Today I bring the findings reported by Chou and colleagues  (open-access available here) to the blogging table and how the research might of the Taiwan National Health Insurance Database (NHIRD) brought it's 'big data' ("n = 23 422 955") to bear on the question: what is the risk of developing schizophrenia where one or more first-degree or other relatives are affected?
The answer: "Having an affected co-twin, first-degree relative, second-degree relative, or spouse was associated with an adjusted RR [relative risk] (95% CI) of 37.86 (30.55-46.92), 6.30 (6.09-6.53), 2.44 (1.91-3.12), and 1.88 (1.64-2.15), respectively. Compared with the general population, individuals with one affected first-degree relative had a RR (95% CI) of 6.00 (5.79-6.22) and those with 2 or more had a RR (95% CI) of 14.66 (13.00-16.53) for schizophrenia."
To translate the science-talk: if one twin is diagnosed with schizophrenia, there is a hugely increased risk of the other twin also being affected. If a mother or father, sister or brother, or your child(ren) are diagnosed with schizophrenia, there is an enhanced risk but nothing like the risk to twins. As you move outwards to other outlying family members (uncles, aunts, grandparents, etc) affected, your risk continues to diminish albeit still noticeable. Interestingly, when it comes to spouses (husband or wife), there is a small but increased risk that if they are diagnosed with schizophrenia so the other partner is at some small, enhanced risk. This tallies with the concept of assortative mating  but does not necessarily rule out other shared non-genetic factors either.
The final sentence in that quote provides some evidence for a cumulative effect too. So if one of your close family members is diagnosed with schizophrenia, so the risk to yourself might be heightened. If two or more close family members are diagnosed, the relative risk to yourself jumps quite a bit more.
"A family history of schizophrenia is therefore associated with a higher risk of developing schizophrenia, mood disorders, and delusional disorders. Heritability and environmental factors each account for half of the phenotypic variance of schizophrenia."
To close, Yoda don't like seagulls...
 Chou IJ. et al. Familial Aggregation and Heritability of Schizophrenia and Co-aggregation of Psychiatric Illnesses in Affected Families. Schizophr Bull. 2016 Nov 21. pii: sbw159.
 Parnas J. Assortative mating in schizophrenia: results from the Copenhagen High-Risk Study. Psychiatry. 1988 Feb;51(1):58-64.
Chou IJ, Kuo CF, Huang YS, Grainge MJ, Valdes AM, See LC, Yu KH, Luo SF, Huang LS, Tseng WY, Zhang W, & Doherty M (2016). Familial Aggregation and Heritability of Schizophrenia and Co-aggregation of Psychiatric Illnesses in Affected Families. Schizophrenia bulletin PMID: 27872260
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