A dog can smell diseases

Metabolomics involves the study of the body’s metabolic products, also known as metabolites, and their structure and operation in cells, the blood and secretions. The key issue is understanding the significance of metabolites and their effect on human wellbeing and health. Soile Rummukainen is using metabolomics to study canine and human cancers. Her goal is to identify olfactory molecules related to cancer.

Susanna Paavilainen, the Managing Director of the Wise Nose association, which specialises in training dogs to distinguish between various smells, found her dog Kössi sniffing a specific area of another dog’s skin. She realised that something was wrong. Eventually, it was discovered that the other dog had gum cancer. She figured out that, thanks to its acute sense of smell, a trained dog could detect cancer in other dogs.

Cancer detected with 100% accuracy

A multidisciplinary research project was started between the University of Helsinki’s Faculty of Veterinary Medicine, Wise Nose, Aqsens Health Ltd. and the University of Eastern Finland. First, the dogs are trained to identify signs of canine mammary tumours from urine samples. According to tests, sniffer dogs’ results were good, with cancer detection of almost 100 per cent. This method will now be extended to detecting prostate and breast cancer.

Dogs have an extremely acute sense of smell. An average-size dog has up to 220 million olfactory receptors, compared to just 5 million in humans. This means that dogs’ sense of smell is thousands of times better than that of humans. A mass spectrometer used for the detection of organic matter generally needs some ten billion molecules before anything shows in the reading. A dog can smell out a disease from a much smaller number. In a test conducted at the University of Eastern Finland, Kössi only needed a sample with ten molecules.

Dogs’ findings analysed with mass spectrometer

Metabolites are compounds of low molecular weight that are involved in various cell metabolism functions. These small molecules cannot be seen or detected directly. Instead, you need measuring devices, such as mass spectrometers, which create signals for subsequent analysis.

Soile Rummukainen, an early stage researcher at the School of Pharmacy at the University of Eastern Finland in Kuopio, uses a mass spectrometer to study cancer samples sniffed out by dogs.

“We study these cancer samples and control group samples by using the non-targeted metabolomics method. The mass spectrometer will reveal tens of thousands of molecular features of metabolic products in urine samples. We use statistical methods to compare differences between groups, trying to identify the most interesting metabolites, that is, those that are different between groups.”

With a mass spectrometer and liquid chromatography, it is possible to separate the compounds from the sample and create a mass spectrum for each of them. The x-axis indicates the ion mass formed by the molecules, and the peak height (y-axis) their relative abundance. On the other hand, the molecular fragmentation product is used to determine the molecular structure. Liquid chromatography (LC) combined with mass spectrometry (MS) is an efficient analysis technique for the definition of metabolites. LC-MS methods are used extensively in pharmaceutical research and clinical diagnostics.

Difficulty of metabolite identification

According to Rummukainen, the difficult part of metabolomics is the identification of molecules. You have to be able to identify the molecular structure that corresponds to the fragmentation spectrum. Fragmentation ions are compared to global databases, their fragmentation spectrum library and our own standards.

“Our own standard library consists of standards we have analysed at the university. These will enable us to identify metabolites with maximum accuracy, since they have been analysed using the same methods and analytical devices and include retention time data, which is a key identification component. However, our library is limited, so we must also use other databases.”

The retention time refers to the time it takes for the compound to travel through the chromatography equipment to the detector.

“A biological sample may contain thousands of metabolic products. When we analyse a sample with a mass spectrometer, we get data that results in tens of thousands of molecular features. These features must then be combined into molecules. By means of the exact mass, fragmentation spectra and retention time, we can perhaps identify 100 or 200 metabolites from the sample, which is quite a small number.”

That is where the dogs come in again. We move on to fractioning, that is, taking partial samples from samples. Then we use the dogs to test whether the smell is still present in the fractions. Rummukainen points out that most work is involved in making and analysing the fractions.

“In the future, we will study these partial samples and analyse the compounds they contain using mass spectrometer methods and nuclear magnetic resonance (NMR) spectroscopy. With the aid of sniffer dogs and mammary tumour samples, our goal is to create a method that can also be applied to determine metabolites related to human cancers.”

Dogs are currently trained to sniff out prostate and breast cancer.

Data processing is also important. Processing raw data obtained from a mass spectrometer requires plenty of computing power and disk space.

“A single metabolite may be linked to a dozen of intracellular signal routes. In this respect, we would benefit from computer simulation to improve our understanding of the biological significance of any changes identified. It would also be interesting to combine data obtained through genomics and proteomics with metabolomics, once the necessary software and tools become more advanced.”

Ari Turunen

Read article in PDF