The developing human brain may carry out a large-scale form of genomic quality control around the time of birth. A study of individual cortical neurons found that cells with widespread copy-number variants were considerably more common before birth than afterward, suggesting that many genetically abnormal neurons are selectively removed during the perinatal period.
The research, published in Nature, examined 2,357 single neurons from postmortem human brain samples spanning fetal development through old age. Using two independent single-cell whole-genome amplification methods, the team reported that an average of 25.9% of prenatal cortical neurons carried widespread copy-number changes involving multiple chromosome regions. The proportion was about 4.5 times lower after birth.
Following genomic variation one neuron at a time
Copy-number variants, or CNVs, are gains and losses of sections of DNA. Large CNVs can change the dosage of many genes at once and are associated with neurodevelopmental and neuropsychiatric conditions. The researchers used Tn5-transposase-based amplification and primary template amplification to compare CNVs in prenatal and postnatal neurons.
The analysis distinguished neurons with limited changes from those carrying broad patterns of gains or losses across their genomes. The largest decline occurred around birth, roughly parallel to a known wave of programmed neuronal cell death. Neurons with smaller CNV burdens appeared to decline more slowly across postnatal life.
Additional microscopy identified micronuclei—small structures containing missegregated chromosomal material—in the fetal cortical plate. Sequencing of micronuclear fractions indicated that this aberrant genetic material also became less prominent with age, providing a separate line of evidence consistent with clearance.
Synaptic gene activity may influence survival
To investigate why some neurons might be eliminated, the researchers also analyzed chromatin accessibility and gene expression in 44,861 nuclei. Postnatal neurons with higher CNV burdens showed disrupted activity in synaptic gene modules. A subset combined reduced synaptic-module expression with increased activity in cell-death pathways.
The authors propose that, as neural circuits form, neurons with extensive genomic disruption may be less able to establish appropriate synaptic connections and may therefore be more likely to undergo programmed cell death. Surviving neurons were relatively depleted of CNVs affecting dosage-sensitive genes and genes linked to neurodevelopmental disorders, which is consistent with selective pressure against damaging variants.
What the study does and does not establish
The findings are based on postmortem tissue collected across different individuals rather than repeated observations in the same developing brain. Third-trimester samples were limited, making it difficult to define the exact timing and mechanism of the proposed selection process. The data therefore support an inferred developmental pattern, not direct observation of individual neurons being removed.
CNVs also represent only one form of somatic mutation. Neurons can carry point mutations, insertions and deletions that were not the main focus of this analysis. The study does not imply that every neuron with a CNV is defective or destined to die; size, location and gene content are likely to matter.
A developmental window for genome selection
The results suggest that mature human neurons emerge from a more genetically diverse prenatal population than previously appreciated. Around birth, when synapses and circuits are rapidly forming, the brain may preferentially retain cells whose genomes and gene-expression programs are compatible with effective neuronal function.
Understanding that process could help researchers investigate whether genetic or environmental factors that disrupt developmental cell selection allow damaging neuronal variants to persist. The work also raises questions about whether related quality-control mechanisms continue to operate in ageing and neurodegenerative disease.