Risk for Cancer in Parents of Patients With Schizophrenia
Oksbjerg Dalton S, Munk Laursen T, Mellemkjaer L, Johansen C and Mortensen PB;
Commented by , 28 Jun 2004
Aim of the study
There is some scientific evidence in the literature that patients with schizophrenia have a reduced risk for different kinds of cancers, in particular lung cancer. There are two general theories that may explain this finding: The first theory is that treatment with phenothiazines leads to some protection against malignancies. The other theory is that there is a genetic factor that is associated with both schizophrenia and reduced cancer risk.
The common link may be apoptosis (programmed cell death). Increased apoptosis may on the one hand lead to schizophrenia, but may on the other hand prevent cancers from growing in this population. If there were a genetic reason for the reduced cancer risk of patients with schizophrenia, then relatives of schizophrenics should have decreased cancer risk, as well. The objective of the study by Dalton and colleagues therefore was to determine whether there is a genetic protection against cancer in parents of offspring with schizophrenia.
Methods
The study was carried out in Denmark. The authors identified 1,999,072 parents of children born after 1935 in the Danish Central Population Registry. This Denmark wide population-based parent cohort was linked to the Danish Psychiatric Central Register so that 19,856 parents of offspring with schizophrenia were identified. The date of birth of the oldest child or April 1, 1969 was used for the start of follow-up for cancer in the Danish Cancer Registry and the follow-up was ended on the date of cancer diagnosis, death, or Dec. 31, 1997.
Thus, a total of 48,343,430 person-years at risk and 211,681 cases of cancer were available for the analysis. In the statistical analysis, Poisson regression analysis adjusted for age, period, and number of children was used to estimate the relative risk for cancer among parents with schizophrenic children compared to those parents with no schizophrenic children.
Results
The overall cancer risk in parents of children with schizophrenia was not different compared to parents without schizophrenic children. For fathers of schizophrenics the relative risk for all cancer was 1.01 and for mothers it was 1.00 indicating virtually no difference in risk compared to the controls. The risk for lung cancer of mothers of schizophrenic patients was increased rather than decreased (relative risk 1.2). The only reduced cancer risk was found for leukemia in both mothers and fathers of schizophrenics, while none of the other forms of cancer examined showed any significant differences.
Discussion
The main strength of epidemiological studies from the Nordic countries is that in some of them population based registers are available for medical research. The main result of the study was that it did not show a reduced risk for cancer in patients with schizophrenia. Thus it did not provide support the theory of genetic protection against cancer in schizophrenics and their families. In their discussion the authors mainly explain differences between their findings and a previous Finnish study that did show a reduced cancer risk in parents of schizophrenic patients (ref. 1).
The main methodological difference in the Finnish study was that the authors compared the cancer risk in parents of schizophrenics with the cancer risk in the general population rather than in parents without schizophrenic children. Dalton and colleagues thus explain the reduction of cancer risk in the Finnish study by a positive effect of being a parent rather than being a first-degree relative of a schizophrenic patient. They speculate that parents in general have a healthier and more regular life style than the general population.
The only form of cancer that was reduced in the current study was leukaemia. Since previous studies had not shown a reduced risk for leukaemia, the authors recommend cautious interpretation of this finding. Namely, it might just have been an artefact of the multiple statistical tests that were applied in their analysis.
References
1. Lichtermann D, Ekelund J, Pukkala E, Tanskanen A, Lönnqvist J: Incidence of cancer among persons with schizophrenia and their relatives. Archives of General Psychiatry 2001; 58:573–578