3-methylglutaconic aciduria type 2, BTHS, Cardioskeletal myopathy with neutropenia and abnormal mitochondria, Cardioskeletal myopathy-neutropenia, MGA2, X-linked cardioskeletal myopathy and neutropenia
Barth syndrome (BTHS) is an inborn error of phospholipid metabolism characterized by dilated cardiomyopathy (DCM), skeletal myopathy, neutropenia, growth delay and organic aciduria.Prevalence is estimated at 1/454,000 and incidence at 1/140,000 (South-West England, South Wales) to 1/300,000-1/400,000 live births (USA). BTHS affects male patients.Clinical presentation is highly variable. Most boys will develop DCM during the first decade, generally during the first year of life, which may be accompanied by endocardial fibroelastosis (EFE) and/or left ventricular noncompaction (LVNC). It may start in utero, causing cardiac failure, fetal hydrops and miscarriage or stillbirth during the 2nd/3rd trimester of pregnancy. Ventricular arrhythmia, especially during adolescence, can lead to sudden cardiac death. There is a significant risk of stroke. Skeletal (mostly proximal) myopathy causes delayed motor milestones, hypotonia, severe lethargy or exercise intolerance. There is a tendency to hypoglycemia during the neonatal period. Ninety percent of patients show mild to severe intermittent or persistent neutropenia with a risk of septicemia, severe bacterial sepsis, mouth ulcers and painful gums. Lactic acidosis and mild anemia may occur. Affected boys usually show delayed puberty and growth delay that is observed until the late teens or early 20s, when a substantial growth spurt often occurs. Patients may also present severe difficulties with adequate food intake. Episodic diarrhea is common. Many patients have a similar facial appearance with chubby cheeks, deep-set eyes and prominent ears.BTHS is caused by mutations in the TAZ gene (tafazzin; Xq28) which encodes Taz1p acyltransferase involved in the metabolism of cardiolipin, a major phospholipid in inner mitochondrial membranes. Defective Taz1p function results in abnormal remodelling of cardiolipin which ultimately compromises mitochondrial structure or respiratory chain function.Diagnosis was historically based on metabolic screening of urine showing elevated excretion of organic acids (typically 3-methylglutaconic acid (3-MGCA)), followed by TAZ gene sequencing. However, 3-MGC excretion may be normal even in severe cases. Analysis of the ratio of monolysocardiolipin (MLCL) / cardiolipin (CL) on blood, tissue, fibroblasts or stored neonatal bloodspots is therefore the diagnostic test of choice.Differential diagnosis includes hereditary, dilated and nutritional cardiomyopathy and idiopathic/cyclic neutropenia (see these terms).Prenatal diagnosis (chorionic villus biopsy and/or amniocentesis) is possible in families in which the mutation is known.Transmission is X-linked recessive. A son born to a female carrier has a 50% risk of inheriting the mutation and developing the disease, while a daughter has a 50% risk of being a carrier. All daughters of an affected male will be carriers but none of his sons will be affected.Treatment is essentially supportive and multidisciplinary. Cardiac failure is treated with conventional drugs or by cardiac transplantation if refractory. The risk of bacterial sepsis in cases of intermittent neutropenia can be reduced by the use of prophylactic antibiotics and/or intermittent use of granulocyte-colony stimulating factor (G-CSF). Difficulties in feeding may necessitate nasogastric or gastrostomy tube feeding.Prognosis has greatly improved with early detection and improvements in treatment and management. Patients are already surviving into their 40s and are expected to live beyond this age.