Samar H. Ibrahim, William F. Balistreri
A wide variety of mitochondrial disorders are associated with liver disease. Hepatocytes contain a high density of mitochondria because the liver, with its biosynthetic and detoxifying functions, is highly dependent on adenosine triphosphate. Defects in mitochondrial function can lead to impaired oxidative phosphorylation, increased generation of reactive oxygen species, impairment of other metabolic pathways, and activation of mechanisms of cellular death.
Mitochondrial disorders can be divided into primary, in which the mitochondrial defect is the primary cause of the disorder, and secondary, in which mitochondrial function is affected by exogenous injury or a genetic mutation that affects nonmitochondrial proteins (see Chapter 105.4 ). Primary mitochondrial disorders can be caused by mutations affecting mitochondrial DNA (mtDNA) or by nuclear genes that encode mitochondrial proteins or cofactors (see Chapter 383 —Table 383.3 and Table 388.1 ). Specific patterns may be noted (Table 388.2 ). Secondary mitochondrial disorders include diseases with an uncertain etiology, such as Reye syndrome; disorders caused by endogenous or exogenous toxins, drugs, or metals; and other conditions in which mitochondrial oxidative injury may be involved in the pathogenesis of liver injury.
Table 388.1
Genotypic Classification of Primary Mitochondrial Hepatopathies and Organ Involvement
| GENE | RESPIRATORY CHAIN COMPLEX | HEPATIC HISTOLOGY | OTHER ORGANS INVOLVED | CLINICAL FEATURES |
|---|---|---|---|---|
| Deletion | Multiple (Pearson) | Steatosis, fibrosis | Kidney, heart, CNS, muscle | Sideroblastic anemia, variable thrombocytopenia and neutropenia, persistent diarrhea |
| MPV17 | I, III, IV | Steatosis | CNS, muscle, gastrointestinal tract | Adult-onset multisystemic involvement: myopathy, ophthalmoplegia, severe constipation, parkinsonism |
| DGUOK | I, III, IV | Steatosis, fibrosis | Kidneys, CNS, muscle | Nystagmus, hypotonia, renal Fanconi syndrome, acidosis |
| MPV17 | I, III, IV | Steatosis, fibrosis | CNS, PNS | Hypotonia |
| SUCLG1 | I, III, IV | Steatosis | Kidneys, CNS, muscle | Myopathy, sensorineural hearing loss, respiratory failure |
| POLG1 | I, III, IV | Steatosis, fibrosis | CNS, muscle | Liver failure preceded by neurologic symptoms, intractable seizures, ataxia, psychomotor regression |
| C10orf2/Twinkle | I, III, IV | Steatosis | CNS, muscle | Infantile-onset spinocerebellar ataxia, loss of skills |
| BCS1L | III (GRACILE) | CNS ±, muscle ±, kidneys | Fanconi-type renal tubulopathy | |
| SCO1 | IV | Steatosis, fibrosis | Muscle | |
| TRMU | I, III, IV | Steatosis, fibrosis | Infantile liver failure with subsequent recovery | |
| EFG1 | I, III, IV | Steatosis | CNS | Severe, rapidly progressive encephalopathy |
| EFTu | I, III, IV | Unknown | CNS | Severe lactic acidosis, rapidly fatal encephalopathy |
CNS, central nervous system; GRACILE, growth restriction, aminoaciduria, cholestasis, iron overload, lactic acidosis, and early death; PNS, peripheral nervous system.
From Lee WS, Sokol RJ: Mitochondrial hepatopathies: advances in genetics, therapeutic approaches and outcomes, J Pediatr 163:942–948, 2013 (Table 2, p. 944).
Table 388.2
Hepatic Phenotypes of Mitochondrial Cytopathies
From Wyllie R, Hyams JS, Kay M, editors: Pediatric gastrointestinal and liver disease , ed 5, Philadelphia, 2016, Elsevier (Box 71.2, p. 876).
Mitochondrial respiratory chain disorders of all types affect 1 in 20,000 children younger than 16 yr of age; liver involvement has been reported in 10–20% of patients with respiratory chain defect. Primary mitochondrial disorders, including mtDNA depletion syndromes (MDSs), occur in 1 in 5,000 live births and are a known cause of acute liver failure in children <2 yr of age.
More than 200 pathogenic point mutations, deletions, insertions, and rearrangements that involve mtDNA and nuclear DNA and encodes mitochondrial proteins are identified. Mitochondrial genetics are unique because mitochondria are able to replicate, transcribe, and translate their mitochondrial-derived DNA independently. A typical hepatocyte contains approximately 1,000 copies of mtDNA. Oxidative phosphorylation (the process of adenosine triphosphate production) occurs in the respiratory chain located in the inner mitochondrial membrane and is divided into 5 multienzyme complexes: reduced nicotinamide adenine dinucleotide coenzyme Q reductase (complex I), succinate–coenzyme Q reductase (complex II), reduced coenzyme Q–cytochrome-c reductase (complex III), cytochrome-c oxidase (complex IV), and adenosine triphosphate synthase (complex V). The respiratory chain peptide components are encoded by both nuclear and mtDNA genes, hence mutations in either genome can result in disorders of oxidative phosphorylation. Thirteen essential polypeptides are synthesized from the small 16.5-kilobase circular double-stranded mtDNA. mtDNA also encodes the 24 transfer RNAs required for intramitochondrial protein synthesis, whereas nuclear genes encode more than 70 respiratory chain subunits and an array of enzymes and cofactors required to maintain mtDNA, including DNA polymerase-γ (POLG), thymidine kinase 2, and deoxyguanosine kinase.
The expression of mitochondrial disorders is complex, and epidemiologic studies are hampered by technical difficulties in collecting and processing the tissue specimens needed to make accurate diagnoses, the variability in clinical presentation, and the fact that most disorders display maternal inheritance with variable penetrance (see Chapter 97 ). mtDNA mutates 10 times more often than nuclear DNA due to a lack of introns, protective histones, and an effective repair system in mitochondria. Mitochondrial genetics also displays a threshold effect in that the type and severity of mutation required for clinical expression varies among people and organ systems; this is explained by the concept of heteroplasmy, in which cells and tissues harbor both normal and mutant mtDNA in various amounts because of random partitioning during cell division. Mutations, deletions, or duplications in either mitochondrial or nuclear genes can cause disease, and mutations in nuclear genes that control mtDNA replication, transcription, and translation may lead to MDS or to a translational disorder.
Defects in oxidative phosphorylation can affect any tissue to a variable degree, with the most energy-dependent organs being the most vulnerable. One should consider the diagnosis of a mitochondrial disorder in a patient of any age who presents with progressive, multisystem involvement that cannot be explained by a specific diagnosis. Gastrointestinal complaints include vomiting, diarrhea, constipation, failure to thrive, and abdominal pain; certain mitochondrial disorders have characteristic gastrointestinal presentations. Pearson marrow-pancreas syndrome manifests with sideroblastic anemia and exocrine pancreatic insufficiency, whereas mitochondrial neurogastrointestinal encephalomyopathy manifests with chronic intestinal pseudo-obstruction and cachexia. Hepatic presentations range from chronic cholestasis, hepatomegaly, cirrhosis, and steatosis to fulminant hepatic failure and death. Patients with certain mitochondrial diseases may have normal or minimally elevated lactate levels even in the setting of a metabolic crisis. The lactate-to-pyruvate molar ratio (L:P) has been proposed as a screening test for mitochondrial disorders because it reflects the equilibrium between the product and substrate of the reaction catalyzed by lactase dehydrogenase. An L:P ≥ 25 has been considered to be highly suggestive of respiratory chain dysfunction; however, an elevated lactate or an elevated L:P can also represent secondary mitochondrial dysfunction occurring as a result of severe liver disease.
A common presentation of respiratory chain defects is severe liver failure manifested as jaundice, hypoglycemia, coagulopathy, renal dysfunction, and hyperammonemia, with onset within the first few weeks to months of life. Cytochrome-c oxidase (complex IV) is the most common deficiency in these infants, although complexes I and III and MDSs are also implicated (see Tables 388.1 and 383.3 ). The key biochemical features include a markedly elevated plasma lactate concentration, an elevated molar ratio of plasma lactate to pyruvate (L:P) (>25), and a raised ratio of β-hydroxybutyrate to acetoacetate (>4.0). Symptoms are nonspecific and include lethargy and vomiting. Most patients additionally have neurologic involvement that manifests as a weak suck, recurrent apnea, or myoclonic epilepsy. Liver biopsy shows predominantly microvesicular steatosis, cholestasis, bile duct proliferation, glycogen depletion, and iron overload. With standard therapy the prognosis is poor, and most patients die from liver failure or infection in the first few months of life.
Diagnostic criteria include refractory mixed-type seizures with a focal component; psychomotor regression that is episodic and triggered by intercurrent infections; and hepatopathy with or without acute liver failure. Alpers syndrome manifests from infancy up to 8 yr of age with seizures, hypotonia, feeding difficulties, psychomotor regression, and ataxia. Patients develop hepatomegaly and jaundice and have a slower progression to liver failure than those with cytochrome-c oxidase deficiency. Elevated blood or cerebrospinal fluid lactate and pyruvate levels are supportive of the diagnosis, in addition to characteristic electroencephalographic findings (high-amplitude slow activity with polyspikes), asymmetric abnormal visual evoked responses, and low-density areas or atrophy in the occipital or temporal lobes on computed tomography scanning of the brain. In some patients complex I deficiency has been found in liver or muscle mitochondria. The disease is inherited in an autosomal recessive fashion; mutations in the catalytic subunit of the nuclear gene mtDNA POLG have been identified in multiple families with Alpers syndrome, leading to the advent of molecular diagnosis for Alpers syndrome. Patients with POLG mutations are susceptible to valproate-induced liver dysfunction.
MDS is characterized by a tissue-specific reduction in mtDNA copy number, leading to deficiencies in complexes I, III, and IV. MDS manifests with phenotypic heterogeneity; multisystem and localized disease forms include myopathic, hepatocerebral, and liver-restricted presentations. Infants with the hepatocerebral form present in the neonatal period. The first symptoms are metabolic; these rapidly progress to hepatic failure with hypoglycemia and vomiting. This stage is followed by neurologic involvement affecting the central and peripheral systems. Laboratory studies are characterized by lactic acidosis, hypoglycemia, and markedly elevated α-fetoprotein in plasma. In some patients, iron overload has been found with elevated transferrin saturation, high ferritin levels, and iron accumulation in hepatocytes and Kupffer cells. Death usually occurs by 1 yr of age. Spontaneous recovery has been reported in a patient with liver-restricted disease. Inheritance is autosomal recessive and mutations in the nuclear deoxyguanosine kinase gene (DGUOK) have been identified in many patients with hepatocerebral MDS. Thymidine kinase 2 has been implicated in the myopathic form; no known genetic defect has been identified in liver-restricted MDS. Multiple other nuclear genes including POLG, MPV17, Twinkle helicase gene, and SUCLG1 have been implicated in hepatocerebral MDS. Liver biopsies of patients with MDS show microvesicular steatosis, cholestasis, focal cytoplasmic biliary necrosis, and cytosiderosis in hepatocytes and sinusoidal cells. Ultrastructural changes are characteristic, with oncocytic transformation of mitochondria, which is characterized by mitochondria with sparse cristae, granular matrix, and dense or vesicular inclusions. If the native DNA-encoded complex II is normal and the activities of the other complexes are decreased, one should investigate mtDNA copy numbers for a MDS. Diagnosis is established by the demonstration of a low ratio of mtDNA (<10%) to nuclear DNA in affected tissues and/or genetic testing. Importantly, the sequence of the mitochondrial genome is normal.
Navajo neurohepatopathy (NNH) is an autosomal recessive sensorimotor neuropathy with progressive liver disease found only in Navajo Indians of the southwestern United States. The incidence is 1 in 1,600 live births. Diagnostic criteria include sensory neuropathy; motor neuropathy; corneal anesthesia; liver disease; metabolic or infectious complications including failure to thrive, short stature, delayed puberty, or systemic infection; and evidence of central nervous system demyelination on radiographic imaging and peripheral nerves biopsies. An MPV17 gene mutation is implicated in the pathogenesis of NNH. Interestingly, this is the same gene implicated in MDS (see earlier), demonstrating that NNH may be a specific type of MDS found only in Navajos. NNH is divided into three phenotypic variations based on age of presentation and clinical findings.
First, classic NNH appears in infancy with severe progressive neurologic deterioration manifesting clinically as weakness, hypotonia, loss of sensation with accompanying acral mutilation, corneal ulcerations, and poor growth. Liver disease, present in the majority of patients, is secondary and variable; it includes asymptomatic elevations of liver function tests, Reye syndrome–like episodes, and hepatocellular carcinoma or cirrhosis. γ-Glutamyl transpeptidase levels tend to be higher than in other forms of NNH. Liver biopsy might show chronic portal tract inflammation and cirrhosis, but there is shows less cholestasis, hepatocyte ballooning, and giant cell transformation than in other forms of NNH.
Infantile NNH manifests between the ages of 1 and 6 mo with jaundice and failure to thrive and progresses to liver failure and death by 2 yr of age. Patients have hepatomegaly with moderate elevations in aspartate aminotransferase, alanine aminotransferase, and γ-glutamyl transpeptidase. Liver biopsy demonstrates pseudoacinar formation, multinucleate giant cells, portal and lobular inflammation, canalicular cholestasis, and microvesicular steatosis. Progressive neurologic symptoms are not usually noticed at presentation but develop later.
Childhood NNH manifests from age 1-5 yr with the acute onset of fulminant hepatic failure leading to death within months. Most patients also have evidence of neuropathy at presentation. Liver biopsies are similar to those in infantile NNH except for significant hepatocyte ballooning and necrosis, bile duct proliferation, and cirrhosis, which are also seen.
There is no effective treatment for any of the forms of NNH, and neurologic symptoms often preclude liver transplantation. The identical MPV17 mutation is seen in patients with both the infantile and classic forms of NNH, highlighting the clinical heterogeneity of NNH.
Pearson marrow-pancreas syndrome has a neonatal-onset with severe macrocytic anemia, variable neutropenia and thrombocytopenia, and ringed sideroblasts in the bone marrow. Diarrhea and fat malabsorption develop in early childhood secondary to extensive pancreatic fibrosis, acinar atrophy, and partial villous atrophy of the small intestine. The liver involvement includes hepatomegaly, steatosis, and cirrhosis. Liver failure and death have been reported before the age of 4 yr. Other features of the syndrome include renal tubular disease, photosensitivity, diabetes mellitus, hydrops fetalis, and the late development of visual impairment, tremor, ataxia, proximal muscle weakness, external ophthalmoplegia, and a pigmentary retinopathy. Methylglutaconic aciduria is a useful diagnostic marker. Large deletions of mtDNA are reported in most patients, resulting in deficiency of complexes I and III. mtDNA deletions can be detected in patients’ cultured fibroblasts as well as in peripheral blood lymphocytes.
Children with this disease present with severe anorexia, vomiting, chronic diarrhea, and villous atrophy in the 1st yr of life. Hepatic involvement includes mild elevation of aminotransferase levels, hepatomegaly, and steatosis. Lactic acidosis is worsened with high-dextrose intravenous infusions or enteral nutrition. Diarrhea improves by 5 yr of age in association with the normalization of intestinal biopsies. Subsequently patients develop retinitis pigmentosa, cerebellar ataxia, sensorineural deafness, and proximal muscle weakness, with eventual death late in the 1st decade of life. The disease is attributed to a mtDNA rearrangement defect. A complex III deficiency was found in the muscle of affected patients.
The acronym GRACILE summarizes the most important clinical features, namely fetal growth restriction (birth weight about −4 SD), aminoaciduria (caused by Fanconi-type tubulopathy), cholestasis (with steatosis and cirrhosis), iron overload, severe lactic acidosis, and early death. The syndrome is associated with mutations of the complex III assembly factor BCS1L. The liver histology shows microvesicular steatosis and cholestasis with abundant iron accumulation in hepatocytes and Kupffer cells. The liver iron content decreases slightly with age, concomitantly with increasing fibrosis and cirrhosis. Abnormal aminotransferase levels and coagulation are noted, but the cause of death seems to be related more to energy depletion than to liver failure. About half of these patients die within the first 2 wk of life.
Mutations in nuclear translation factor genes (TRMU) of the respiratory chain enzyme complexes have been identified as the etiology of acute liver failure manifesting at ages 1 day to 6 mo. The respiratory chain deficit was similar to that seen in MDS, where the activity of the native DNA-encoded complex II was normal whereas complexes I, III, and IV were decreased. The elongation factor EFG1 (gene GFM1 ) mutation was associated with fetal growth restriction, lactic acidosis, liver dysfunction that progresses into liver failure and death. The mutation in the elongation factor EFTu manifests as severe lactic acidosis and lethal encephalopathy with mild hepatic involvement.
Secondary mitochondrial hepatopathies are caused by exposure to a hepatotoxic metal, drug, toxin, or endogenous metabolite. In the past, the most common secondary mitochondrial hepatopathy was Reye syndrome, the prevalence of which peaked in the 1970s and had a mortality rate of >40%. Although mortality has not changed, the prevalence has decreased from >500 cases in 1980 to approximately 35 cases per year since. The decline in the reported incidence of Reye syndrome may be partially related to more accurate modern diagnosis of infectious, metabolic, or toxic disease, thus reducing the percentage of idiopathic or true cases of Reye syndrome. Reye syndrome is precipitated in a genetically susceptible person by the interaction of a viral infection (influenza, varicella) and salicylate and/or antiemetic use. Clinically it is characterized by a preceding viral illness that appears to be resolving and the acute onset of vomiting and encephalopathy (see Table 388.3 ). Neurologic symptoms can rapidly progress to seizures, coma, and death. Liver dysfunction is invariably present when vomiting develops, with coagulopathy and elevated serum levels of aspartate aminotransferase, alanine aminotransferase, and ammonia. Importantly, patients remain anicteric and serum bilirubin levels are normal. Liver biopsies show microvesicular steatosis without evidence of liver inflammation or necrosis. Death is usually secondary to increased intracranial pressure and cerebral herniation. Patients who survive have full recovery of liver function but should be carefully screened for fatty-acid oxidation and fatty-acid transport defects (Table 388.4 ).
Table 388.3
Clinical Staging of Reye Syndrome and Reye-Like Diseases
Table 388.4
Diseases That Present a Clinical or Pathologic Picture Resembling Reye Syndrome
Acquired abnormalities of mitochondrial function can be caused by several drugs and toxins, including valproic acid, cyanide, amiodarone, chloramphenicol, iron, the emetic toxin of Bacillus cereus, and nucleoside analogs. Valproic acid is a branched fatty acid that can be metabolized into the mitochondrial toxin 4-envalproic acid. Children with underlying respiratory chain defects appear more sensitive to the toxic effects of this drug, and valproic acid is reported to precipitate liver failure in patients with Alpers syndrome and cytochrome-c oxidase deficiency. Nucleoside analogs directly inhibit mitochondrial respiratory chain complexes. The reverse transcriptase inhibitors zidovudine, didanosine, stavudine, and zalcitabine―used to treat HIV-infected patients―inhibit DNA POLG of mitochondria and can block elongation of mtDNA, leading to mtDNA depletion. Other conditions that can lead to mitochondrial oxidative stress include cholestasis, nonalcoholic steatohepatitis, α1 -antitrypsin deficiency, and Wilson disease.
Screening tests include common biochemical tests (comprehensive metabolic profile, INR, α-fetoprotein, CPK, phosphorus, complete blood cell count, ammonia, lactate, pyruvate, serum ketone bodies: both quantitative 3-hydroxybutyrate and quantitative acetoacetate, total free fatty acids, serum acylcarnitine profile; serum-free and total carnitines, urine organic acids, and serum amino acids) (Table 388.5 ). These results will guide subsequent confirmatory testing to establish a molecular diagnosis. Genotyping, including single gene or panel screening for common mitochondrial disease, is used in clinical practice. Whole exome or genome sequencing is also helpful and is replacing single gene or gene panel testing. However, the identification of multiple gene variants of uncertain significance will require detailed clinical and biochemical confirmation for interpretation. Tissue (liver biopsy, skin fibroblast, and muscle biopsy) may be needed to make a specific biochemical diagnosis.
Table 388.5
From Wyllie R, Hyams JS, Kay M, editors: Pediatric gastrointestinal and liver disease , ed 5, Philadelphia, 2016, Elsevier (Box 71-3, p. 876).
There is no effective therapy for most patients with mitochondrial hepatopathies; neurologic involvement often precludes orthotopic liver transplantation. Patients with mitochondrial disorders remain at risk for transplant-related worsening of their underlying metabolic disease, especially patients with POLG -related disease. Several therapeutic drug combinations―including antioxidants, vitamins, cofactors, and electron acceptors―have been proposed, but no randomized controlled trials have been completed to evaluate them. Treatment strategies are supportive and include the infusion of sodium bicarbonate for acute metabolic acidosis, transfusions for anemia and thrombocytopenia, and exogenous pancreatic enzymes for pancreatic insufficiency. It is important to discontinue or avoid medications that may exacerbate hepatopathy, including sodium valproate, tetracycline, and macrolide antibiotics, azathioprine, chloramphenicol, quinolones, and linezolid. Ringer lactate should be avoided because patients with liver dysfunction may not be able to metabolize lactate. Propofol should be avoided during anesthesia because of potential interference with mitochondrial function. In patients with lactic acidosis, lactate levels should be monitored during procedures. It is important to maintain anabolism using a balanced intake of fat and carbohydrates while avoiding unbalanced intakes (e.g., glucose only at a high intravenous rate) or fasting for >12 hr.