Unlocking the Archive: Extracting DNA and RNA from FFPE Tissue Blocks

For decades, hospital archives have acted as silent repositories of medical history. Billions of FFPE tissue blocks, containing samples dating back generations, sit in storage rooms. Historically, these blocks were viewed primarily as diagnostic records—useful for looking at cells, but useless for studying genetics. However, the question arose: Can DNA or RNA be extracted from these chemically treated blocks? The answer is a resounding yes, though it is a process fraught with unique challenges that distinguish it from standard extraction protocols.

The Chemistry of the Challenge

To understand why extracting nucleic acids from FFPE is difficult, one must understand what formalin does. Formalin fixation creates methylene bridges—covalent bonds that link proteins to each other and to nucleic acids. This effectively locks the DNA and RNA inside a protein web. Furthermore, over time, the formaldehyde can oxidize into formic acid, leading to acid hydrolysis. This causes strand breaks and base modifications (such as the deamination of cytosine to uracil).

Therefore, extracting DNA or RNA from FFPE is not simply a matter of separation; it is a matter of reversal and repair.

The Extraction Process

The standard protocols for fresh tissue involve simple lysis and purification. For FFPE, the process must be significantly more aggressive and nuanced.

1. Deparaffinization: The first hurdle is the wax itself. Before the cells can be accessed, the paraffin must be removed. This is typically done using organic solvents like xylene or limonene. Without this step, the hydrophobic wax would interfere with the aqueous chemical reactions required for extraction.
2. Reversal of Crosslinks: Once the tissue is exposed, a high-temperature incubation step is required. Often using a detergent buffer containing proteinase K, the sample is heated (often above 90°C) to break the methylene bridges created by formalin. This releases the nucleic acids from the protein mesh.
3. Purification: Following the release, standard silica-membrane or magnetic-bead technologies are used to capture the DNA or RNA, washing away the degraded proteins and remaining chemical contaminants.

Quality and Quantity

While DNA and RNA can indeed be extracted, the quality is distinct from fresh tissue. The DNA recovered from FFPE is often fragmented, usually ranging from 100 to 400 base pairs, compared to the high-molecular-weight DNA obtained from fresh samples.

RNA is even more sensitive. Ribosomal RNA (rRNA) is often degraded, and the extracted total RNA typically presents with a shifted peak on a Bioanalyzer trace, indicating fragmentation. However, this fragmentation does not render the sample useless. Modern library preparation kits for sequencing are specifically designed to work with fragmented DNA or RNA.

The Value of the Archive

The ability to extract nucleic acids from FFPE has revolutionized retrospective studies. Researchers can now correlate patient outcomes from 20 years ago with specific genetic markers found in their archived tissue blocks. This turns a static diagnostic record into a dynamic molecular resource.

While fresh tissue remains the optimal source for high-quality, long-read sequencing, the extraction of DNA and RNA from FFPE has democratized access to vast clinical datasets. It allows science to reach back into the past, decoding the genetic secrets preserved in wax.