Protean manifestations of mitochondrial diseases: a minireview

Corneal endothelial dysfunction in Pearson syndrome

Mitochondrial disorders are associated with well recognized ocular manifestations. Pearson syndrome is an often fatal, multisystem, mitochondrial disorder that causes variable bone marrow, hepatic, renal and pancreatic exocrine dysfunction. Phenotypic progression of ocular disease in a 12-year-old male with Pearson syndrome is described. This case illustrates phenotypic drift from Pearson syndrome to Kearns-Sayre syndrome given the patient's longevity. Persistent corneal endothelial failure was noted in addition to ptosis, chronic external ophthalmoplegia and mid-peripheral pigmentary retinopathy. We propose that corneal edema resulting from corneal endothelial metabolic pump failure occurs within a spectrum of mitochondrial disorders.

Celiac disease in siblings with Pearson syndrome

Pearson syndrome (PS) is a rare mitochondrial disorder characterized by sideroblastic anemia and exocrine pancreas deficiency as a result of mitochondrial DNA deletion or deletion-duplication. A 21-year-old woman and 11-year-old brother who had been diagnosed as PS at the age of 18 and 8, respectively, were admitted to our hospital with the complaints of chronic diarrhea while using exogenous pancreas enzymes. Diarrhea was present in both for a long time and attributed to existing PS. Endoscopic and laboratory examinations revealed celiac disease (CD). After gluten-free diet, their symptoms were resolved. Detailed family investigation revealed that parents and the sister of the siblings were free of both PS and CD. To our knowledge, there has been no previous report of the presence of these 2 disorders, PS and CD, in the same patient. CD should be screened and treated appropriately in cases with PS.

Mechanisms of Disordered Granulopoiesis in Congenital Neutropenia

Neutrophils are critical components of the innate immune response, and persistent neutropenia is associated with a marked susceptibility to infection. There are a number of inherited clinical syndromes in which neutropenia is a prominent feature. A study of these rare disorders has provided insight into the mechanisms regulating normal neutrophil homeostasis. Tremendous progress has been made at defining the genetic basis of these disorders. Herein, progress in understanding the genetic basis and molecular mechanisms of these disorders is discussed. We have focused our discussion on inherited disorders in which neutropenia is the sole or major hematopoietic defect.

Gastrointestinal manifestations of mitochondrial disease

Although non-specific gastrointestinal and hepatic symptoms are commonly found in most mitochondrial disorders, they are among the cardinal manifestations of several primary mitochondrial diseases, such as: mitochondrial neurogastrointestinal encephalomyopathy; mitochondrial DNA depletion syndrome; Alpers syndrome; and Pearson syndrome. Management of these heterogeneous disorders includes the empiric supplementation with various "mitochondrial cocktails," supportive therapies, and avoidance of drugs and conditions known to have a detrimental effect on the respiratory chain. There is a great need for improved methods of treatment and controlled clinical trials of existing therapies. Liver transplantation is successful in acquired cases; however neuromuscular involvement in primary mitochondrial disorders should be a contraindication for liver transplantation.

Diagnosis and management of mitochondrial diseases

Mitochondrial dysfunction should be considered in the differential diagnosis of any progressive multisystem disorder. The diagnosis is most challenging when only one symptom is present. In contrast, the diagnosis is easier to consider when two or more seemingly unrelated symptoms are present, involving more than one organ system. It is important to consider the diagnosis of a mitochondrial disorder when dealing with an unexplained association of symptoms, with an early onset and progressive course involving seemingly unrelated organs. The investigation can be relatively straightforward if a person has a recognizable phenotype and if it is possible to identify a known pathogenic mtDNA mutation. The difficulty arises when no known mtDNA defect can be found or when the clinical abnormalities are complex and not easily matched to those of more common mitochondrial disorders. In summary: A full mitochondrial evaluation often is warranted in children with a complex neurologic picture or a single neurologic symptom and other system involvement. When the presentation is classic for a maternally inherited mitochondrial syndrome, such as MELAS, MERRF, or Leber's hereditary optic neuropathy, appropriate mtDNA studies should be obtained first. When the clinical picture is classic for a nuclear DNA inherited syndrome and the gene or linkage is known, such as MNGIE, the clinician should proceed with genetic studies. When the clinical picture is nonspecific but highly suggestive of a mitochondrial disorder, the clinician should start with plasma or CSF lactic acid, ketone bodies, plasma acylcarnitines, and urinary organic acids. If these studies are abnormal, the clinician should proceed with muscle biopsy and assessment of the respiratory chain enzymes. Normal plasma or CSF lactic acid does not rule out a mitochondrial disorder.

Sensitive Assay for Mitochondrial DNA Polymerase γ

Background: The mitochondrial DNA polymerase γ is the principal polymerase required for mitochondrial DNA replication. Primary or secondary deficiencies in the activity of DNA polymerase γ may lead to mitochondrial DNA depletion. We describe a sensitive and robust clinical assay for this enzyme.

Methods: The assay was performed on mitochondria isolated from skeletal muscle biopsies. High-molecular weight polynucleotide reaction products were captured on ion-exchange paper, examined qualitatively by autoradiography, and quantified by scintillation counting.

Results: Kinetic analysis of DNA polymerase γ by this method showed a Km for dTTP of 1.43 μmol/L and a Ki for azidothymidine triphosphate of 0.861 μmol/L. The assay was linear from 0.1 to 2 μg of mitochondrial protein. The detection limit was 30 units (30 fmol dTMP incorporated in 30 min). The linear dynamic range was three orders of magnitude; 30–30 000 units. Imprecision (CV) was 6.4% within day and 12% between days. Application of this assay to a mixed population of 38 patients referred for evaluation of mitochondrial disease revealed a distribution with a range of 0–2506 U/μg, reflecting extensive biologic variation among patients with neuromuscular disease.

Conclusion: This assay provides a useful adjunct to current laboratory methods for the evaluation of patients with suspected mitochondrial DNA depletion syndromes.