As much as 79% of schizophrenia risk may be explained by genetic factors, researchers at the University of Copenhagen, Denmark, estimate in the largest study of twins in schizophrenia research to date.
The study used a new statistical approach to address one of the factors that contributes to inconsistencies across previous studies.
Usually studies of heritability require that people be classified as either having schizophrenia or not, but some people at risk could still develop the disease after the study ends. Drs. Hilker, Helenius and colleagues applied a new method to take this problem into account, making the current estimates likely the most accurate to date.
“The new estimate of heritability of schizophrenia, 79%, is very close to the high end of prior estimates of its heritability. It supports the intensive efforts in place to try to identify the genes contributing to the risk for developing schizophrenia,"
said Dr. John Krystal, referring to previous estimates that have varied between 50% and 80%.
The Schizophrenia Spectrum
The study took advantage of the nationwide Danish Twin Register — a record of all twins born in Denmark since 1870 — coupled with information from the Danish Psychiatric Central Research Register, to assess genetic liability in over 30,000 pairs of twins.
Because the diagnosis of schizophrenia is based on a narrow definition of symptoms, the researchers also estimated heritability using a broader illness category including related disorders on the schizophrenia spectrum. They found a similar estimate of 73%, indicating the importance of genetic factors across the full illness spectrum.
Dr. Hilker explained,
“This study is now the most comprehensive and thorough estimate of the heritability of schizophrenia and its diagnostic diversity. It is interesting since it indicates that the genetic risk for disease seems to be of almost equal importance across the spectrum of schizophrenia,"
even though the clinical presentation may range from severe symptoms with lifelong disability to more subtle and transient symptoms.
“Hence, genetic risk seems not restricted to a narrow illness definition, but instead includes a broader diagnostic profile,"
Hilker, Rikke et al.
Heritability of Schizophrenia and Schizophrenia Spectrum Based on the Nationwide Danish Twin Register