Questions: Copy Number Variation and Structural Variants

Question 1

A patient with intellectual disability is found to carry a deletion of one copy of a gene on chromosome 22 involved in neural development. What primarily determines whether this CNV causes the patient's symptoms?

• A. The size of the deletion in base pairs — larger deletions are always more harmful
• B. Whether the deleted gene is dosage-sensitive — if two copies are required for normal function, losing one copy will have consequences; if one copy is sufficient, the deletion may be benign
• C. Whether the deletion is inherited or arose de novo — de novo CNVs are always pathogenic while inherited ones are benign
• D. The number of segmental duplications flanking the deletion — more segmental duplications indicate a more harmful variant

Answer: (B)

Gene dosage sensitivity is the key determinant. Many genes function normally with one copy (haploinsufficiency tolerance); others require two functional copies for adequate protein production. CNVs are pathogenic when they alter the copy number of dosage-sensitive genes. The 22q11.2 deletion (DiGeorge syndrome) is severe because it removes dosage-sensitive developmental regulators. Size alone doesn't determine pathogenicity — a large CNV in a gene-poor region may be benign while a tiny deletion of a critical gene is catastrophic. De novo status increases suspicion of pathogenicity but inheritance doesn't guarantee benignity.

Question 2

Why do CNV 'hotspots' — genomic regions that frequently generate new CNVs — cluster in areas rich in segmental duplications?

• A. Segmental duplications are AT-rich and therefore more prone to double-strand breaks during replication
• B. Segmental duplications are transcribed at high rates, creating R-loops that destabilize the nearby genomic regions
• C. The highly similar sequences in flanking segmental duplications can misalign during meiosis, causing the recombination machinery to cross over between non-allelic copies, generating duplications on one chromosome and deletions on the other
• D. Segmental duplications contain transposons that actively excise and reinsert, carrying adjacent sequences with them

Answer: (C)

The mechanism is nonallelic homologous recombination (NAHR). Homologous recombination normally occurs between paired alleles — the same position on sister chromatids or homologs. When flanking segmental duplications are highly similar in sequence, the recombination machinery can mistake one copy for the other and cross over between them instead of between the actual alleles. This misalignment produces one chromosome with a duplication and one with a deletion. The 22q11.2 region, 17p12, and other CNV hotspots are all flanked by segmental duplications that facilitate this misalignment.

Question 3

CNVs collectively affect more base pairs of the human genome than single-nucleotide polymorphisms (SNPs) do, even though SNPs are more numerous as individual variants.

• T. True
• F. False

Answer: True

This is a striking and counterintuitive finding from human genomics. There are far more SNPs than CNVs in any two human genomes, but each CNV affects a stretch of DNA typically 1 kilobase or larger — some span megabases. SNPs, by definition, each affect a single base pair. When you sum the total base pairs affected, the larger size of CNVs outweighs their lower count, so CNVs account for more total genomic variation. This is why CNVs, despite being discovered later than SNPs, are recognized as a major form of human genetic diversity.

Question 4

Copy number variations are rare mutations affecting a small percentage of the human genome, and the vast majority of CNVs detected in the population are associated with disease.

• T. True
• F. False

Answer: False

Both claims are wrong. CNVs are common — approximately 12% of the human genome consists of CNV-prone regions, and most individuals carry dozens to hundreds of CNVs. The majority of common CNVs are benign, falling in non-coding regions or affecting dosage-insensitive genes. Some are adaptive: AMY1 copy number variation correlates with starch digestive capacity. Disease-associated CNVs are a subset, typically involving large deletions or duplications of dosage-sensitive genes. Conflating 'structural variant' with 'pathogenic variant' is a fundamental misunderstanding.

Question 5

Why might populations with a long history of high-starch diets carry more copies of the AMY1 gene, and what general principle about CNVs does this example illustrate?

Model answer: AMY1 encodes salivary amylase, which begins starch digestion in the mouth. More AMY1 copies produce more salivary amylase protein, increasing starch breakdown efficiency and potentially improving caloric extraction from starch-rich foods. In populations where starchy tubers, grains, or roots were dietary staples for many generations, individuals with higher AMY1 copy number may have had a survival or reproductive advantage, leading to positive selection for higher copy number. This illustrates that CNVs are not inherently pathological — they are a form of heritable genetic variation that can be neutral, harmful, or beneficial depending on context and gene function.

The AMY1 example is important because it demonstrates CNVs as a mechanism of adaptive evolution, not just disease. It shows that copy number — how many times a gene is present — can be a quantitative tuning mechanism for gene expression levels. This connects CNVs to the broader concept of dosage sensitivity: just as some genes are sensitive to reduced copy number (haploinsufficiency), others respond to increased copy number with proportionally increased protein output, which can be adaptive.