Why am I having problems isolating RNA from peripheral blood mononuclear cell (hMNC) pellets?

Difficulties in obtaining RNA with good yield and purity from hMNCs are quite common.
 
Underlying reason
The main cause is usually the presence of large amounts of free genomic DNA in hMNC preparations. This DNA originates primarily from granulocytes, which:

• Undergo lysis during the isolation process
• Are not present in the final hMNC fraction as intact cells
• But release genomic DNA that remains in the preparation and can adhere to the mononuclear cells

This residual DNA can significantly interfere with RNA isolation, leading to low purity or poor RNA yield.
 
Recommended solution
To address this issue, it is recommended to remove DNA prior to RNA purification by including a DNase digestion step.
Most commercial RNA isolation kits offer an optional DNase treatment, which is well suited for this purpose.