Metabolites across the colorectal cancer continuum: A clinical and nutritional perspective

Colorectal cancer is one of the most prevalent cancers worldwide. Genetic, environmental, nutritional, and other lifestyle factors play a large role in the development of colorectal cancer, but the role in colorectal cancer recurrence and survival is largely unknown. Measuring metabolites, i.e. molecules present in biospecimens, such as blood and tissue samples, can help to unravel the underlying biology of the colorectal cancer continuum. The colorectal cancer continuum reflects the process in which normal cells progress into colorectal cancer cells. Therefore, the overall aim of this thesis is to better understand the association between circulating metabolites and the colorectal cancer continuum from a clinical and nutritional perspective in two international consortia.

In Chapter 2, the association between plasma metabolites and colorectal cancer occurrence is studied using an untargeted metabolomics approach. Data of 268 stage I-IV colorectal cancer patients and 353 colorectal cancer-free individuals of an international consortium are included. Data are split into an independent discovery and replication set. After replication, 28 molecules are associated with colorectal cancer, including 15 molecules that could be identified as known metabolites. Levels of taurine, hypoxanthine, lysophosphatidylethanolamines (LysoPEs) (20:4) and (22:6) are shown to be higher among colorectal cancer patients compared to the colorectal cancer-free individuals. The opposite is observed for valine, leucine, bilirubin, 1-methylnicotinamide, and seven phosphatidylcholines (LysoPCs), namely LysoPC(15:0), LysoPC(16:0), LysoPC(16:0) isomer, LysoPC(P-16:0), LysoPC(16:1), LysoPC(17:0), and  LysoPC(18:0), which show lower levels among colorectal cancer patients compared to colorectal cancer-free individuals.

The association between plasma metabolites at diagnosis and colorectal cancer stage is investigated using a targeted metabolomics approach in Chapter 3. In total, 744 patients with stage I-IV colorectal cancer from an international consortium are included in the analysis. Sphingomyelin (SM) C26:0 show lower concentrations among patients with stage III compared to stage I colorectal cancer. SM C18:0 and phosphatidylcholine diacyl (PC aa) C32:0 concentrations are higher, whereas citrulline, histidine, PC aa C34:4, PC acyl-alkyl (ae) C40:1 and LysoPC a C16:0 and C17:0 are lower among stage IV compared to stage I colorectal cancer patients. No differences in plasma metabolite concentrations are observed between stage II and stage I colorectal cancer patients.

In Chapter 4, two diet quality indices and three dietary patterns, and their associations with plasma metabolites in colorectal cancer patients are shown. Plasma metabolites of 195 stage I-IV colorectal cancer patients are quantified using a targeted metabolomics approach. Two widely acknowledged diet quality indices are used, the 2018 dietary cancer prevention recommendations of the World Cancer Research Fund (WCRF), i.e. the WCRF dietary score, and the Dutch Healthy Diet Index (DHD15-index), based on the Dutch dietary guidelines of 2015. A higher intake of a healthier diet is reflected by a higher WCRF score and DHD15-index. The three dietary patterns are identified based on the reported habitual dietary intake of the colorectal cancer patients by Food Frequency Questionnaire at cancer diagnosis, i.e. a Western, Carnivore and Prudent pattern. The Western and Carnivore pattern reflect high intake of items usually present in an unhealthier diet. The Western dietary pattern is characterized by a high intake of snacks, savory sauces and spreads, refined grains, pizza, high and medium-fat dairy, nuts and seeds, beer, and hard fats, and a low intake of whole grain products and potatoes. The Carnivore pattern is characterized by a high intake of red and processed meat, poultry, fish, eggs, and potatoes, and a low intake of soy and vegetarian products, and medium and high-fat dairy. The Prudent pattern reflects a healthy diet, with high consumptions of vegetables, fruits, fish, nuts and seeds, and low-fat dairy and low intakes of pastry and biscuits, and beer. Better adherence to healthier diets is associated with lower concentrations of plasma phosphatidylcholines (Chapter 4). More specifically, patients with better adherence to the WCRF dietary score, DHD guidelines and higher consumptions of the Prudent pattern show lower concentrations of plasma phosphatidylcholines. Contrary, patients with higher intakes of the Western and Carnivore patterns show higher concentrations of plasma phosphatidylcholines.

Chapter 5 describes the associations between circulating concentrations of folate species at diagnosis and colorectal cancer recurrence and survival. The folate species folate, folic acid, and folate catabolites p-aminobenzoylglutamate (pABG) and p-acetamidobenzoylglutamate (apABG) are quantified using a targeted metabolomics approach in blood samples of 2024 stage I-III colorectal cancer patients within an international consortium. Circulating concentrations of folate, pABG and apABG are not associated with colorectal cancer recurrence and survival. Higher compared to lower concentrations of folic acid are associated with an increased risk of recurrences, but folic acid concentrations are not associated with colorectal cancer survival.

The findings of this thesis suggest that amino acid and lipid metabolism may play important roles in the colorectal cancer continuum. In addition, higher circulating concentrations of folic acid may be associated with an increased risk of colorectal cancer recurrence. Based on the observations in this thesis, investigating metabolite concentrations across the colorectal cancer continuum may provide important leads to further study the underlying biology of colorectal cancer development and progression.

This thesis also showed that national and international collaborations are valuable because of the availability of data from colorectal cancer patients from several countries and the availability of a wide variability in exposures and outcomes. Future national and international collaborations are, therefore, encouraged to further study the underlying biology of the complex interplay between metabolites, nutrition, and the colorectal cancer continuum from a clinical and nutritional perspective.