By Charlene N. Rivera-Bonet, Waisman Science Writer
Brain structure isn’t always the first thing that comes to mind when we think about what we inherited from our biological parents or share with our siblings. We mostly think about facial features, hair color, and even personality. Investigators at the Waisman Center, University of Wisconsin-Madison found that genetics explains individual differences in even small details of the brain such as the development of axons– the part of a neuron that carries signals for brain communication.
Brain development can be influenced by both genetics and the environment – like nature and nurture. A study led by Ross Luo, MS, and published in Brain Structure and Function found that some microstructural characteristics of the brain, in particular axons, are highly heritable, being shaped more by genetics than by the environment. This genetic influence is observed in differences in brain microstructure between individuals. Understanding how brain structure is influenced by either genetics or the environment helps scientists better understand how the brain develops both typically and atypically.
Although the influence of genetics versus environments can be hard to disentangle, studying twins helps understand specific effects of genetics, shared environments, and unique environments on the brain. Particularly, this study recruited both monozygotic or identical twins – who share the same genes – and dizygotic or fraternal twins – who have about half of their genes overlapping. Comparing the twins’ brain characteristics allowed the researchers to understand how much of the differences were influenced by genetics versus environment. Characteristics for which identical twins are more similar than fraternal are considered heritable. On the other hand, when the characteristics are equally similar between identical and fraternal twins, they are attributed to a shared environment. The microstructural characteristics of the brain that contribute to making each person different are not fully understood yet.
Bundles of axons make up what’s known as white matter in the brain. White matter helps carry information from one region of the brain to another. This study by Luo and his colleagues is one of the largest to look at genetic and environmental effects on white matter microstructure in the brain, says Andy Alexander, PhD, professor of medical physics and psychiatry and a Waisman Center investigator. It is also the first study to use a novel magnetic resonance imaging (MRI) technique that measures the density and organization of axons, called neurite orientation dispersion and density imaging (NODDI).
Identical twin pairs had the most similar white matter microstructure compared to fraternal pairs. This points to genetics being the biggest contributor to organization and density of axons. It also means that individual differences in white matter microstructure are explained primarily by genetic factors, not environmental factors.
The levels of heritability, however, varied throughout the brain, with unequal genetic influence in different brain regions. In particular, the white matter tracts most heavily influenced by genetics were those connecting the cortex – the outermost layer of the brain involved in many functions including cognition, emotion, and many others – with deeper brain structures. Regions with more complex organization of axons were also primarily influenced by genetics.
The regions with lowest heritability levels were found in the midbrain and brainstem associated with sensory and motor information processing, and might be heavily influenced by experience or other aspects of the environment.
The researchers also looked at the influence of shared environments in axonal development. Shared environmental influences can include shared family experiences, schools and communities. “Because they’re twins and living in the same household, they probably get more shared experiences than most people. But each person has their own unique experiences as well,” says Alexander. The shared environmental influence appeared to be minimal in the majority of the white matter tracts assessed.
The majority of this research was conducted by Zhan “Ross” Luo, MS, who passed away shortly after the completion of this study. Ross was a graduate student in the Neuroscience Training Program at UW-Madison. He was co-mentored by both Andy Alexander, PhD, and Hill Goldsmith, PhD, Antoine Bascom Professor & Leona Tyler Professor of Psychology and a Waisman investigator. This study illustrates his important scientific contributions to the study of genetic and environmental influences on the brain and behavior development. To learn more about Luo, click here.