liver failure with subsequent recovery
EFG138	I, III, IV	Mitochondrial translation elongation factor	Precise function unknown	Steatosis	CNS	Severe, rapidly progressive encephalopathy
EFTu39	I, III, IV	Isoform of α-subunit of elongation factor-1 complex	Responsible for enzymatic delivery of aminoacyl transfer RNAs to the ribosome	Unknown	CNS	Severe lactic acidosis, rapidly fatal encepaphalopathy

CNS, central nervous system; dGK, deoxyguanosine kinase; TK2, thymidine kinase 2; PNS, peripheral nervous system; ADP, adenosine diphosphate; GDP, gluanosine diphosphate; ATP, adenosine triphosphate; GTP, gluanosine triphosphate; GRACILE, growth restriction, aminoaciduria, cholestasis, iron overload, lactic acidosis, and early death.

Neonatal Liver Failure

In previous studies, this presentation was noted in 13 of 57 infants (22.8%) with neonatal mitochondrial cytopathies with hepatic manifestation,¹⁸ and in 8 of 32 infants (25.0%) with mitochondrial oxidative phosphorylation disorders with neonatal presentation.²⁴ Approximately 12% of the affected infants were born preterm, and another 30% had intrauterine growth retardation.²⁴ Onset may occur within the first weeks to months of life, and may manifest as mild hepatic dysfunction that progresses to liver failure, vomiting, poor feeding, hypotonia, seizures, and lethargy.^10-12 In these patients, laboratory investigations generally reveal an elevated serum lactate level (>2.5 mmol/L), increased lactate/pyruvate molar ratio (>25 mol/mol), hypoglycemia, prolonged prothrombin time, hyperammonemia, and variably elevated aminotransferase and bilirubin concentrations.¹¹

Alpers-Huttenlocher Disease

The inheritance of Alpers-Huttenlocher disease is autosomal recessive, although clinically more males are affected than females.²⁵ Symptom onset occurs between age 3 months and 8 years (Table II).²⁶ The typical clinical course includes intractable seizures and liver failure after a period of developmental regression. In some children, seizures may be the initial manifestation.^25,26 The disease is usually fatal within 3 years of onset; however, some patients survive into their teens and even early adulthood. Genetic heterogeneity and exposure to environmental factors (including medications) may explain the wide variation in clinical presentation. Assays of respiratory chain complex concentrations reveal complex 1 deficiency,²⁷ and genotyping shows 2 pathologic mutations in the POLG gene.^25,27

Variable Later Presentations

Clinical presentations include failure to thrive with elevated aminotransferase levels, lactic acidosis, hepatic steatosis, portal hypertension and cirrhosis, acute (or recurrent) liver failure presenting at any age, and intestinal pseudo-obstruction.^23,24,28 Any of the foregoing features can present at any age, not infrequently in combination with neurologic, muscular, renal, pancreatic, cardiac, hearing, visual, and other involvement.^23,24,29

Genetics of Mitochondrial Hepatopathies

More than 1000 nuclear genes and 37 mtDNA genes encode mitochondrial proteins, ribosomal RNA, and transfer RNA.^1,28 To date, mutations in more than 228 protein-encoding nuclear DNA genes and 13 mtDNA have been identified as human mitochondrial disease-causing genes.¹ Mitochondrial hepatopathies result more frequently from autosomal recessive mutations in nuclear genes than from mutations in the mtDNA genome.¹² At least 14 nuclear gene defects have been linked to primary mitochondrial hepatopathies; these can be classified into several categories: mtDNA depletion syndrome, nuclear assembly factors—respiratory chain complex, and nuclear translation factor genes (Table II).

mtDNA Depletion Syndrome

In this syndrome, nuclear gene mutations responsible for mtDNA replication and maintenance lead to impaired synthesis of mtDNA or reduced capacity of salvaging activity of deoxyridbonucleotide triphosphate, resulting in abnormally low amounts of mtDNA relative to the normal levels of nuclear DNA.^11,12 This results in marked impairment of respiratory chain complexes I, III, and IV, increased oxidative stress, and adenosine triphosphate depletion. The clinical presentation is diverse, but generally 3 clinical forms are recognized: myopathic, encephalomyopathic, and hepatocerebral.¹² Mutations in various genes have been implicated,²⁸ including DGUOK,²⁹ MPV17,³⁰ SUCLG1,³¹ POLG1,²⁵ and C10ORF2/Twinkle³² (Table II).

Depletion of mtDNA also can result from large deletions of mtDNA, affecting the production of several respiratory chain components, such as in the Pearson marrow-pancreas syndrome.¹ Clinical presentation depends on the length of mtDNA deletions.³³ More recently, multiple mtDNA deletions also have been shown in MPV17 mutations, causing an adult-onset multisystemic disorder with steatohepatopathy (Table II).³⁴

Nuclear Assembly Factors—Respiratory Chain Complex

Nuclear genes coding for respiratory chain assembly factors can cause hepatopathy. Lactic acidosis, liver involvement, and Fanconi type renal tubulopathy are common when the complex III assembly factor gene BCS1L harbors mutations. The most severe form is the GRACILE syndrome (growth restriction, aminoaciduria, cholestasis, iron overload, lactic acidosis, and early death).³⁵ Mutations in SCO1 cause complex IV deficiency with severe acidosis, hypotonia, hypoglycemia, enlarged liver, and liver failure with hepatic steatosis.³⁶

Nuclear Translation Factor Genes

Mutations in nuclear translation factor genes (TRMU, EFG1, and EFTu) of the respiratory chain enzyme complexes have been linked to neonatal liver failure or dysfunction and chronic liver disease.^37-39

Common Forms of mtDNA Depletion Syndrome

The most common genetic causes of mtDNA depletion are discussed below. Details of other genetic causes of mitochondrial hepatopathies are presented in the references cited in Table II.

DGUOK Deficiency

The most common form of hepatocerebral mtDNA depletion, DGUOK deficiency can present as neonatal liver failure along with elevated serum transferrin saturation and serum ferritin and alpha-fetoprotein levels, resembling neonatal hemochromatosis.³⁹ Death due to liver failure generally occurs at age 2-18 months⁴⁰; however, there have been reports of spontaneous survival in the absence of significant neurologic involvement.⁴¹ Hepatocellular carcinoma has been reported in survivors; thus, surveillance is required for survivors with native liver.⁴¹

POLG1 Deficiency

The enzyme DNA polymerase γ is essential for mtDNA (but not nuclear DNA) replication and repair. POLG1 mutations are a common cause of mtDNA depletion,¹⁵ resulting in low concentrations of respiratory chain complexes I, III, and IV¹⁵ and presenting as Alpers-Huttenlocher syndrome or neonatal liver failure.⁴² Liver transplantation (LT) is generally contraindicated because of its systemic nature and the inevitable progression of severe central nervous system lesions and symptoms despite LT.¹⁰ Treatment of complex seizures in patients with Alpers-Huttenlocher syndrome with valproic acid may precipitate ALF.⁴³ In these patients, the respiratory chain enzyme activities and mtDNA content in skeletal muscle may be normal; thus, liver tissue analysis or genotyping is necessary to establish the diagnosis.⁴³

A prospective study of Drug-Induced Liver Injury Network data over a 5-year period identified 17 patients (including 10 adults) with valproate hepatic toxicity.⁴⁴ Eight of the 14 patients with probable valproate hepatotoxicity had a heterozygous POLG mutation, and 1 patient was homozygous.⁴⁴ It is currently recommended that POLG genotying be considered before administering valproate as an anticonvulsant in children under age 2 years.⁴⁴ The success rate of LT after sodium valproate-induced liver failure is dismal.⁴⁵ According to the Organ Procurement and Transplantation Network data on pediatric LT in the US, the 1-year survival rate post-LT in patients with valproate-induced ALF was 18%, compared with 69% in those with non–valproate-induced ALF.⁴⁵ Thus, LT is not indicated in patients with Alpers-Huttenlocher syndrome with valproate-induced liver failure.

MPV17 Disease

Originally described in 4 infants with fatal liver failure and hypotonia, autosomal recessive mutations in MPV17 were subsequently linked to Navajo neurohepatopathy, a mtDNA depletion syndrome comprising sensorimotor neuropathy associated with neonatal cholestasis, hepatic steatosis, cirrhosis, or ALF in full-blooded Native Americans of the Navajo tribe.³⁰ MPV17 disease also has been described as isolated hepatic involvement in infants. Extrahepatic manifestations can be severe and terminal and do not respond to LT. Hepatocellular carcinoma has been reported in a young Caucasian child with a MPV17 mutation.³⁰ More recently, a fatal hepatocerebral form of mitochondrial hepatopathy caused by a MPV17 mutation was described in a Brazilian child.⁴⁶

Medical Therapy for Mitochondrial Hepatopathies

Currently, no curative therapy for mitochondrial hepatopathies is available.^1,47 The greatest challenge to performing randomized controlled trials in the affected population is the heterogeneity of mitochondrial disorders. In neonatal ALF, supportive therapy includes prevention of hypoglycemia and correction of acidosis and hyperammonemia.¹⁰ In children with chronic liver disease, management includes feeding a formula enriched with medium-chain triglycerides, providing a diet with 30%-40% of energy as fat, preventing hypoglycemia, and providing adequate fat-soluble vitamin supplementation.¹⁰

Various pharmacologic therapies have been advocated for mitochondrial disorders. However, a recent Cochrane systemic review examining the efficacies of coenzyme Q10, dichloroacetate, creatine monohydrate, and a whey-based, dietary supplement failed to show any clear evidence supporting their use in treating mitochondrial disorders.⁴⁸ Nonetheless, some disorders appear to respond to treatment, such as coenzyme Q deficiency.⁴⁹ Clearly, future research should identify novel agents for testing in homogenous study populations with clinically relevant endpoints.⁴⁸

Potential future therapies for mitochondrial disorders include genetic therapy, enzyme replacement therapy, small-molecule therapy, bypass of electron transport complexes, nutritional therapies, and for myopathies, exercise and conditioning.¹ Recently, interest has focused on the potential of small molecules, a novel class of small peptide molecules that selectively target the inner mitochondrial membrane and protect mitochondrial function, as a mitochondrial protective agent.⁵⁰

LT for Mitochondrial Hepatopathies

Long-term outcomes of patients with primary mitochondrial hepatopathies are very poor.^19,51 In the study of Cormier-Daire et al⁵¹ of 22 patients with mitochondrial hepatopathies, the mortality rate was higher in those with the neonatal-onset form compared with those with the delayed-onset form (66% vs 38%). Another French study reported a mortality rate of 94%.¹⁹ In patients with DGUOK mutations, survival into childhood may occur in the absence of neurologic features.⁴¹

The role of LT in treating mitochondrial hepatopathies remains controversial.¹⁰ In many mitochondrial diseases, post-LT progression of neuromuscular, cardiac, or other symptomatology may prove fatal despite excellent results of the LT procedure, and thus the outcomes after LT have not been encouraging (Table III).^30,40,49-52 In the 54 patients with respiratory chain disease who underwent LT reported in the literature to date (many of whom did not have obvious extrahepatic involvement before LT), the overall survival rate was only 30% (Table III). However, there is a subset of patients who do not exhibit progression of respiratory chain dysfunction in other organs after LT. It is hoped that with vigilant pre-LT evaluation and selection to exclude neurologic and extrahepatic involvement, post-LT deterioration and death can be avoided in most, if not all, patients. However, it should be emphasized that in young patients with ALF and some forms of mitochondrial hepatopathy, even thorough evaluation might not detect extrahepatic involvement.

Table III. Outcome of LT in mitochondrial hepatopathies secondary to RC disorders.

Authors	Number of patients (age)	Presentation	Diagnosis	Survival	Follow-up	Comments
Sokal et al (1999; Europe and US)52	11 (1-7 mo)	ALF	RC enzyme assay and clinical	5 of 11	5 mo to 8 y	All 3 with diarrhea and vomiting died
Durand et al (2001; France)19	5 (<1 y)	ALF	RC enzyme assay and clinical	2 of 5	3.5 years	No extrahepatic involvement pre-LT
Dubern et al (2001; France)53	5 (<1 y)	ALF	RC enzyme assay and clinical	2 of 5	Not specified	No extrahepatic involvement pre-LT
Rabinowitz et al (2004)54	1 (neonate)	Liver failure, neurologic symptoms	DGUOK genotyping	Died		Died a few months after LT
Dimmock et al (2008; literature)41	10 (<10 mo; one 3 y)	Liver failure, neurologic symptoms	DGUOK genotyping	2 of 10		No benefit of LT if neurologic features present
El-Hattab etal (2010; literature and US)30	10 (infancy)	Liver failure, neurologic symptoms	MPV17 genotyping	5 of 10; 2 of 3 NNH, 3 of 7 other presentations	4-21 y	Patients with NNH had progression of neurologic features post-LT
Iwama etal (2010; literature)55	12 (<10 mo; one 3 y)	Liver failure	RC enzyme assay and clinical	0 of 7	4 mo to 5 y	No benefit of LT if neurologic features present
Total	54			16 (30%)

Indications for LT include patients with ALF and respiratory chain involvement limited to the liver only. Significant involvement of the central nervous system, heart, skeletal muscle, retina, and/or intestines usually is predictive of poor outcome and thus are considered contraindications to transplantation. Outcome of LT in patients with DGUOK disease without significant neurologic involvement may be better than that in patients with other forms of mitochondrial hepatopathy.⁴⁰ LT is usually contraindicated in patients with mitochondrial disease caused by mutations in POLG and MPV17.^30,52 Family members should be counseled about the possibility of other organ involvement after LT that may be potentially devastating and fatal. The most difficult cases are those in which a young infant presents in ALF without definable extrahepatic involvement and with a previously normal history. Ideally, a thorough evaluation with magnetic resonance imaging/magnetic resonance spectroscopy (and possibly electroencephalography) of the brain, echocardiography, retinal examination, cerebrospinal fluid analysis, respiratory chain testing of muscle and/or liver, and genotyping for the likely genetic defects should be performed if time permits.

Summary

An ever-increasing number of nuclear gene mutations have been identified as responsible for mitochondrial hepatopathies. A diagnosis of such a hepatopathy is suggested by lactic acidosis, hepatic steatosis, liver failure without markedly increased aminotransferase levels, and features of extrahepatic involvement. At present, genotyping is available for many of the mutations identified. Although medical therapy is currently inadequate, selective LT may be life-saving in carefully selected cases, with special caution regarding the presence of extrahepatic disease.

Glossary

ALF

Acute liver failure

LT

Liver transplantation

mtDNA

Mitochondrial DNA