Tissue-Specific Expression
Definition
The pattern by which certain genes are transcribed at high levels in one tissue and at negligible levels in others, forming the molecular basis for mRNA-based fluid identification.
- Basis
- Differential transcription of mRNA/miRNA across tissue types
- Used for
- Identifying body fluid source from RNA markers
- Target molecules
- mRNA and microRNA (miRNA) transcripts
- Contrast with
- STR profiling, which identifies the donor, not the fluid
Common questions
Why use RNA instead of DNA to identify a body fluid?+
DNA is essentially identical across a person's tissues, so it cannot say which fluid a stain came from. RNA transcription levels differ sharply by tissue, so a marker gene switched on in saliva but silent in blood lets an examiner assign the stain to a fluid type.
Does tissue-specific expression work on old or degraded stains?+
RNA is chemically less stable than DNA, so recovery drops with age and poor storage. Forensic mRNA assays target short amplicons and often pair with a co-extracted DNA profile from the same stain to offset this limitation.
Can one marker gene identify a fluid on its own?+
Single markers can cross-react with related tissues, so validated assays use a panel of several tissue-specific genes per fluid and require multiple markers to score positive before reporting an identification.
Related terms
- mRNA (Messenger RNA)
- Transcripts encoding proteins, synthesised in the nucleus from DNA templates and translated by ribosomes. mRNA molecules vary greatly in length (hundreds to...
- NanoString nCounter
- A hybridisation and digital counting platform that tallies individual mRNA molecules using fluorescent barcodes, avoiding amplification bias and enabling simultaneous profiling of...
- OSAC Validation
- The framework for forensic assay validation published by the US Organisation of Scientific Area Committees, requiring demonstrations of sensitivity, specificity, mixture performance,...
- RNase
- Ribonuclease enzymes that cleave RNA phosphodiester bonds. Present in blood plasma (RNase A family) and released from cells on death. The activity...
- RT-PCR (Reverse Transcription PCR)
- A two-stage process: RNA is first reverse-transcribed into complementary DNA (cDNA), then the cDNA is amplified by PCR. Quantitative RT-PCR (RT-qPCR) measures...