A tool for predicting pathogenicity of novel mitochondrial tRNA mutations, developed by Neal Sondheimer
and Sanjay Sonney
The pathogenicity scoring from MitoTIP is intended as a starting point for analysis of newly observed tRNA mutations
. It is based only on database frequencies, the nature of the nucleotide change (transversion/transition/deletion), and conservation scoring. No clinical histories, heteroplasmy data, or functional studies are included. Mutations must be further evaluated by the end user in the context of the actual patient heteroplasmy, and the heteroplasmy seen in affected and unaffected family members. Other additional lines of evidence are suggested below.
MitoTIP integrates multiple sources of information to provide a prediction for the likelihood that novel point mutations or deletion in tRNA-encoding sequences would cause disease. The sources of information used include:
Each possible mutation was scored and the scores have been interpreted within quartiles. Current MitoTIP scores range from 1.0 to 20.8, with a median score of 14.1. MitoTIP scores will be recalculated on a regular basis.
MitoTIP's scoring of tRNA variants is embedded into Mitomaster and may be viewed by using Mitomaster's SNV query or Sequence query tools. Raw scores may be found here. Please note that raw scores and rankings may change as sequences are added to our database.
- GenBank deposited mitochondrial sequence
- Annotations of pathogenicity from MITOMAP
- Conservation across species
- The position of the mutation within the tRNA
- Whether the mutation is a transition or transversion
Current Raw Scores
Quartile Ranking /
14.1 – 16.2
10.7 – 14.1
Additional lines of evidence should be pursued to confirm pathogenicity for any tRNA variant: determination of heteroplasmy levels, correlation of heteroplasmy with phenotype, functional cybrid studies, tRNA steady state levels, single fiber studies.
Screenshot of Mitomaster query details showing the MitoTIP prediction |
(information in the tx effect column will soon be replaced with raw numerical score)
Users can input any tRNA-encoding position into MITOMAP’s point mutation search or into MITOMASTER’s SNV Query and retrieve the predicted pathogenicity score of any possible change at that position. Any mutation whose status is not already confirmed as pathogenic will be displayed with a prediction (likely polymorphic/possibly polymorphic/possibly pathogenic/likely pathogenic) derived from quartiles of the pathogenicity score. The use of this platform will simultaneously direct users to underlying literature supporting the assignment of confirmed or suspected mutations.
Weaknesses of this system include its reliance on databases, which may fall out of date, contain errors or have sequence entries from patients with mitochondrial disorders. The underlying calls of pathogenicity represent a best effort at identifying all legitimate reports of mitochondrial variants but may have missed some reports and may include other reports that inaccurately ascribe pathogenicity to neutral variants. Providing MitoTIP data in the context of other information will allow users to integrate multiple sources of information when assessing unfamiliar variants. The system’s strength is also in its use of databases as we can easily update it with new information incorporated into MITOMAP so that the system will improve in sensitivity and specificity over time. The system is designed for the evaluation of novel mutations, where previous data confirming pathogenicity is unavailable. Indeed, known pathogenic mutations such as mt.8344A>G score poorly in MitoTIP because the position is neither well conserved nor in a secondary structure location commonly associated with disease. We also have not incorporated the user-detected heteroplasmy of a query variant into our scoring. It is widely accepted that heteroplasmic mutations are more likely to be pathogenic and low penetrance mutations that are homoplasmic are less common. The pathogenicity scoring from MitoTIP for newly observed mutations can and should be evaluated by the end user in the context of the actual patient heteroplasmy, and the heteroplasmy seen in affected and unaffected family members.
A brief paper describing the MitoTIP system has been submitted for publication. An online PDF or PubMed link will be provided when available.