Leigh syndrome

The symptoms of Leigh syndrome were classically described as beginning in infancy and leading to death within a span of several years; Excess lactate may be seen in the urine, cerebrospinal fluid, and blood of a person with Leigh syndrome. Hypertrichosis is seen in Leigh syndrome caused by mutations in the nuclear gene SURF1. ==Genomics==

Mutations in mitochondrial DNA (mtDNA) and over 30 genes in nuclear DNA (gene SURF1 and some COX assembly factors) have been implicated in Leigh disease. Several mitochondrial genes involved in creating the first complex of the oxidative phosphorylation chain can be implicated in a case of Leigh syndrome, including genes MT-ND2, MT-ND3, MT-ND5, MT-ND6 and MT-CO1. Mitochondrial DNA is passed down matrilineally in a pattern called maternal inheritance—a mother can transmit the genes for Leigh syndrome to both male and female children, but fathers cannot pass down mitochondrial genes. Some types of SURF1 mutations cause a subtype of Leigh syndrome that has a particularly late onset but similarly variable clinical course. Many of these genes affect the first oxidative phosphorylation complex. X-linked Leigh syndrome pattern of inheritance seen occasionally in cases of Leigh syndrome Leigh syndrome can also be caused by deficiency of the pyruvate dehydrogenase complex (PDHC), the x-linked gene being PDHA1. The metabolic form presents as severe lactic acidosis in the newborn period, and many die as newborns. Both compound heterozygosity and homozygous mutations have been observed in French Canadian Leigh syndrome. This subtype of the disease was first described in 1993 in 34 children from the region, all of whom had a severe deficiency in cytochrome c oxidase (COX), the fourth complex in the mitochondrial electron transport chain. Though the subunits of the protein found in affected cells were functional, they were not properly assembled. The deficiency was found to be almost complete in brain and liver tissues and substantial (approximately 50% of normal enzyme activity) in fibroblasts (connective tissue cells) and skeletal muscle. Kidney and heart tissues were found to not have a COX deficiency. French Canadian Leigh syndrome has similar symptoms to other types of Leigh syndrome. The age of onset is, on average, 5 months and the median age of death is 1 year and 7 months. Children with the disease are developmentally delayed, have mildly dysmorphic facial features, including hypoplasia of the midface and wide nasal bridge, chronic metabolic acidosis, and hypotonia (decreased muscular strength). Other symptoms include tachypnea (unusually quick breathing rate), poor sucking ability, hypoglycemia (low blood sugar), and tremors. Severe, sudden metabolic acidosis is a common cause of mortality. Estimates of the rate of genetic carriers in the Saguenay–Lac-Saint-Jean region range from 1 in 23 to 1 in 28; the number of children born with the disease has been estimated at 1 in 2063 to 1 in 2473 live births. Genealogic studies suggest that the responsible mutation was introduced to the region by early European settlers. ==Pathophysiology==

The characteristic symptoms of Leigh syndrome are at least partially caused by bilateral, focal lesions in the brainstem, basal ganglia, cerebellum, and other regions of the brain. The lesions take on different forms, including areas of demyelination, spongiosis, gliosis, necrosis, and capillary proliferation. Demyelination is the loss of the myelin sheath around the axons of neurons, inhibiting their ability to communicate with other neurons. The brain stem is involved in maintaining basic life functions such as breathing, swallowing, and circulation; the basal ganglia and cerebellum control movement and balance. Damage to these areas therefore results in the major symptoms of Leigh syndrome—loss of control over functions controlled by these areas. The lactic acidosis sometimes associated with Leigh syndrome is caused by the buildup of pyruvate, which is unable to be processed in individuals with certain types of oxidative phosphorylation deficiencies. The pyruvate is either converted into alanine via alanine aminotransferase or converted into lactic acid by lactate dehydrogenase; both of these substances can then build up in the body. ==Diagnosis==

Leigh syndrome is suggested by clinical findings and confirmed with laboratory and genetic testing. Clinical findings Dystonia, nystagmus, and problems with the autonomic nervous system suggest damage to the basal ganglia and brain stem potentially caused by Leigh syndrome. Other symptoms are also indicative of brain damage, such as hypertrichosis and neurologically caused deafness. Laboratory findings of lactic acidosis or acidemia and hyperalaninemia (elevated levels of alanine in the blood) can also suggest Leigh syndrome. Assessing the level of organic acids in urine can also indicate a dysfunction in the metabolic pathway. Differential diagnosis Other diseases can have a similar clinical presentation to Leigh syndrome; excluding other causes of similar clinical symptoms is often a first step to diagnosing Leigh syndrome. Conditions that can appear similar to Leigh disease include perinatal asphyxia, kernicterus, carbon monoxide poisoning, methanol toxicity, thiamine deficiency, Wilson's disease, biotin-thiamine-responsive basal ganglia disease (BTBGD), and some forms of encephalitis. Perinatal asphyxia can cause bilateral ganglial lesions and damage to the thalamus, which are similar to the signs seen with Leigh syndrome. When hyperbilirubinemia is not treated with phototherapy, the bilirubin can accumulate in the basal ganglia and cause lesions similar to those seen in Leigh syndrome. This is not common since the advent of phototherapy. ==Treatment==

Succinic acid has been studied, and shown effective for both Leigh syndrome, and MELAS syndrome. A high-fat, low-carbohydrate diet may be followed if a gene on the X chromosome is implicated in an individual's Leigh syndrome. Thiamine (vitamin B1) may be given if pyruvate dehydrogenase deficiency is known or suspected. The symptoms of lactic acidosis are treated by supplementing the diet with sodium bicarbonate (baking soda) or sodium citrate, but these substances do not treat the cause of Leigh syndrome. Dichloroacetate may also be effective in treating Leigh syndrome-associated lactic acidosis; research is ongoing on this substance. In 2016, John Zhang and his team at New Hope Fertility Center in New York, USA, performed a spindle transfer mitochondrial donation technique on a mother in Mexico who was at risk of producing a baby with Leigh disease. A healthy boy was born on 6 April 2016. However, it is not yet certain if the technique is completely reliable and safe. == Prognosis ==

Different genetic causes and types of Leigh syndrome have different prognoses, though all are poor. The most severe forms of the disease, caused by a full deficiency in one of the affected proteins, cause death at a few years of age. If the deficiency is not complete, the prognosis is somewhat better and an affected child is expected to survive 6–7 years, and in rare cases, to their teenage years. ==Epidemiology==

Leigh syndrome occurs in at least 1 of 40,000 live births, though certain populations have much higher rates. In the Saguenay–Lac-Saint-Jean region of central Quebec, Leigh syndrome occurs at a rate of 1 in 2000 newborns. ==History==

Leigh syndrome was first described by Denis Leigh in 1951 and distinguished from similar Wernicke's encephalopathy in 1954. ==See also==