Is brain size linked to two common gene variants?

BACKGROUND: Human brain size is hereditary, but the genes that influence brain size in healthy people are unknown. The genes microcephalin (MCPH1) and abnormal spindle-like microcephaly-associated (ASPM) are potential candidates, because mutations in each have been linked to microcephaly, a disease causing smaller head size and mental retardation.

Specific normal variants of the MCPH1 and ASPM genes have attracted scientific attention, because they are now widespread in some human populations -- despite evidence that these variants first arose only 5,000 to 37,000 years ago. This rapid increase suggests that the variants provided some evolutionary selective advantage. While scientists are uncertain why these variants are beneficial, they have interpreted the findings as evidence that the human brain continues to evolve.

FINDINGS: UCLA scientists aimed to evaluate whether these variants are associated with differences in brain size. The team used MRI scans to measure the brain size of 120 healthy people and then identified individuals with the MCPH1 and ASPM variants. The researchers found no evidence that either gene variant was related to increases or decreases in brain size.

IMPACT: The UCLA findings illustrate the need for scientific caution when interpreting evidence that nature has favored one gene variant over another during human evolution. Incorrect interpretations may lead to inaccurate assumptions about people who do or do not possess the favored variant.

AUTHORS: Dr. Roger Woods, associate professor of neurology at the David Geffen School of Medicine at UCLA, and a researcher at the Ahmanson-Lovelace Brain Mapping Center; and Dr. Nelson Freimer, professor of psychiatry and human genetics at the Center for Neurobehavioral Genetics, Semel Institute for Neuroscience and Human Behavior at UCLA, are available for interviews.

JOURNAL: The research appears in the May 10 online version of the peer-reviewed journal Human Molecular Genetics.

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