Study finds mental illness may have unusual evolutionary basis

New Delhi: A new study has found that the common syndromesof severe mental illness, such as schizophrenia, bipolar disorder,dementia, obsessive-compulsive disorder, and addiction, may have anunusual evolutionary basis. The constant prevalence of mental illness across the history of the humanspecies suggests a Darwinian paradox. Despite causing significant disease,there seems little impact on selection for fitness. […]

New Delhi: A new study has found that the common syndromes
of severe mental illness, such as schizophrenia, bipolar disorder,
dementia, obsessive-compulsive disorder, and addiction, may have an
unusual evolutionary basis.

The constant prevalence of mental illness across the history of the human
species suggests a Darwinian paradox. Despite causing significant disease,
there seems little impact on selection for fitness. Genetic variation that
predisposes to psychiatric disease is seen across all populations and is
quite common. Previous selection, over evolution, may produce a bias
towards certain kinds of variation that may cause disease later in life.

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These mechanisms may protect against inflammation or infection when
young, only to predispose towards disease when one is older.
In the new study, researchers at the National Institute of Mental Health
and Neurological Sciences (NIMHANS), Bengaluru, and Institute of
Genomics at the University of Tartu, Estonia, explored these aspects
through the genetic analysis of individuals from families who have
multiple affected members, considering that the increased prevalence of
illness in a particular family should point to even more extreme evidence
of the risky alleles.

The study compared the genetic sequences of exomes (the regions that
are translated into proteins) from individuals in 80 such families and
compared them against African and South Asian populations. They also
looked for evidence of Neanderthal genes in the sample, as the

persistence of these ancient genomes in modern humans seems to
correspond to some traits as well as the risk of disease.

“We observed evidence of selection in 74 genes, which were mainly
involved in immunological and defense responses, including activation
and regulation of interferon-gamma, cytokine and immune system, and
different signaling pathways”, said Dr. Ajai K. Pathak, one of the lead
authors of the study.

The study inferred that almost one-fourth, 20 out of the 74 putatively
selected genes were implicated in the risks of illnesses such as
schizophrenia, dementia, and Parkinson’s disease and general attributes
such as intelligence and cognitive abilities.

“There was also evidence of Neanderthal genes, but their variation and
distribution were much the same in the south Indian population and did
not correlate with the risk of disease or demonstrated signs of adaptive
selection” stated one of the senior authors of the paper, Dr. Mayukh
Mondal.

Noting that the human population has expanded from 1 billion 200 years
ago, to more than 6 billion now, and that over the same period human
longevity has also increased three-fold, the scientists said that perhaps, in
common with many other diseases like diabetes and hypertension that
begin later in life, the risk of psychiatric disease was embedded in the
biology and the nature and extent of genetic variation of our species.

Another lead author, Dr. Jayant Mahadevan, said that the study adds to
the growing evidence that the risk of psychiatric disease may be linked to
variations in the shared gene pool, which in turn depends on the
evolutionary history of the species.

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