Development of a diagnostic variable number tandem repeat marker and dual TaqMan genotyping assay to distinguish Lophophora species.

A new DNA marker in the trnL intron region distinguishes the two cactus species Lophophora williamsii (which contains the psychotropic agent mescaline) and Lophophora diffusa. The marker is a variable number tandem repeat (VNTR) where one repeat unit (13 nucleotides) occurs in L. williamsii and two in L. diffusa. A TaqMan genotyping assay using two probes detected the marker with high sensitivity (detection limits of 0.000063 ng and 0.000066 ng). Testing 16 crime-scene samples presumed to be L. williamsii matched results from gas chromatography-mass spectrometry, confirming the method's forensic applicability for species identification.

Study at a glance

Abstract

The Lophophora genus of the Cactaceae family includes Lophophora diffusa and Lophophora williamsii, which has traditionally been used as a natural analgesic; however, its use is now under strict regulation worldwide as it contains mescaline, a unique psychotropic agent. Recently, non-medical and illegal distribution and abuse of L. williamsii have increased worldwide; thus, effective species identification methods are urgently needed. Here, we identified a new variable number tandem repeat (VNTR) marker in the trnL intron region to identify and characterize species in forensic analyses. The VNTR marker has a unique structure of tandem repeats, each with 13 nucleotides; one repeat unit was found in L. williamsii and two in L. diffusa. Phylogenetic and length polymorphism analyses confirmed that this novel VNTR marker could distinguish between Lophophora species. Furthermore, our newly developed TaqMan genotyping assay utilizes two probes; the color and position of dots on the discrimination plot differ according to the tandem repeat count within the VNTR marker. The limits of detection of the assay were 0.000063 ng (LW-VNTR probe-1) and 0.000066 ng (LW-VNTR probe-2), indicating high sensitivity. Moreover, when crime scene samples of 16 presumed L. williamsii species were analyzed, the results coincided with those of gas chromatography-mass spectrometry, confirming the applicability of our marker for Lophophora species identification. Thus, the tandem repeats within the trnL intron region can be exploited as a VNTR marker to identify L. williamsii and L. diffusa. Our dual TaqMan genotyping assay based on a novel marker demonstrates potential for forensic applications.