Genome-Wide Detection of Single-Nucleotide and Copy-Number Variation of a Single Human Cell(1)
Reporter, Writer: Stephen J. Williams, Ph.D.
Most tumors exhibit a level of diversity, at the cellular, histologic, and even genetic level (2). This genetic heterogeneity within a tumor has been a focus of recent research efforts to analyze the characteristics, expression patterns, and genetic differences between individual tumor cells. This genetic diversity is usually manifested as single nucleotide variations (SNV) and copy number variations (CNV), both of which provide selection pressures in both cancer and evolution.
As cancer research and personalized medicine is focused on analyzing this tumor heterogeneity it has become pertinent view the tumor as a heterogeneous population of cells instead of as a homogenous mass. In, fact, studies have suggested that cancer cell lines growing on plastic in culture, even though thought of as clonogenic, can actually display a varied degree of expression differences between neighboring cells growing on the same dish. Indeed, cancer stem cells show an asynchronous cell division, for example a parent CD133-positive cell will divide into a CD133-positive and a CD133-negative cell(3). In addition, the discovery that circulating tumor cells (a rare population of circulating cells in the blood) can be prognostic of outcome in cancer such as inflammatory breast cancer(4), it is ever more important to develop methods to analyze single cell populations.
Harvard University researchers, Dr. Chenghang Zong, Sijia Lu, Alec Chapman and Sunney Xie developed a new amplification method utilizing multiple annealing and looping-based amplification cycles (MALBAC)(1). A quasilinear preamplification process is used on pictograms of DNA genomic fragments (form 10 to 100 kb) isolated from a single cell. This is performed to reduce the bias associated with nonlinear DNA amplification. A series of random primers (which the authors termed MALBAC primers, constructed with a common sequence tags) are annealed at low temperature (0 °C). PCR rounds produce semiamplicons. Further rounds of amplification, after a step of looping the amplicons, result in full amplicons with complementary ends. When the two ends hybridize to form the looped DNA, this prevents use of this loop structure as a template, therefore leading to a close-to–linear amplification. The process allows for a higher fidelity of DNA replication and the ability to amplify a whole genome. The amplicons are then sequenced either by whole-genome sequencing methods using Sanger-sequencing to verify any single nucleotide polymorphisms. This procedure of MALBAC-amplification resulted in coverage of 85-93% of the genome of a single cell.
As proof of principle, the authors used MALBAC to amplify the DNA of single SW480 cancer cells (picked from a clonally expanded population of a heterogeneous population (the bulk DNA). Comparison of the MALBAC method versus the MDA method revealed copy number variations (CNV) between three individual cells, which had been picked from the clonally expanded pool. Their results were in agreement with karyotyping studies on the SW480 cell line. Meticulous quality controls were performed to limit contamination, high false positive rates of SNV detection due to amplification bias, and false positives due to amplification or sequencing errors.
Interestingly, the authors found 35 unique single nucleotide variations which h had occurred from 20 cell divisions from a single SW480 cancer cell. This resulted in an estimated 49 mutations which occurred in 20 generations, yielding a mutation rate of 2.5 nucleotides per generation. In addition, the authors were able to map some of these mutations on various chromosomes and perform next-gen sequencing (deep sequencing) to verify the nucleotide mutations and found an unusually high purine-pyrimidine exchange rate.
In a subsequent paper, investigators from the same group at Harvard used this technology to sequence 99 sperm cells from a single individual to study genetic diversity created during meiotic recombination, a mechanism involved in evolution and development(5).
Reference:
1. Zong, C., Lu, S., Chapman, A. R., and Xie, X. S. (2012) Science 338, 1622-1626
2. Cooke, S. L., Temple, J., Macarthur, S., Zahra, M. A., Tan, L. T., Crawford, R. A., Ng, C. K., Jimenez-Linan, M., Sala, E., and Brenton, J. D. (2011) British journal of cancer 104, 361-368
3. Guo, R., Wu, Q., Liu, F., and Wang, Y. (2011) Oncology reports 25, 141-146
4. Giuliano, M., Giordano, A., Jackson, S., Hess, K. R., De Giorgi, U., Mego, M., Handy, B. C., Ueno, N. T., Alvarez, R. H., De Laurentiis, M., De Placido, S., Valero, V., Hortobagyi, G. N., Reuben, J. M., and Cristofanilli, M. (2011) Breast cancer research : BCR 13, R67
5. Lu, S., Zong, C., Fan, W., Yang, M., Li, J., Chapman, A. R., Zhu, P., Hu, X., Xu, L., Yan, L., Bai, F., Qiao, J., Tang, F., Li, R., and Xie, X. S. (2012) Science 338, 1627-1630
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PUT IT IN CONTEXT OF CANCER CELL MOVEMENT
The contraction of skeletal muscle is triggered by nerve impulses, which stimulate the release of Ca2+ from the sarcoplasmic reticuluma specialized network of internal membranes, similar to the endoplasmic reticulum, that stores high concentrations of Ca2+ ions. The release of Ca2+ from the sarcoplasmic reticulum increases the concentration of Ca2+ in the cytosol from approximately 10-7 to 10-5 M. The increased Ca2+ concentration signals muscle contraction via the action of two accessory proteins bound to the actin filaments: tropomyosin and troponin (Figure 11.25). Tropomyosin is a fibrous protein that binds lengthwise along the groove of actin filaments. In striated muscle, each tropomyosin molecule is bound to troponin, which is a complex of three polypeptides: troponin C (Ca2+-binding), troponin I (inhibitory), and troponin T (tropomyosin-binding). When the concentration of Ca2+ is low, the complex of the troponins with tropomyosin blocks the interaction of actin and myosin, so the muscle does not contract. At high concentrations, Ca2+ binding to troponin C shifts the position of the complex, relieving this inhibition and allowing contraction to proceed.
Figure 11.25
Association of tropomyosin and troponins with actin filaments. (A) Tropomyosin binds lengthwise along actin filaments and, in striated muscle, is associated with a complex of three troponins: troponin I (TnI), troponin C (TnC), and troponin T (TnT). In (more ) Contractile Assemblies of Actin and Myosin in Nonmuscle Cells
Contractile assemblies of actin and myosin, resembling small-scale versions of muscle fibers, are present also in nonmuscle cells. As in muscle, the actin filaments in these contractile assemblies are interdigitated with bipolar filaments of myosin II, consisting of 15 to 20 myosin II molecules, which produce contraction by sliding the actin filaments relative to one another (Figure 11.26). The actin filaments in contractile bundles in nonmuscle cells are also associated with tropomyosin, which facilitates their interaction with myosin II, probably by competing with filamin for binding sites on actin.
Figure 11.26
Contractile assemblies in nonmuscle cells. Bipolar filaments of myosin II produce contraction by sliding actin filaments in opposite directions. Two examples of contractile assemblies in nonmuscle cells, stress fibers and adhesion belts, were discussed earlier with respect to attachment of the actin cytoskeleton to regions of cell-substrate and cell-cell contacts (see Figures 11.13 and 11.14). The contraction of stress fibers produces tension across the cell, allowing the cell to pull on a substrate (e.g., the extracellular matrix) to which it is anchored. The contraction of adhesion belts alters the shape of epithelial cell sheets: a process that is particularly important during embryonic development, when sheets of epithelial cells fold into structures such as tubes.
The most dramatic example of actin-myosin contraction in nonmuscle cells, however, is provided by cytokinesisthe division of a cell into two following mitosis (Figure 11.27). Toward the end of mitosis in animal cells, a contractile ring consisting of actin filaments and myosin II assembles just underneath the plasma membrane. Its contraction pulls the plasma membrane progressively inward, constricting the center of the cell and pinching it in two. Interestingly, the thickness of the contractile ring remains constant as it contracts, implying that actin filaments disassemble as contraction proceeds. The ring then disperses completely following cell division.
Figure 11.27
Cytokinesis. Following completion of mitosis (nuclear division), a contractile ring consisting of actin filaments and myosin II divides the cell in two.
http://www.ncbi.nlm.nih.gov/books/NBK9961/
This is good. I don’t recall seeing it in the original comment. I am very aware of the actin myosin troponin connection in heart and in skeletal muscle, and I did know about the nonmuscle work. I won’t deal with it now, and I have been working with Aviral now online for 2 hours.
I have had a considerable background from way back in atomic orbital theory, physical chemistry, organic chemistry, and the equilibrium necessary for cations and anions. Despite the calcium role in contraction, I would not discount hypomagnesemia in having a disease role because of the intracellular-extracellular connection. The description you pasted reminds me also of a lecture given a few years ago by the Nobel Laureate that year on the mechanism of cell division.
PUT IT IN CONTEXT OF CANCER CELL MOVEMENT
The contraction of skeletal muscle is triggered by nerve impulses, which stimulate the release of Ca2+ from the sarcoplasmic reticuluma specialized network of internal membranes, similar to the endoplasmic reticulum, that stores high concentrations of Ca2+ ions. The release of Ca2+ from the sarcoplasmic reticulum increases the concentration of Ca2+ in the cytosol from approximately 10-7 to 10-5 M. The increased Ca2+ concentration signals muscle contraction via the action of two accessory proteins bound to the actin filaments: tropomyosin and troponin (Figure 11.25). Tropomyosin is a fibrous protein that binds lengthwise along the groove of actin filaments. In striated muscle, each tropomyosin molecule is bound to troponin, which is a complex of three polypeptides: troponin C (Ca2+-binding), troponin I (inhibitory), and troponin T (tropomyosin-binding). When the concentration of Ca2+ is low, the complex of the troponins with tropomyosin blocks the interaction of actin and myosin, so the muscle does not contract. At high concentrations, Ca2+ binding to troponin C shifts the position of the complex, relieving this inhibition and allowing contraction to proceed.
Figure 11.25
Association of tropomyosin and troponins with actin filaments. (A) Tropomyosin binds lengthwise along actin filaments and, in striated muscle, is associated with a complex of three troponins: troponin I (TnI), troponin C (TnC), and troponin T (TnT). In (more ) Contractile Assemblies of Actin and Myosin in Nonmuscle Cells
Contractile assemblies of actin and myosin, resembling small-scale versions of muscle fibers, are present also in nonmuscle cells. As in muscle, the actin filaments in these contractile assemblies are interdigitated with bipolar filaments of myosin II, consisting of 15 to 20 myosin II molecules, which produce contraction by sliding the actin filaments relative to one another (Figure 11.26). The actin filaments in contractile bundles in nonmuscle cells are also associated with tropomyosin, which facilitates their interaction with myosin II, probably by competing with filamin for binding sites on actin.
Figure 11.26
Contractile assemblies in nonmuscle cells. Bipolar filaments of myosin II produce contraction by sliding actin filaments in opposite directions. Two examples of contractile assemblies in nonmuscle cells, stress fibers and adhesion belts, were discussed earlier with respect to attachment of the actin cytoskeleton to regions of cell-substrate and cell-cell contacts (see Figures 11.13 and 11.14). The contraction of stress fibers produces tension across the cell, allowing the cell to pull on a substrate (e.g., the extracellular matrix) to which it is anchored. The contraction of adhesion belts alters the shape of epithelial cell sheets: a process that is particularly important during embryonic development, when sheets of epithelial cells fold into structures such as tubes.
The most dramatic example of actin-myosin contraction in nonmuscle cells, however, is provided by cytokinesisthe division of a cell into two following mitosis (Figure 11.27). Toward the end of mitosis in animal cells, a contractile ring consisting of actin filaments and myosin II assembles just underneath the plasma membrane. Its contraction pulls the plasma membrane progressively inward, constricting the center of the cell and pinching it in two. Interestingly, the thickness of the contractile ring remains constant as it contracts, implying that actin filaments disassemble as contraction proceeds. The ring then disperses completely following cell division.
Figure 11.27
Cytokinesis. Following completion of mitosis (nuclear division), a contractile ring consisting of actin filaments and myosin II divides the cell in two.
http://www.ncbi.nlm.nih.gov/books/NBK9961/
This is good. I don’t recall seeing it in the original comment. I am very aware of the actin myosin troponin connection in heart and in skeletal muscle, and I did know about the nonmuscle work. I won’t deal with it now, and I have been working with Aviral now online for 2 hours.
I have had a considerable background from way back in atomic orbital theory, physical chemistry, organic chemistry, and the equilibrium necessary for cations and anions. Despite the calcium role in contraction, I would not discount hypomagnesemia in having a disease role because of the intracellular-extracellular connection. The description you pasted reminds me also of a lecture given a few years ago by the Nobel Laureate that year on the mechanism of cell division.
I actually consider this amazing blog , âSAME SCIENTIFIC IMPACT: Scientific Publishing –
Open Journals vs. Subscription-based « Pharmaceutical Intelligenceâ, very compelling plus the blog post ended up being a good read.
Many thanks,Annette
I actually consider this amazing blog , âSAME SCIENTIFIC IMPACT: Scientific Publishing –
Open Journals vs. Subscription-based « Pharmaceutical Intelligenceâ, very compelling plus the blog post ended up being a good read.
Many thanks,Annette
I actually consider this amazing blog , âSAME SCIENTIFIC IMPACT: Scientific Publishing –
Open Journals vs. Subscription-based « Pharmaceutical Intelligenceâ, very compelling plus the blog post ended up being a good read.
Many thanks,Annette