Researchers reveal new way to determine THC levels in marijuana

In Research Topics, Top Stories by Dr. Tim Sandle, Ph.D.

Cannabis is a multifunctional plant that occupies a unique role in human culture. Some strains are used for hemp, which has a variety of uses; others are used to manufacture marijuana. One problem that has confounded scientists is how to tell different types of cannabis apart without destroying the plant. A research team based at the University of Frankfurt has come up with the answer.

The science group has developed a genetic screening method that identifies whether tetrahydrocannabinol (THC) is present in a cannabis plant. THC is the principal psychoactive constituent (or cannabinoid) in marijuana; although there are currently 70 cannabinoids found only in cannabis, THC accounts for virtually all the psychoactive effects. The newly devised test is non-destructive and allows THC-rich cannabis plants to be differentiated from their hemp cousins.

The Medical Marijuana Review spoke with Dr. Christina Staginnus, the lead scientist behind the study and an expert in plant molecular biology, to find out more about the research.

The reason behind the research, Dr. Staginnus explained, is to “differentiate between marijuana and hemp. The aim was to identify, individualize, and determine genetic relatedness among and within different species.” This is not always straightforward, because both marijuana and hemp are produced from the same plant genus (Cannabis sativa) and are very similar in appearance. Marijuana and hemp samples are too genetically similar and cannot be distinguished by conventional biological techniques.

Cannabis has been used throughout history for its stems in the production of hemp fiber and its buds and leaves as a psychoactive drug. Recently, the issue of telling hemp and marijuana plants apart has become more complex because there are several hundred strains of cannabis owing to selective breeding, often designed to increase the THC content.

Hemp typically refers to high-growing varieties of the cannabis plant and its products, which include fiber (an alternative to cotton fabric), oil, and seed. Hemp has relatively low levels of THC, unlike the herb that is used to produce marijuana. The marijuana and hemp plants both contain enzymes that produce cannabinoids. The difference between them is at the last step of the cannabinoid production process, where cannabigerol (CBG) is converted to either THC or cannabidiol (CBD). Unlike hemp, marijuana plants produce high quantities of THCA synthase. This enzyme converts CBG to THCA. CBDA and THCA are converted to CBD and THC upon heating.

Hemp plants and marijuana plants are grown differently: the “drug varieties” are grown much like a Christmas tree farm, with its spacing, pruning, and early harvest, whereas the oilseed and fiber varieties (hemp) are grown more like pulp wood trees. However, once out of the ground it can be difficult to tell the plants apart.

The way that Dr. Staginnus has got around the difficulty of telling the hemp and marijuana plants apart is through devising a genetic marker. This is, she explained, “a gene or DNA sequence with a known location on a chromosome that can be used to identify individuals or species.”

The genetic marker was identified using a technique called polymerase chain reaction (PCR), a molecular technique that allows scientists to amplify a single or a few copies of a piece of DNA across several orders of magnitude. PCR is widely used in forensic analysis because of its sensitivity, speed, and amenability to analyze minute and even degraded plant DNA.

Using this method, Dr. Staginnus stated that “with methods to screen for specific phenotypes within the THCA synthase gene, they could identify Cannabis sativa plants as being either THC-rich or CBD-rich.”

This comes down to the fact that the “CBD/THC ratio remains constant throughout the plant’s life cycle; it is independent of environmental factors.”

Once established, the researchers tested the robustness of the method by screening a batch of over 200 mixed-type plants. The study found the method could even distinguish between plant seeds and roots.

The main advantage of the technique is that a cannabis plant can be assessed as being rich in THC or not without actually testing the THC directly. The main limitation with the method is that it cannot determine how much THC is present. This requires techniques that result in the destruction of the plant. Nonetheless, the genetic marker method is rapid and non-destructive and can be used when no other test is available, as Dr. Staginnus explained: “The marker represents a valuable tool for the evaluation of cannabis material, for which a THC assessment is not possible.”

In terms of application, the technique can be used three ways, Dr. Staginnus surmises. The genetic markers have practical utility for establishing “whether the cultivation and distribution of marijuana is legitimate; two, identifying geographic sources of seized drugs; and three, discriminating potent marijuana cultivars from hemp where the cultivation of industrial hemp is permitted.”

The genetic marker and the results of the study have been published in the Journal of Forensic Science. The paper is titled, “A PCR marker Linked to a THCA synthase Polymorphism is a Reliable Tool to Discriminate Potentially THC-Rich Plants of Cannabis sativa L.”