[Mitochondrial medicine for internists]

Human mitochondrial DNA (mtDNA) is a small, circular molecule, encoding for the translational machinery of the mitochondrion, as well as for 13 structural proteins that are all subunits of the respiratory chain. Point mutations, deletions, and copy-number variations are now functionally and genetically linked to human disease. Despite the fact that mtDNA is solely transmitted from the mother to the offspring, e.g. is maternally inherited, some mutations may occur spontaneously or may be acquired due to defects in nuclear DNA, e.g. are inherited in a mendelian fashion. The internist encounters predominantly myopathies, cardiomyopathies, lactic acidosis or diabetes mellitus but mtDNA-changes are also present with neurologic, hematologic and renal symptoms. Acquired mtDNA alterations are responsible for important drug side effects, such as ifosfamide, carboplatin, doxorubicin or nucleoside-analog reverse-transcriptase inhibitors. A specific mtDNA point-mutation predisposes to aminoglycoside-induced sensorineural deafness.

Neurotrophin-4 is up-regulated in ragged-red fibers associated with pathogenic mitochondrial DNA mutations

Ragged-red fibers (RRFs) are found more frequently in highly oxidative type I fibers than in glycolytic type II fibers in the muscle of many patients with mitochondrial myopathies. Neurotrophin-4 (NT-4), a neuronal signaling molecule, is also expressed in skeletal muscle, predominantly in type I fibers. We found that NT-4 protein and mRNA were present in both type I and type II fibers but were up-regulated in RRFs of patients with mitochondrial encephalomyopathies; it is noteworthy that NT-4 was not up-regulated in muscle fibers from healthy aerobically trained athletes. Thus, NT-4 might represent a member of a new class of candidate molecules involved in the compensatory adjustments of muscle fibers to oxidative dysfunction, and may even play a role as a signaling molecule for mitochondrial proliferation.

Mitochondrial mobility in differentiating muscle heterokaryons

Ragged-red fibers, a morphological hallmark of many patients with mitochondrial encephalomyopathies who harbor mitochondrial DNA (mtDNA) mutations, usually contain varying ratios of mutated and wild-type mtDNAs. Deficient respiratory function in muscle is almost invariably segmental. To investigate whether this observation may be explained by restricted lateral movement of mitochondria within myofibers, we studied the spatial and temporal behavior of two different mitochondrial populations within multinucleate myotubes. We co-cultured normal human and mouse myoblasts, allowed them to fuse into muscle heterokaryons and investigated whether the mitochondria remained segregated, or migrated and intermixed. Human and mouse nuclei were identified by their differential staining pattern with the dye Hoechst 33 258 and mitochondria were distinguished immunologically and by in situ hybridization. Although we observed some territoriality at very early time points after myoblast fusion, there was rapid intermixing of the mitochondrial populations, as early as 48 h after myoblast fusion. We conclude that mitochondria, unlike many other muscle components, lack territorial organization in cultured, differentiating heterokaryons.

Respiratory chain encephalomyopathies: a diagnostic classification

Mitochondrial encephalomyopathies are a heterogenous group of disorders with various biochemical defects of the respiratory chain (RC). Due to the considerable phenotypic diversity of the RC encephalomyopathies, they are included in the differential diagnosis of many cases of multisystem disease. Aside from clinical evaluation and family history, diagnosis can be supported by many other sources. The complexity of diagnostic information can make it difficult for the clinician to establish the level of certainty at which a RC cytopathy is diagnosed in atypical cases. We review the parameters (clinical, pathological, biochemical and molecular) which are used to aid diagnosis of RC encephalomyopathy and identify levels of abnormality in each that strongly or less strongly support the diagnosis. A system is developed that allows classification of the diagnosis in possible, probable and definite categories of certainty, which will be of value to the clinician.

The therapy of respiratory chain encephalomyopathy: a critical review of the past and current perspective

The mitochondrial respiratory chain encephalomyopathies represent an important group of multisystem disorders. No curative treatment is currently available. A number of measures have been reported to have a theoretical potential to improve respiratory function. These treatment strategies have variable scientific support, many reports being anecdotal. We critically review the various therapeutic measures employed and suggest future treatment directions.

Intravascular lymphoma simulating vasculitis

The case history of a patient with intravascular lymphoma (IL) is reported. Signs of a systemic illness including fever, muscular weakness, telangiectasias, nephrotic syndrome, and neurologic manifestation suggested vasculitis. Cyclophosphamide treatment produced almost complete remission, but the patient died of respiratory failure 13 months after presentation due to lung involvement. The diagnosis was not revealed until postmortem examination. The differential diagnosis of IL is provided.

[Reduced "oxidative burst" in a granulocyte subpopulation in a case of Sweet syndrome]

Oxidative burst was defective in 50% of peripheral blood neutrophils in a case of Sweet syndrome with leukemoid granulocytosis. Phagocytosis was normal. We suggest that the decreased ability to produce oxygen radicals observed in this patient might lead to a compensatory recruitment of hematopoietic growth factors. Consecutive activation, increased chemotaxis and adhesion of polymorphonuclear granulocytes might be the cause of the neutrophilic dermal infiltrate of Sweet syndrome.