Tumor mutational burden (TMB) refers to the number of somatic gene mutations present in a tumor.
TMB is defined as the total number of somatic mutations per coding area of a tumor genome, and is a measure of all non-synonymous coding mutations in a tumor exome thst varies widely from patient to patient and across tumor types.
Tumor mutational burden (TMB) refers to a genetic characteristic of malignant tissue.
It is defined as the number of non-inherited mutations per million bases (Mb) of genomic sequence.
TMB is associated with microsattelite instability (MSI) and that each can result from deficiencies in mismatch repair proteins.
TMB is defined as the number of somatic mutations/megabase.
Its measurement has been enabled by next generation sequencing, using approximately 200 or more genes.
TMB varies across different cancer types.
TMB-high is correlated with response to immunotherapy in certain tumor types.
High TNB is defined as TNB 10 mutation/Mb or greater.
The prevalence of somatic mutations among cancers ranges from .01 mutations/megabase to more than 400 mutations/megabase.
It is suspected that these tumor mutations can result in proteins expressed by tumor cells that are recognized by the immune system, and are called neoantigens.
Increased antigens may lead to increased detection of cancer cells by the immune system and more robust activation of cytotoxic T-lymphocytes.
Activation of T-cells is further regulated by immune checkpoints that can be displayed by cancer cells, thus treatment with immune checkpoint inhibitors can lead to improved patient survival.
It has potential as a predictive biomarker in patient response to immune checkpoint inhibitor (ICI) therapy in a variety of cancers.
Mutational signatures are distinct mutational patterns of single base substitutions, double base substitutions, or small insertions and deletions in tumors.
It is postulated that high TMB is associated with an increased amount of neoantigens, which are tumor specific markers displayed by cells.
These mutations lead to the translation of novel peptide epitropes, referred to as neo antigens, that enhance the immunogenicity of the tumor by eliciting T cell activity.
A greater number of mutations can result in a greater number of neoantigens, with more immune system activation.
Highly mutated tumors produce many neoantigens, some of which may increased T cell reactivity.
Tumors with a high TMB, may be more likely to respond to immune checkpoint inhibitor (ICI) therapies because ICIs work by enhancing the immune response to tumor cells.
A high mutational burden is associated with a high neoantigens burden, and is associated with a high T-cell infiltration, a high immune checkpoint inhibitor response across different tumor types.
It is a bio marker for predicting response to immune checkpoint inhibitors.
Blood based tumor mutational burden is measured with targeted next generation sequencing using cell free DNA.
There is a positive correlation with tissue-based TMB and is a biomarker for better prognosis free survival in patients with malignancies.
In a study of more than 100,000 human cancer genomes it was found that the level of TMB varies widely across cancer types.
Adults with cancer carried a higher TMB than children with cancer.
TMB was highest in skin cancers, including squamous cell carcinoma and melanoma, and cancers of the lung, bladder, cervix, and kidney.
The lowest levels of TMB were found in myelodysplastic or myeloproliferative disorders and cancers of the bone or soft tissue, adrenal gland, and thymus.
Some patients harbored very high or very low TMBs regardless of their specific disease type.
TMB is measured differently between studies, using different assays: testing for certain mutations, the number of genes in the assay, the type of mutations included, and different thresholds for defining mutation burden.
The above differences can lead to variations in the scoring of TMB.
The turnaround time for TMB is approximately 2 to 3 weeks.
Presently, no prospective clinical trials had demonstrated that selection of immune checkpoint inhibitor treatment based on a high TMB over low TMB improves overall survival.
Pembrolizumab is used treat any advanced solid-tumor cancers with a TMB greater than 10 mutations per Mb and continued growth following prior treatments.
Mutations in the genome are reflected in proteins produced from them through transcription and translation.
Some proteins are fragmented into peptides that can then be presented as antigens on the surface of cell membranes by major histocompatibility complexes (MHCs).
If these antigens accumulate enough mutations, they can bind and activate T-cells which can then initiate immune mediated cell death.
However, a mechanism in tumors increase the expression of immune checkpoint molecules that can bind to tumor-specific T-cells and inactivate them, so tumor cells cannot be detected and killed.
Immune checkpoint inhibitors improve patients’ response and the survival rates as they help the immune system to target tumor cells.
The expression of PD-L1 (programmed death-ligand 1) has been demonstrated to be a good biomarker of PD-L1 blockade therapy in some cancers.
There is an association between patients’ outcome by immune checkpoint inhibitors (ICI) and the TMB value.
TMB can be used as a predictive marker of response in ICI therapy across many cancer types.
Tumors with higher TMB values usually result in a higher number of neoantigens, the antigens that are presented on the tumor cells surface that are usually a result of missense mutations.
TMB can help find the patients who can benefit from ICI therapy by increasing the chance of detecting the neoantigens.
Higher TMB levels (≥ 20 mutations/Mb) corresponded to a 58% response rate to ICIs while lower TMB levels (<20 mutations/Mb) reduced response to 20%.
55% of the differences in the objective response rate across cancer types is explained by TMB.
Differences in survival rates, with high TMB individuals having a median progression-free survival of 12.8 months and a median overall survival not reached, compared to 3.3 months and 16.3 months respectively for individuals with lower TMB.
In patients who have not received ICI therapy found that intermediate levels of TMB (>5 and <20 mutations/Mb) correlated with significantly decreased survival, likely as a result of the accumulation of mutations in oncogenes.
This relationship does not appear to be significantly disparate across different tissues types and is only modestly affected by corrections for confounders such as smoking, sex, age, and ethnicity.
TMB is a reliable indicator of poor patient outcomes in the absence of ICI therapy.
It is hypothesized that the decreased risk of death under very high TMB could result from reduced cell viability due to genetic instability or increased production of neoantigens recognized by the immune system.
There is a large variation in TMB values across different cancer types.
TMB of somatic mutations can span from 0.01 to 400 mutations per megabase of genome.
Melanoma, NSCLC and other squamous carcinomas have the highest levels of TMB, while leukemias and pediatric tumors have the lowest levels of TMB and other cancers like breast, kidney, and ovary have intermediate TMB values.
Different cut-offs are needed for different cancer types to find the patients who can benefit from ICI therapy.
Tumor heterogeneity, that can affect TMB and consequently the response to ICIs.
Most metastatic samples have been shown to be monoclonal, with only one cluster of cells in the tumor, while primary tumors usually consist of a higher number of clusters and have higher overall genetic diversity.
Metastatic tumors usually have a higher TMB level compared to primary tumors and this can be due to monoclonal nature of metastatic lesions.
Whole genome sequencing, whole exome sequencing, and panel based approaches can be used to help to calculate TMB.
TMB can also be calculated directly from H&E stained pathology images.
Different studies have assigned different cut-offs to delineate between high and low TMB status.
In the lung, the median TMB across more than 18,000 lung cancer cases was 7.2 mutations/Mb, with approximately 12% of the patients showing more than 20 mutations/Mb.
A tumor mutational burden greater than or equal to 10 mutations/Mb as the optimal cut-off to benefit from combination immunotherapy.
However, in other cancer types, high TMB status has been classified as >20 mutations/Mb.
TMB should be used as a complementary marker with other biomarkers such as PD-L1.