Pseudogenes: a novel source of trans-acting antisense RNAs.

While long thought to represent only "junk" DNA, several recent studies support a functional role for pseudogenes. Several hundreds of pseudogenes have been reported as transcribed into RNA in a large variety of tissues and tumors. Most studies have focused on pseudogenes expressed in the sense direction, but some reports suggest that pseudogenes can be also transcribed as antisense RNAs (asRNAs). A few examples of key regulatory genes, such as PTEN and OCT4, have in fact been reported to be under the regulation of pseudogene-expressed asRNAs. Here, we review what is known about pseudogene-expressed asRNAs and we discuss the functional role that these transcripts may have in gene regulation. Finally, we discuss some technical challenges when characterising the function of pseudogene asRNAs.