Test of an Improved DNA and RNA Purification Protocol—Importance of Proteinase K and Co-Purified Small RNAs

Optimized and reliable DNA/RNA extraction protocols are a vital tool in clinical practice in the context of molecular testing. Here, we present our successful attempt to enhance the quantity of RNA isolated from clinical specimens, which we originally found challenging (breast and testis). We compared several purification methods with special focus on two AllPrep system-based protocols (QIAGEN). Our data suggest that addition of proteinase K may markedly increase RNA and, in some cases, also DNA yield. The extraction kit used, AllPrep DNA/RNA/miRNA universal kit, provides RNA amounts comparable with the phenol-chloroform extraction method; however, part of the final yield consisted of small RNAs, visible as a thick band in the bioanalyzer gel-like image (5S peak). The 5S peak, albeit in some cases dominating the bioanalyzer image, plays only a small role in RT-qPCR analysis, and Qubit or NanoDrop measurements can still be used as a reliable estimate of starting amounts of mRNA for downstream analyses. In conclusion, we showed that implementing a protocol containing a step of proteinase K digestion markedly increases RNA yield. The AllPrep DNA/RNA/miRNA Universal Kit can be successfully used for simultaneous extraction of DNA and total RNA, irrespective of the tissue of origin, and does not present inconveniences related to phenol-chloroform extraction.