Mutational load and mutational patterns in relation to age in head and neck cancer

Abstract

Head and neck squamous cell carcinoma (HNSCC) is a cancer with well-defined tumor causes such as HPV infection, smoking and drinking. Using The Cancer Genome Atlas (TCGA) HNSCC cohort we systematically studied the mutational load as well as patterns related to patient age in HNSCC. To obtain a homogenous set we excluded all patients with HPV infection as well as wild type TP53. We found that the overall mutational load is higher in patients of old age. Through unsupervised hierarchical clustering, we detected distinct mutational clusters in very young as well as very old patients. In the group of old patients, we identified four enriched pathways ("Axon Guidance", "ECM-Receptor Interaction", "Focal Adhesion" and "Notch Signaling") that are only sporadically mutated in the other age groups. Our findings indicate that the four pathways regulate cell motility, tumor invasion and angiogenesis supposedly leading to less aggressive tumors in older age patients. Importantly, we did not see a strict pattern of genes always mutated in older age but rather an accumulation of mutations in the same pathways. Our study provides indications of age-dependent differences in mutational backgrounds of tumors that might be relevant for treatment approaches of HNSCCs patients.

Keywords: Gerotarget; aging; genomics; head and neck squamous cell carcinoma; sequencing; somatic mutations.

Figure 1. CONSORT diagram of original and selected patient cohort

Figure 2. Correlation of average mutations and age considering one mutation per gene

Upper graph number of patients in the respective age group. Lower graph, average number of mutations for all patients in the respective age groups. Linear regression analysis was done using F-statistics and shows a significant increase in mutations in older patients (p = 0.000161, adjusted r² = 0.24). Grey area around regression line indicates 95% confidence interval. Regression without the patient having an extreme number of mutations (TCGA-D6-6516, see Figure S1) also yields a significant connection between average mutations and age p = 0.000291, r² = 0.22 (data not shown).

Figure 3. Unsupervised hierarchical clustering of gene mutation frequencies of specific age groups with pooled young and old ages

Patients were grouped into age groups of very young (ages 19‐40) and very old (ages 81‐87) with 10‐year bins in between, then clustered according to the mutations frequencies of the quantified genes. Only genes with a minimum frequency difference of 0.15 between at least two of the age groups are displayed. Black boxes indicate extracted genes for A., age group 81 to 87 and B., age group 19 to 40 (for genes see Table S3).

Figure 4. Unsupervised hierarchical clustering of gene mutation frequencies of specific age groups with separate old ages

Patients were grouped into age groups of very young (ages 19‐40), decades in-between and two separate old ages groups (ages 81‐85 and 87), then clustered according to the mutations frequencies of the quantified genes. Only genes with a minimum frequency difference of 0.15 between two of the age groups are displayed. Black boxes indicate extracted genes for A., age group 81 to 85 upper cluster (shared with ages 86 to 87), B., age group 81 to 85 lower cluster (separate genes from ages 86 to 87), C., two clusters of ages 19 to 40 and D., ages 86 to 87 (for genes see Table S3).

Figure 5. Unsupervised hierarchical clustering of mutation frequencies of genes involved in the “Axon Guidance” pathway (according to the KEGG database)

Patients were grouped into age groups of very young (ages 19‐40), decades in-between and two separate old ages groups (ages 81‐85 and 86 to 87), then clustered according to the mutation frequencies of all quantified genes of the “Axon Guidance” pathway. Genes that did not make the original cut off of 0.15 difference in mutation frequency but are found mutated in this pathway in age group 81‐85 are LRRC4C, DPYSL2, EPHA8, LIMK1, NCK1, NGEF, RAC1, ROBO2, ROCK1 and SLIT2.