Am J Med Genet C Semin Med Genet 15;133(1):43-7
History and geography of suicide: could genetic risk factors account for the variation in suicide rates?
The current state of knowledge of genetic predisposition towards the suicidal behavior allows for a question whether genetic risk factors account for the variation in suicide rates through time and space. Accordingly, the presented paper will attempt to tackle the genetics behind suicidal behavior from the perspective of the populational genetics. First, the variability of suicide rates across Europe is discussed. This is followed by a brief discussion of the J curve (on a map of Europe, the countries with a higher suicide rate form a so-called J curve, which starts in Finland and extends down to Slovenia), which maps on to the second principal component identified for European gene distribution, representing the ancestral adaptation to cold climates and the Uralic language dispersion. Furthermore, we will discuss whether the group of people living within the J-curve could share genes, which may not tolerate excessive amounts alcohol, the combination of which is more likely to end in suicidal behavior. Further along we list possible ways in which suicidal behaviour could have been selected for genetically in populations and identify those specific populations in which it may have appeared. Finally, we point at other locations in the world where a similar interplay of genes and environment has probably occurred, Greenland being the best example of the malignant interaction of alcohol consumption and the trait-like characteristics, which might constitute the vulnerability to suicidal behaviour.
Slovenia is a good example to illustrate that such prediction would not be valid. According to environmental predictors, Slovenia should share the same low rate with its Mediterranean neighbours such as Italy. Also about 70 per cent of its population are Roman Catholic; a religion where suicide is strictly forbidden. However, Slovenia has a suicide rate above 30 per 100,000 per year. Also, the annual suicide rate in Slovenia has remained remarkably stable, despite recent profound political and social change. Moreover, high rates of suicide in the native rather than the immigrant population of Slovenia also contradicts the theory that social and ecological risk factors are the major influence on suicide statistics [Marusic and Farmer, ]. Finally, Slovenia does differ from other suicide-prone nations in some environmental predictors. This suggests that factors other than environmental predictors may be implicit in determining national suicide rates in Europe. Such factors could include some shared genetic vulnerability to suicidal behaviour among some European nations.
An example of similar genetic origins could be the Hungarians of central Europe (living in Hungary, Transylvania in Romania and Vojvodina in Serbia) and the Finns and Karelians in the North, who belong to the same (Finno-Ugrian) ethno-lingual group. Among all these groups, suicide is widespread, and we find even higher suicide rates among individuals in the original territory of both ethnic groups, which is in the Urals, specifically between Mari and Udmurts with the suicide rates three times higher than those in Hungary and Finland. Kondrichin [] was the first to suggest that the high rates, which we see among individuals of Finno-Ugrian ethnicity, extend to the early development of ethnic groups in whom a collection of shared genes contributed to an elevated susceptibility to suicide. It is interesting that in their genetic inheritance, both Finns and Hungarians have approximately the same ratio of European (90 %) and Uralic genes (10 %) [Cavalli-Sforza et al., ]. Mogyorosi [] claimed that there is no evidence that so-called Finno-Ugrian nations are genetically closer to each other than to their geographical neighbors. Melhus [] pointed at recent analyses of mitochondrial lineages in European population, which show that the Finnish population belongs to a gene pool common to most other European groups [Sajantila and Paabo, ]. The mitochondrial DNA will, however, only show maternal DNA. Hence, it remains possible that it is the male DNA that discriminates between the Finnu-Ugrians and other Europeans. For example, according to some theories, Hungary was occupied by 100.000-500.000 Magyar invaders, herders who spoke a Uralic language in 862. They formed a fraction of 10-50% of the total population after the invasion; subsequent gene flow from neighbors is likely to have decreased this initial proportion. The current estimate of admixture is 13%. Other Finnu-Ugrians are Finns and Lapps. The Finns have a proportion of 90% European genes and 10% Uralic [Guglielmino et al., ].
A historical review of the traditions associated with wine in Slovenia and the discerning of inheritance and environmental factors can shed some light on this question. Wine-growing, which the Romans first introduced to the area now encompassing Slovenia, nearly died out when the Slavs and Magyars conquered the area. The production of wine was very limited until the 12th century. A theory exists that the members of these tribes, whose inheritance or genetic structure was not compatible with the consumption of alcohol, are the primary ancestors of those whose present suicide rate is among the highest in Europe. Once religious (environmental) factors had led to the re-establishment of vine growing, the region's inhabitants who were genetically unsuited to alcohol consumption, soon developed the highest rates of alcohol related mental disorders. This adverse combination of genetic propensity and high alcohol consumption led to the high rates of suicide noted in the region today. It is worth noting that current suicide rate in the north-east of Slovenia is the same as the Hungarian rates, and about twice as high as those in more western regions of Slovenia [Marusic and Farmer, ].
In the mid-13th century, Genghis Khan's Mongol hordes swept across the Russian plain, seizing Moscow and imposing their rule over much of the country for the next 250 years. Almost eight centuries later, it seems that genetic traces the Mongols left behind are partly to blame for Russia's traditional weakness for alcohol. The gene, known as ADH2-2, is common in Asian countries but almost non-existent among Europeans. Carriers of the gene are more susceptible to the effects of drinking and more likely to become alcoholics. Since alcohol consumption was rare among Asiatic tribes, they evolved differently from Europeans, who have a long tradition of producing and consuming alcohol. The culture of drinking has existed for centuries in Europe, dating back to before the Roman Empire, so Europeans have built up a natural resistance. Scientists have long known that the ADH2-2 gene is common among Russians, but Ogurtsov et al. [] were the first to test how it affects alcohol consumption. There is a negative correlation between the ADH2-2 allele and alcohol misuse (both alcoholic dependence and alcoholic cirrhosis). This correlation is expressed more in alcoholic dependence. Among many other negative characteristics, it has also been found that those with the gene were more likely to have negative emotional responses (like aggression and depression) and more likely to suffer withdrawal symptoms [Mainville, ].
Further north-east, in Alaska, an average of 130 suicides occurred annually in the last decade [Anonymous, ]. The most recent suicide rate was above 21 per 100.000 per year, which is the highest in the United States and twice the national average. Most striking is the data among Alaskan Natives whose rate of suicide (42.7) is four times the national rate. Furthermore, among Native American males the rate (68.5) is six times the national average [Anonymous, ].
Another population of the Inuit have been faced with high suicide rates, namely the Inuit of Greenland [Bjerregaard and Curtis, ]. This is however a much clearer example of the malignant interaction of alcohol consumption and the trait-like characteristics, which might constitute the vulnerability to suicidal behavior. The majority of the population of Greenland are Inuit, the ancestors of whom migrated from Asia about 20-30 000 years ago. In Greenland, alcohol consumption has recently increased dramatically. In 1950, the estimated average consumption among adults, expressed as litres of alcohol, was 7 litres per year. By 1987 this had increased to a maximum of 22 litres. Accordingly, suicides were uncommon in the 1950s with an estimated rate of 4.6 per 100.000 per year. During the following years, suicides increased dramatically and in the early nineties the suicide rate was 107 per 100.000 per year [Bjerregaard and Curtis, ].