The human, F-actin-based cytoskeleton as a mutagen sensor.

BACKGROUND: Forty years ago the actin cytoskeleton was determined to be disrupted in fibroblasts from persons with DNA repair-defective, hereditary colon cancer, with no clear connection between the cytoskeleton and DNA repair defects at that time. Recently, the large number of sequenced genomes has indicated that mammalian mutagenesis has a large stochastic component. As a result, large coding regions are large mutagen targets. Cytoskeletal protein-related coding regions (CPCRs), including extra-cellular matrix proteins, are among the largest coding regions in the genome and are indeed very commonly mutated in cancer. METHODS: To determine whether mutagen sensitivity of the actin cytoskeleton could be assessed experimentally, we treated tissue culture cells with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and quantified overall cytoskeleton integrity with rhodamine-phalloidin stains for F-actin. RESULTS: The above approach indicated cytoskeletal degradation with increasing mutagen exposure, consistent with increased mutagenesis of CPCRs in TCGA, smoker samples, where overall mutation rates correlate with CPCR mutation rates (R2 = 0.8694; p < 0.00001). In addition, mutagen exposure correlated with a decreasing cell perimeter to area ratio, raising questions about potential decreasing, intracellular diffusion and concentrations of chemotherapy drugs, with increasing mutagenesis and decreasing cytoskeleton integrity. CONCLUSION: Determination of cytoskeletal integrity may provide the opportunity to assess mutation burdens in nonclonal cell populations, such as in intact tissues, where DNA sequencing for heterogeneous mutation burdens can be challenging.

Flat cells come full sphere: Are mutant cytoskeletal-related proteins oncoprotein-monsters or useful immunogens?

Osteogenesis imperfecta is inherited as a dominant disease because if one allele is mutated, it contributes a mutant, destructive subunit polypeptide to collagen, which requires many subunits to form normal, polymeric, collagenous structures. Recent cancer genome atlas (TCGA) data indicate that cytoskeletal-related proteins are among the most commonly mutated proteins in human cancers, in distinct mutation frequency groups, i.e., including low mutation frequency groups. Part of the explanation for this observation is likely to be the fact that many of the coding regions for these proteins are very large, and indeed, it is likely these coding regions are mutated in many cells that never become cancerous. However, it would not be surprising if mutations in cytoskeletal proteins, when combined with oncoprotein or tumor suppressor protein mutations, had significant impacts on cancer development, for a number of reasons, including results obtained almost 5 decades ago indicating that well-spread cells in tissue culture, with well-formed cytoskeletons, were less tumorigenic than spherical cells with disrupted cytoskeletons. This raises the question, are mutant cytoskeletal proteins, which would likely interfere with polymer formation, a new class of oncoproteins, in particular, dominant negative oncoproteins? If these proteins are so commonly mutant, could they be the bases for common cancer vaccines?

A Novel Approach to Evaluating Cancer Driver Gene Mutation Densities: Cytoskeleton-related Gene Candidates.

BACKGROUND: Oncoprotein genes are over-represented in statically defined, low mutation-frequency fractions of cancer genome atlas (TCGA) datasets, consistent with a higher driver mutation density. MATERIALS AND METHODS: We developed a "continuously variable fraction" (CVF) approach to defining high and low mutation-frequency groups. RESULTS AND CONCLUSION: Using the CVF approach, an oncoprotein set was shown to be associated with a TCGA, low mutation-frequency group in nine distinct cancer types, versus six, for statically defined sets; and a tumor-suppressor set was over-represented in the low mutation-frequency group in seven cancer types, notably including BRCA. The CVF approach identified single-mutation driver candidates, such as BRAF V600E in the thyroid cancer dataset. The CVF approach allowed investigation of cytoskeletal protein-related coding regions (CPCRs), leading to the conclusion that mutation of CPCRs occurs at a statistically significant, higher density in low mutation-frequency groups. Supporting online material for this article can be found at www.universityseminarassociates.com/Supporting_online_material_for_scholarly_pubs.php.

Big genes are big mutagen targets: a connection to cancerous, spherical cells?

We determined the most commonly mutated genes in five cancer genome atlas (TCGA) datasets. Many of these genes were extraordinarily large, as are many cancer fusion gene partners. And many of these genes had cytoskeletal related functions. We further determined that these genes were distributed into high and low frequency mutation groups largely according to overall rate of gene-occurrence in the high and low mutation frequency groups, as was also the case with common metastasis and tumor suppressor genes. Oncoproteins were selectively mutated in the low mutation frequency groups in colon and lung datasets. Thus, genes that have very large coding regions and may impact the cytoskeleton are more commonly mutated than are common metastasis and tumor suppressor genes in both high and low frequency mutation groups. These analyses raise questions related to cell shape: (i) Are cancer cells often spherical because cytoskeletal-related proteins are large mutagen targets? (ii) Is drug-resistance facilitated by relatively common mutant proteins that lead to round cells, with altered cell physiology or reduced surface to volume ratios that could reduce intra-cellular drug concentrations?