Genetic influences on alexithymia and their relationship with depressive symptoms
Introduction
The term ‘alexithymia’ (from the Greek a = lack, lexis = words and thymos = emotion, mood) was introduced in the early seventies to describe some cognitive and affective characteristics that were often observed among patients with so-called psychosomatic disorders [1], [2]. The alexithymia construct, which has gradually taken on a key role within psychosomatic medicine, encompasses limited ability to identify and verbally express emotions, to distinguish emotions from bodily sensations, impoverished fantasy life, and reduced symbolic thinking with a concrete, stimulus-bound, action-oriented thinking style similar to the ‘pensée opératoire’ [3].
While considerable research has been devoted to the association between alexithymia and physical and mental health problems and its possible causal implications, only relatively few studies have attempted to identify which factors are involved in the etiology of alexithymia. Most of these studies pointed to a role of childhood family factors [4], [5], [6], [7], [8], [9], [10], [11]. Although such factors seem to be entirely environmental in nature, in recent years behavioral genetic studies revealed that the exposure to many features of the family environment is at least partly under genetic influence [12], [13].
There are several mechanisms that may account for the contribution of genetic factors to individual differences in alexithymic characteristics. The genes potentially involved in variation in alexithymia may influence the transcription of neuronal receptors or neurotransmitters, or may affect neurodevelopment. Genetic factors significantly contribute to variation in brain structures [14], [15], and a recent study reported an association between alexithymia and the catechol O-methyltransferase Val108/158Met gene polymorphism [16]. Some family studies found a significant association of mothers' [17], [18], [19] and fathers' [19] alexithymia scores with alexithymia scores in the offspring. While these studies suggest an intergenerational transmission of alexithymia, they cannot establish whether the observed familial aggregation originates from genetic or environmental influences shared by family members.
The twin study design is the most suitable to disentangle the role of nature and nurture. Given that monozygotic (MZ) twin pairs share all genes, while dizygotic (DZ) pairs share on average 50% of them, a higher phenotypic resemblance observed among MZ pairs for a trait suggests a substantial role of genetic factors in its expression. To date, only three twin studies of alexithymia have been performed. Two pioneer studies suggested genetic influences on alexithymia [20] or at least its externally oriented thinking facet [21]. However, these studies suffered from limitations in sample size [20], [21] and measurement [20]. Recently, a very large population-based study of 8785 twin pairs strongly supported a genetic contribution to individual differences in all facets of alexithymia [22].
While these studies suggest genetic effects on alexithymia, there are several related issues that deserve further investigation. First, no study used multivariate models to examine whether there are genetic or environmental influences common to alexithymia facets. Second, it remains to be determined if genetic influences on alexithymia are independent of genetic influences on other mental health variables, such as depression, which are correlated with alexithymia [23], [24], [25] and are known to be genetically influenced [26], [27].
In this study, we measured alexithymia and depression in a general population sample of MZ and DZ twins reared together, with the aim of confirming previous findings of a genetic contribution to individual differences in alexithymia, examining whether there are genetic or environmental influences common to alexithymia facets, and investigating whether genetic influences on alexithymia are independent of genetic influences on depression.
Section snippets
Participants
The study sample was derived from the population-based Italian Twin Register (ITR) [28], which currently contains information on approximately 23,000 Italian twins belonging to different age groups and geographical areas and is involved in both general population- and clinical-based studies on various complex phenotypes, some conducted within large European networks [29].
A total of 2930 twins enrolled in the ITR and aged 23–24 years were contacted by mail and invited to participate in a study
Sample descriptives
Table 1 shows means and standard deviations of the total TAS-20 and its subscales, computed for twins as individuals, according to gender and zygosity.
The TAS-20 total score was moderately correlated (r=0.44) with PDS score. Females scored, on average, significantly higher on the DIF dimension and significantly lower on the EOT dimension compared to males. No significant zygosity differences were detected.
Correlations
Table 2 provides the observed correlation structure for the total TAS-20 and its subscales.
Discussion
We found a substantial contribution of genetic factors to individual differences in alexithymia and all its facets as measured by the TAS-20. This finding is consistent with a recent large Danish study, though our heritability estimates are slightly higher than those reported in that study [22]. This discrepancy may be due to age differences between study samples, as all our twins were of young age whereas the Danish twins had a broad age range of 20–71 years. Our finding of substantial genetic
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