A simple method for detection of rabies viral sequences in 16-year old archival brain specimens with one-week fixation in formalin

Archival formalin-fixed and paraffin-embedded brain tissues are important source for diagnosis and molecular analysis. However, nucleic acids are particularly vulnerable to degradation during tissue processing. The brain cutting process usually is performed after 1 week of brain storage in formalin followed by embedding of each particular neuro-anatomical specimen in paraffin. A simple method of deparaffinization, proteinase K digestion and RNA extraction using the Boom technique to obtain rabies RNA in unbuffered, formalin-fixed and paraffin-embedded brain tissues kept at 30 °C for 16 years is described. Reverse transcription-polymerase chain reaction (RT-PCR) can be used to identify rabies viral N gene sequences of 150 bases in length in all patients, but not from every immunohistochemical (IHC)-positive specimen. Direct sequencing of 301 bp of N gene was achieved in 4 of 7 patients. Results of sequencing a single sample of 1432 bases of N gene from a 24 h processed formalin-fixed and paraffin-embedded rabies infected brain tissue after 1 month storage were in accord with those from frozen specimen analysis. It is strongly suggested that for further molecular analysis, a piece of fresh brain tissue should be saved prior to the brain sectioning process and stored no longer than 24 h in formalin before embedding.