Subacute Necrotizing Encephalopathy in Yorkshire Terriers is Associated with a Combined Respiratory Chain Complex I and IV Defect
A. Fischer1; K. Baiker2; S. Hofmann3; S. Medl4; W. Schmahl3; M.F. Bauer3; K. Matiasek3
Subacute necrotizing encephalopathy (SNE) is a fatal neurodegenerative disorder in Yorkshire terriers and Alaskan huskies that reveals striking similarities to human Leigh- (LS) and Leigh-plus-MELAS syndromes. The resemblance, thereby, bases on morphological observations, whereas the molecular proof of a mitochondrial (mt) defect, known to underlie LS, still is pending in dogs. The present investigation, therefore, was aimed specifically to screen for a mitochondrial involvement in canine SNE.
Fresh muscle samples and brain tissue were available from two female Yorkshire terriers, aged at six and nine months. Muscle probes underwent spectrophotometric assessment of respiratory chain complexes (RCC) I through IV normalized for citrate synthase. Mitochondrial DNA was extracted from muscle and brain tissue and inspected for gene rearrangements via Southern blot analysis. Thereafter, mitochondrial tRNA-genes and ATP6-gene were amplified and sequenced. Levels of heteroplasmia were assessed through restriction fragment length polymorphism.
Both animals presented with a combined defect of RCC I and IV. Mitochondrial DNA analyses revealed a homoplasmic A-to-G transition at position 2691 within mitochondrial tRNA(UUR) gene. However, the same mutation was detected in three out of 43 healthy controls. Major gene rearrangements of mtDNA and ATP6-gene mutations were excluded.
This investigation, for the first time, lends biochemical proof to the suspected mitochondrial dysfunction in canine SNE. Evidence of combined RCC defects is very suggestive of a depressed mitochondrial protein synthesis rather than abnormal structural proteins or assembly factors. Thus, we sequenced all 22 mt-tRNAs and detected a polymorphism in mt-gene for the leucine-bearing tRNA(UUR). Even though this A2691G transition exactly resembles human MELAS-mutation, that causes LS amongst a plethora of other syndromic encephalopathies, it appears to be a neutral polymorphism in canine species. Apart from tRNAs all transcription elements in mitochondria are nuclear encoded. Further investigations of the genetic background of SNE, therefore, will have to address nuclear candidate genes that encode for transcription factors such as the elongation factor gene (EFG)-1.