Past exam of the biochemistry course of the University of Cambridge / 2024 / Paper 3 / Section I

I would establish whether the alteration drives cancer, whether the established cancer depends on it, and whether that dependence offers a selective intervention point. These are different questions. A recurrent alteration can be a passenger; a genuine initiating driver can become dispensable after progression; and an essential protein can be equally essential in normal tissue.

First validate the genomic observation with an independent assay and matched normal DNA. Distinguish a somatic mutation from inherited variation, sequencing error and contamination. For gene amplification, resolve the altered interval, copy number and tumour purity, and determine whether the candidate is the focus of a recurrent, narrowly amplified segment or simply accompanies a neighbouring driver in a broad gain. Measure RNA, protein and relevant activity: increased DNA dosage need not increase active protein. For mutations, inspect the spectrum and allelic configuration. Recurrent clustered missense changes can suggest altered activity, whereas dispersed truncations and loss of the remaining allele can suggest tumour-suppressor inactivation. Neither pattern is conclusive without accounting for sequence context, background mutation rate and selection. Large genes, highly mutable sites and poorly repaired regions can recur without being selected; analyses of mutational heterogeneity and somatic selection show why a simple recurrence ranking is insufficient.^[ref][ref]

Relate the alteration to cancer type, stage, co-occurring lesions and clonal architecture. Single-cell or spatial assays can establish whether it is in the malignant compartment and whether every tumour biological cell carries it. Correct variant fractions for purity and copy number before inferring clonality. A clonal lesion may be early, but clonality alone does not prove causality; a passenger can hitchhike in an expanding clone. Association with outcome should be assessed within appropriate clinical and molecular strata rather than interpreted from a mixture of unrelated tumour types.

For functional necessity, use several independently altered cell lines and patient-derived organoids, together with lineage-matched unaltered cancers and nonmalignant controls. Reduce the candidate with orthogonal perturbations and rescue with a perturbation-resistant allele at a near-physiological level. Measure proliferation, cell-cycle distribution, apoptosis, differentiation and the proposed pathway output separately. In an amplified region, a particularly important control is to avoid equating CRISPR cutting toxicity with gene dependence: many DNA breaks at multiple copies can suppress growth independently of the encoded gene. Aguirre and colleagues demonstrated this copy-number-dependent response.^[ref] CRISPR interference or RNA interference, combined with rescue and appropriately matched cutting controls, helps distinguish protein dependence from this artefact. Guide-specific off-target effects and incomplete depletion also require direct checks.

Test sufficiency and allele specificity by introducing the observed allele into a relevant nonmalignant background, preferably through endogenous editing, or by increasing expression to the measured tumour level. Compare wild type, each recurrent mutant and appropriate inactive variants at matched abundance. For a suspected tumour suppressor, recreate the observed heterozygous or biallelic loss rather than assuming every mutation is equivalent to a knockout. A kinase-domain mutant might activate signalling, a truncation might remove negative regulation, and another allele might abolish function. Physiological assays of acinar organization, growth-factor dependence or invasion can reveal transformation phenotypes missed by a two-dimensional growth assay. Failure to transform one biological cell type alone does not exclude a driver requiring cooperating lesions or the correct developmental state.

Then connect genotype to mechanism. Identify affected enzymatic activity, binding partners, localization and downstream signalling, using time-resolved perturbation to separate primary changes from consequences of dying biological cells. Restore a downstream effector to test whether it bypasses loss of the candidate, or inhibit that effector to test whether it is required for the mutant phenotype. Rescue with an inhibitor-resistant target allele is strong evidence for a drug's intended mechanism. A correlation between mutation and a distant transcriptional signature is weaker. Genetic and pharmacological results should agree at exposures that establish target engagement rather than general cytotoxicity.

Finally test initiation and maintenance in suitable in vivo settings: tissue-appropriate engineered mice, orthotopic organoid transplantation or patient-derived xenografts, with the immune context chosen for the hypothesis. Inducing an allele before tumour development tests initiation; turning it off after established growth tests maintenance. Quantify tumour latency, burden, invasion and metastatic outcome with random allocation, blinded assessment and independent biological replicates. Measure effects on normal tissue to define a therapeutic window, and examine resistant outgrowth for bypass mechanisms. The strongest conclusion combines statistically credible human selection, allele-specific causal perturbation, mechanistic rescue and disease-selective dependence across appropriate models.

cancer largely reuses normal cellular machinery, but its organization and evolutionary behaviour are not adequately described as one homeostatic control simply being turned up or down. Growth, survival, metabolism, plasticity and tissue remodelling all have normal functions. Malignancy arises when their coordination with the needs of the organism is lost, and selection can then generate new combinations, altered molecular activities and pathological ecological interactions.

Normal proliferation depends on extracellular signals and internal cell-cycle controls. Constitutive receptor or Ras signalling, increased cyclin activity or weakened RB–E2F restraint can make a biological cell divide under conditions in which its neighbours would arrest. These changes exploit existing DNA replication and mitotic machinery rather than inventing a new way to duplicate a genome. Equally, normal apoptosis removes damaged or inappropriate biological cells; a tumour can survive by suppressing this response. Loss of p53 can compromise a network linking stress to arrest, repair, death and other restraints. Mice lacking p53 develop spontaneous tumours despite largely normal development, directly demonstrating the importance of an existing protective programme.^[ref] This does not mean that every cancer must eliminate every p53 output or that all p53 mutations have the same effect.

Many other properties have similarly recognizable antecedents. glycolysis, nutrient uptake and autophagy support normal growth and stress adaptation; their deployment can help cancer biological cells tolerate fluctuating resources. Angiogenic signals normally support development and repair, but sustained production can recruit vessels into a growing tumour. Motility, matrix remodelling and reversible changes in adhesion participate in development and wound repair. cancer can recruit them for invasion, with surrounding fibroblasts and immune biological cells contributing signals and physical routes. Collective breast-cancer invasion, for example, can depend on a conserved basal epithelial programme rather than a completely new locomotor apparatus.^[ref] Immune suppression also uses normal regulatory pathways, although suppressing antitumour immunity conflicts with organismal protection.

The homeostatic interpretation is most useful when it emphasizes constraints and feedback. Normal tissue renewal couples production to biological cell loss and spatial organization. A mutation increasing division, reducing death or changing competitive interactions can unbalance this system even without maximal activation of every pathway. Selection acts on net clonal success, not only on instantaneous proliferation. A slowly cycling population that survives therapy or colonizes a new site may be favoured under particular conditions. Conversely, an excessively strong oncogenic signal can trigger senescence or apoptosis, so successful cancers often combine activating changes with escape from compensating restraints. Different combinations can converge on similar phenotypes.

However, “nothing new” becomes misleading at the molecular level. Some alterations generate a qualitatively changed activity. Cancer-associated IDH1 mutants acquire the ability to reduce alpha-ketoglutarate to R-2-hydroxyglutarate, rather than simply increasing the enzyme's ordinary reaction. metabolite profiling and biochemical assays established this altered catalytic activity.^[ref] gene fusions can join recognition and signalling domains in new regulatory arrangements, while new enhancer contacts or altered splicing can change which molecular combinations are expressed. These innovations still obey familiar chemistry and use inherited components, but that is true of biological innovation generally; it does not make them equivalent to a reversible normal homeostatic response.

cancer is also an evolving population within a changing environment. Genetic and epigenetic variation permit selection for survival during hypoxia, immune attack or treatment. The selected phenotype can depend on the surrounding stroma and on interactions among tumour subclones. Normal wound-healing responses may stop when repair is complete, whereas a tumour can maintain the signals or continually regenerate the damage that recruits them. Altered metabolism, inflammation and genomic instability can create feedback that changes the environment itself. This is not a single biological cell merely ignoring one stop signal, and one successful clone need not benefit either neighbouring cancer biological cells or the host.

Experimentally, the distinction matters. If an existing normal programme is misregulated, correcting the upstream input or restoring feedback may normalize behaviour, but normal tissues using the same programme may also be affected. If an altered protein has a tumour-specific pocket or catalytic dependence, that difference may provide selectivity. Genotype-matched perturbations, measurements of the normal counterpart and tests in tissue context discriminate these cases. A list of cancer-associated phenotypes alone does not determine whether the causal change is quantitative, qualitative or ecological. cancer usually repurposes normal processes, but malignancy emerges from altered control, molecular changes and selection among biological cells; “simply deregulation” understates that causal complexity.