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L. Frontali - New complex mitochondrial functions in cell biology
investigate open problems in mitochondrial disease,
such as. elucidation of molecular mechanisms; differentiation
of pathogenic mutations from non-pathogenic
polymorphisms; explanation for the existence
of homoplasmic mutations; understanding of the
reason for the variability of penetrance; and unravelling
of the basis of different pathogenic effects of
the same mutation in different individuals.
We have previously shown that introduction in yeast
mt tRNAleuUUR gene, by biolistic procedure, of base
substitutions equivalent to 3243, 3256 and 3291 found
in MELAS patients, produces very severe growth
defective phenotype in glycerol, with loss of mt DNA.
We have now developed this research line by studying
several new mutations, including the C25Tbase
substitution in tRNAVal, equivalent to the homoplasmic
human mutation 1624 having different penetrance
in various members of the same family.
The phenotypes we observed (growth in glycerol,
respiration, rho° formation, analysis of tRNAs by
Northern blotting on acidic sequencing gels) were
found to be different in different laboratory strains
and interestingly, the severity of all phenotypes varied
with the same trend for all mutations.
Another important aspect of this research is the possibility
of correcting the defects due to the mutations
by the overexpression of some nuclearly
encoded mitochondrial factors which interact with
the mutated tRNA and probably stabilize their structure.
We have previously shown that among these
suppressors there are the mitochondrial protein
elongation factor EF-Tu and the cognate aminoacyltRNA
synthetase. We have now extended these
results to new mutations and we have shown that the
suppression is dependent on the amount of suppres-
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sor available. We also observed that the nuclear context
as well as the endogenous expression level of
the suppressors, affect dramatically the defective
phenotype of the analyzed mutants.
A variable amount of suppressors might be important
for the understanding of the tissue specificity of
cell damage observed in patients and a possible perspective
basis for the correction of the defects.
Actually our results on the suppressive effects of
the above mentioned tRNA interactors have suggested
to several important groups working on
human patients to verify the same effect in human
cells (myoblasts from patient biopsies). Results have
been positive, thus confirming the usefulness of the
yeast model.
Selected publications
Palermo V, Falcone C, Mazzoni C. Apoptosis and
aging in mitochondrial morphology mutants of S.
cerevisiae. Folia Microbiologica 2007, 52:479-83.
Montanari A, Besagni C, De Luca C, Morea V, Oliva
R, Tramontano A, Bolotin-Fukuhara M, Frontali L,
Francisci S. Yeast as a model of human mitochondrial
tRNA base substitutions: investigation of the
molecular basis of respiratory defects. RNA 2008,
14:275-83.
Rinaldi T, Hofmann L, Gambadoro A, Cossard R,
Livnat-Levanon N, Glickman MH, Frontali L,
Delahodde A. Dissection of the carboxyl-terminal
domain of the proteasomal subunit rpn11 in maintenance
of mitochondrial structure and function. Mol
Biol Cell 2008, 19:1022-31.