A new study in the January issue of the Journal of Vascular Surgery, published by the Society for Vascular Surgery, reveals robust epidemiological evidence that hereditary factors contribute to aneurysm formation in twins.
Researchers in Sweden reviewed the data of 172,890 twins born in that country since 1886 and found 265 twins who had developed abdominal aortic aneurysms. These individuals ranged from 48 to 94 years of age; 81% were male. Carl Magnus Wahlgren, from the Department of Vascular Surgery at Karolinska University Hospital in Stockholm, said that this was the largest abdominal aortic aneurysm population-based twin study to date.
“There were seven sets of identical as well as five sets of fraternal concordant pairs,” said Wahlgren. “Concordance rates represent the probability of developing abdominal aortic aneurysms for an individual with an affected twin. The identical pairs had a 24% probability that an identical twin of a person with aneurysms would get the disease. Their risk was 71 times higher than that of the identical twin of a person without aneurysms. The probability rate for the concordant fraternal twins was 4.8%. In contrast, there were 44 identical and 197 fraternal discordant pairs in the abdominal aortic aneurysm group.”
Researchers also reviewed the data for twins with abdominal aortic aneurysms ages 55 years and older, then possibly excluding genetic connective tissue disorders such as Ehler-Danlos and Marfan syndrome, the odds ratio still was significantly higher for identical as opposed to fraternal twins. Heritability of 70% of the total trait variance was estimated and the remaining variance was explained by non-shared environmental factors with no support for a role of shared environmental influences.
“Our twin model provides a powerful means of examining the total genetic contribution to a given disease especially a complex trait such as abdominal aortic aneurysm, and phenotypes (genetic makeup and environmental influences) can be defined to maximize chances of successful gene mapping,” said Wahlgren. “Unlike family or sibling studies, potential confounders such as the variability of disease prevalence with age are removed. This model relies on important assumptions including random mating, no interaction between genes and environment and equivalent environments for identical and fraternal twins.”
The study had only one female concordant twin pair with abdominal aortic aneurysms; researchers said they could not draw any conclusions about a possible stronger heritability in men or women. They noted that in other regions the proportions of type of effects could differ because of environmental factors; also in the cases of aneurysmal disease with several genetic and environmental factors, the liability model assumes that the disease will occur when there are enough contributory factors to push the individual’s liability above the threshold.
According to the authors, this study emphasises the importance of evaluating family history in patients with abdominal aortic aneurysms and supports ultrasound scan screening of unaffected siblings. “The data can improve the information given to first-degree relatives of patients with abdominal aortic aneurysms regarding the risk,” said Wahlgren. “However, many questions in the genetics of familial abdominal aortic aneurysms remain unanswered. The understanding about how risk factors and genotype contribute to abdominal aortic aneurysms development is important for patient selection, screening and more aggressive treatment.”
Source: Society for Vascular Surgery