Chapter 9 Answers to self-check questions

9.1 Dysregulation of haemopoiesis can result in an increased cellular proliferation rate and failure to progress through the normal stages of maturation. This leads to a maturational arrest. Commonly, these dysregulated cells also fail to undergo apoptosis, thereby leading to their accumulation. A failure of maturation and loss of apoptosis leads to a reduction in the numbers of effector cells and the associated consequences of infection, failure of primary haemostasis, and anaemia.

9.2 Acute myeloid leukaemia; acute lymphoblastic leukaemia; chronic myeloid leukaemia; chronic lymphocytic leukaemia.

9.3 Low-grade and high-grade. Low grade lymphomas are associated with a slow cellular proliferation rate and poor response to therapy. Patients with low-grade lymphomas can often survive for months or years without intensive treatment. High-grade lymphomas need to be treated intensively, but generally the malignancy responds to treatment as the cells have a high proliferation rate.

9.4 Findings include a hypercellular bone marrow accompanied by a peripheral blood cytopenia. Patients will often demonstrate a macrocytic anaemia which fails to respond to traditional treatment options.

9.5 Plasma cells.

9.6 Signal transduction is an essential process to allow a cell to respond appropriately to environmental stimuli. A very small stimulus such as a ligand binding to an extracellular receptor causes a large intracellular effect. This effect might be an increase in the transcription and translation of target genes.

9.7 Gap 1 (G1), gap 2 (G2), and synthesis (S) phases. Protein synthesis occurs during G1 phase. S phase represents DNA synthesis. G2 involves the cell making the final preparation for mitosis.

9.8 pRb is a tumour suppressor protein. It acts as a reservoir by binding to E2F-1, preventing its interaction with target genes. pRb also localizes HDAC to the pRb binding site, causing chromatin remodelling and preventing transcription and translation. Phosphorylation of pRb releases E2F-1 and HDAC allowing transcription and translation of target genes to occur.

9.9 The nucleosome is the smallest subunit of chromatin. Four histones H2A, H2B, H3, and H4 are duplicated—forming an octamer. This octamer is wrapped in double-stranded DNA. This complex is called a nucleosome. Nucleosomes attach to one another with linker DNA, stabilized by histone H1.

9.10 G-banding utilizes Giemsa stain to differentiate between heterochromatin (dark areas) and euchromatin (light areas). This forms a banding pattern, much like stripes. Each of these bands can be subdivided into sub-bands. Banding patterns are reported based on the proximity of the band to the centromere—the closer the band to the centromere, the lower the assigned number. By recognizing changes in the banding pattern of chromosomes, we can see how particular genes have been disrupted. We can report the location of a gene based on our knowledge of its location defined by the band it occupies.

9.11 The protein products of proto-oncogenes are involved in cellular drive. Particular roles include increased growth, proliferation, and survival. If mutations within proto-oncogenes cause a loss-of-function, this is not oncogenic as the cells will lose their growth and survival requirements. However, mutation of a proto-oncogene (now called an oncogene) causing a gain-of-function can be associated with malignant transformation as the cell’s original growth and survival requirements will be exacerbated.

Tumour suppressor genes produce proteins which counteract proto-oncogenes. Tumour suppressors ‘put the brakes’ on the cell cycle and are important in screening for and detecting DNA mutations. Tumour suppressors can cause cell cycle arrest, DNA repair, and induction of apoptosis. The mutation of tumour suppressor genes causes a loss-of-function of the protein, therefore DNA mutations will go undetected, the cell cycle continues inappropriately, DNA repair will not occur, and apoptosis will be inhibited.

9.12 Point mutation, missense mutation, nonsense mutation, frameshift mutation.

A point mutation is a substitution of a single nucleotide for another.

A missense mutation substitutes one nucleotide for another. The consequence of a missense mutation is that the mRNA codon sequence is altered, encoding for a different amino acid to that intended.

A nonsense mutation substitutes a single nucleotide for another, but, rather than encoding a different amino acid, fails to encode for an amino acid, forming a stop codon.

A frameshift mutation includes the addition or deletion of nucleotides within a sequence—altering the codon composition of the mRNA. As a consequence, the protein structure will be modified beyond the original location of the mutation.

9.13 47,XY, + 7,-8, + 12,t(15;17)(q22;q12)[20].

9.14 Hypermethylation involves the addition of methyl groups to cytosine residues within CpG islands. These methyl groups allow the binding of transcriptional repressors, such as HDAC, which can cause gene silencing.

Hypomethylation is associated with active genes. The absence of methylated cytosines allows the binding of transcriptional activators.

Acetylation is the process of adding acetyl groups to acceptor molecules. Of particular interest are the lysine residues within the H3 and H4 regions, which, when acetylated cause chromatin to adopt an open conformation and the binding of transcriptional activators via their bromodomains.

9.15 A clone is a group of cells derived from the same progenitor, containing identical genetic and cytogenetic features.

9.16 Knudson’s hypothesis describes the requirement of a single cell to acquire more than one mutation (or hit) in order to become oncogenic. For example, the loss of a tumour suppressor gene will not directly cause cancer, and the mutation of a proto-oncogene should be detected by functional tumour suppressors. However, mutation of a tumour suppressor and the subsequent mutation of a proto-oncogene can be oncogenic when combined within the same cell.

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