Individuals with schizophrenia who test positive for a rare gene mutation will likely have a better prognosis than patients with other forms of the disease, according to researchers from the University of Iowa.
The scientists also found that the linkage disequilibrium associated with the gene may coincide with “overall benefits for human survival.”
“While this polymorphism makes us more vulnerable to a certain illness … overall it is evolutionarily beneficial,” Robert Philibert, associate professor of psychiatry at the University of Iowa and the study’s principal investigator, said in a statement this week. Traditionally, genes that affect human evolution influence resistance to infection and infant survival, he said. “This gene may be involved in both of these positive features.”
Almost one in 50 people of European descent has the polymorphism, called HOPA12pb, and a “small minority” of those with the mutation have schizophrenia, Philibert said. Men are more likely than women to have this form of schizophrenia because HOPA12pb, which was originally identified in 1994 and linked to hypothyroidism, is linked to the X chromosome.
Philibert told Pharmacogenomics Reporter he is completing a study that compares women and men with schizophrenia who test positive for the polymorphism. (He declined to elaborate, saying the research paper describing the results is in press.)
Though patients with HOPA-linked schizophrenia may have hallucinations, they do not have the other “negative symptoms” common to other forms of the disease: decreased attention and loss of emotion.
“We knew the gene causes a specific form of schizophrenia, but we didn’t know that the type was associated with a good prognosis and marked by absence of negative symptoms,” Philibert said, adding that “most individuals with this positive-symptom schizophrenia are able to function in society. … “
There are “at least” 25 genes in the complex in which HOPA resides, several of which are implicated in other diseases, according to Philibert. In fact, the type of evolutionary advantage — resistance to disease — that Philibert’s team identified in the HOPA polymorphism is also common in a polymorphism linked to sickle cell anemia. There, one copy of the polymorphism causes mild cell abnormalities but also provides resistance to malaria, and “thus promotes human survival,” the scientists said. However, two copies of the variant gene cause the debilitating disease sickle cell anemia.
Philibert said he largely inferred the likely effect on human survival illustrated by HOPA12pb after noting the gene’s linkage disequilibrium profile. This portion of the study results is “so greatly influenced by the recombination frequencies at any given marker, and the fact is that we didn’t drop all that many markers down,” he said. “Ideally, if funding was good we would have dropped down three or four more markers.
“I think the most interesting thing about HOPA is, number one, it shows that psychiatric illness is no different than other illnesses,” Philibert told Pharmacogenomics Reporter this week. “Sometimes the genes that make us vulnerable to illness are evolutionarily more beneficial to us, and [they] interact somehow with other genes and the current environment to produce illness.”
Asked whether these data might lead to a molecular diagnostic that could help psychiatrists determine the potential severity of schizophrenia in patients, Philibert said he is “hopeful.”