Aspectos Clínicos, Bioquímicos, Enzimáticos e Moleculares Associados ao Défice do Complexo I

Mariana Ferreira, Tatiana Aguiar, Laura Vilarinho

Laboratório de Investigação, Centro de Genética Médica Jacinto Magalhães, INSA

As citopatias mitocondriais constituem um grupo heterogéneo de doenças que se caracterizam por alterações da estrutura mitocondrial e deficiência da fosforilação oxidativa (OXPHOS). A OXPHOS é constituída por cinco complexos proteicos e dois transportadores de electões. A NADHubiquinona oxidoreductase – complexo I, o primeiro e maior dos cinco complexos é o principal ponto de entrada de electrões, provenientes do ciclo de Krebs, no sistema OXPHOS. Os défices do complexo I são um diagnóstico relativamente frequente de citopatia mitocondrial, sendo causados por mutações no DNA mitocondrial ou no DNA nuclear. Devido a este controlo genético duplo, que contribui para a complexidade do sistema OXPHOS, defeitos no complexo I resultam numa variedade de fenótipos clínicos, que estão geralmente associados a disfunções metabólicas graves da infância, incluindo cardiomiopatia progressiva, encefalomiopatia, leucodistrofia ou síndrome de Leigh. Na investigação dos mecanismos subjacentes aos défices do complexo I, são utilizados vários modelos, tais como a Neuropora crassa e os cíbridos, que conjuntamente com o uso de novas ferramentas bioquímicas (Blue Native Polyacrylamide gel electrophoresis), revelam-se de extrema importância para o processo. Perante a complexidade deste sistema enzimático, quer estrutural como na sua manutenção, é difícil efectuar um diagnóstico molecular na rotina laboratorial. Em Portugal, o estudo destes pacientes tem sido restrito à medição da actividade enzimática dos complexos da cadeia respiratória e pesquisa das mutações pontuais mais comuns e rearranjos do mtDNA, até há alguns meses atrás. Como consequência a maioria dos casos de défices do complexo I continuam por esclarecer sob o ponto de vista molecular, tornando-se assim indispensável avançar para uma investigação mais abrangente, possibilitando desta forma um aconselhamento genético adequado e um diagnóstico pré-natal para as famílias de risco.

Palavras-chave: OXPHOS; complexo I; mtDNA; nDNA; CRM; citopatias mitocondriais.

Mitochondrial Respiratory Chain

Mitochondrial cytopathies are a group of genetically heterogeneous disorders, that is characterized by alterations in the mitochondrial structure and oxidative phosphorylation deficiency (OXPHOS). OXPHOS system consists of five multimeric protein complexes and two electron carriers. NADH-ubiquinone oxidoreductase (complex I), the first and largest of the five complexes, is the major entry point of electrons, from Krebs cycle, of the OXPHOS system. Complex I deficiency is a frequently diagnosed defect of the mitochondrial OXPHOS system, caused by mutations in either the mitochondrial DNA (mtDNA) or the nuclear DNA. Because of this dual genetic control, which contributes to the complexity of the OXPHOS system, defects on complex I results in a broad spectrum of clinical phenotypes, that are usually associated to severe metabolic disorders of childhood, including progressive cardiomyopathy, encephalomyopathy, leukodystrophy or Leigh syndrome. Research on the mechanisms underlying mitochondrial complex I deficiency has used several models, such as Neurospora crassa and human cell cybrids, and has taken advantage by the routinary use of novel biochemical tools, such as Blue Native Polyacrylamide gel electrophoresis. Given the complexity of this OXPHOS enzyme, both in structure and maintenance, it is difficult to achieve the molecular diagnosis of patients in a routine basis. In Portugal, the study of these patients did not go beyond the biochemical activity measurements of respiratory chain enzymes and screening of most common point mutations and rearrangements of mtDNA, until some months ago. As a consequence, at the molecular level, the majority of the complex I deficiency cases remain unsolved, being essential to move forward to a more wide-ranging study. Moreover, a correct diagnosis will permit adequate genetic counselling and prenatal diagnosis.

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Correspondência:

Centro de Genética Médica Jacinto Magalhães

Praça Pedro Nunes, 88

4099-028 Porto

e-mail: laura.vilarinho@igm.min-saude.pt