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1.  Influence of Host Genetics and Environment on Nasal Carriage of Staphylococcus aureus in Danish Middle-Aged and Elderly Twins 
The Journal of Infectious Diseases  2012;206(8):1178-1184.
Background. Nasal carriage is a major risk factor for Staphylococcus aureus infection. Approximately, one-quarter of adults carry S. aureus. However, the role of host genetics on S. aureus nasal carriage is unknown.
Methods. Nasal swabs were obtained from a national cohort of middle-aged and elderly Danish twins. Subjects colonized with S. aureus were identified by growth on selective plates and spa typing. A second sample was obtained from twins initially concordant for carriage. Twins found to again be colonized with S. aureus were defined as persistent carriers.
Results. The prevalence of S. aureus carriage among 617 twin pairs (monozygotic/dizygotic pairs: 112/505) was 26.3% (95% confidence interval [CI], 24.0%–28.9%). The concordance rate for carriage did not differ significantly between pairs of monozygotic (37.5%; 95% CI, 22.3%–53.8%) twins and same sex (24.2%; 95% CI, 15.4%–34.5%), and opposite sex (21.4%; 95% CI, 12.0%–33.4%) dizygotic twins. Despite shared childhoods, only 1 of 617 pairs was concordant with respect to lineage. Although heritability increased for S. aureus and lineage persistency, no significant heritability was detected.
Conclusion. In this study, host genetic factors exhibited only a modest influence on the S. aureus carrier state of middle-aged and elderly individuals.
doi:10.1093/infdis/jis491
PMCID: PMC3448969  PMID: 22872733
2.  Risk of Oral Clefts in twins 
Epidemiology (Cambridge, Mass.)  2011;22(3):313-319.
Background
Small studies have indicated that twinning increases the risk of oral cleft.
Methods
We used data from a Danish national population-based cohort study to investigate whether twinning was associated with isolated oral cleft, and to estimate the twin probandwise concordance rate and heritability. Twins (207 affected/130,710) and singletons (7766 affected/4,798,526) born from 1936 through 2004 in Denmark were ascertained by linkage among the Danish Facial Cleft Database, the Danish Twin Registry and the Civil Registration System. We computed oral cleft prevalence and prevalence proportion ratio for twins versus singletons, stratified for three sub-phenotypes. Probandwise concordance rates and heritability for twins were estimated for two phenotypes—cleft lip with or without cleft palate (CL/P) and cleft palate (CP).
Results
The prevalence of oral cleft was 15.8 per 10,000 twins and 16.6 per 10,000 singletons (prevalence proportion ratio = 0.95; 95% confidence interval = 0.83 – 1.1). This prevalence was similar for monozygotic and dizygotic twins. The probandwise concordance rate was higher for CL/P for monozygotic twins than for dizygotic twins (50 % vs. 8%, respectively). A similar contrast was present for CP. Recurrence risk for both types of clefts was greater in dizygotic twins than in non-twin siblings. Heritability estimates were above 90% for both CL/P and CP.
Conclusion
No excess risk of oral cleft could be demonstrated for twins compared with singletons. The concordance rates and heritability estimates for both types of clefts show a strong genetic component.
doi:10.1097/EDE.0b013e3182125f9c
PMCID: PMC3089024  PMID: 21423016
3.  Recurrence Risk for Offspring of Twins Discordant for Oral Cleft - A Population-based Cohort Study of the Danish 1936–2004 Cleft Twin Cohort 
Our objective in this Danish population-based cohort study was to estimate the recurrence risk of isolated oral cleft (OC) for offspring of the unaffected co-twins of OC discordant twin pairs and to compare this risk to the recurrence risk in the offspring of the affected co-twin as well as to the risk in the background population.
During 1936–2004, 207 twin pairs were ascertained, among whom at least one twin had an OC. The index persons were twins discordant for OC who had children (N=117), and their offspring (N=239). The participants were ascertained by linkage between The Danish Facial Cleft Database, The Danish Twin Registry and The Danish Civil Registration System. In the study OC recurrence risk for offspring of the affected and unaffected twin and relative risk were compared to the background prevalence. We found that among 110 children of the 54 OC affected twins, two (1.8%) children had OC corresponding to a significantly increased relative risk (RR = 10; 95% CI 1.2 to 35) when compared to the frequency in the background population. Among the 129 children of the 63 unaffected twins, three (2.3%) children were affected, corresponding to a significantly increased relative risk (RR = 13; 95% CI 2.6 to 36) when compared the background prevalence. We concluded that in OC discordant twin pairs similar increased recurrence risks were found among offspring of both OC affected and OC unaffected twins. This provides further evidence for a genetic component in cleft etiology and is useful information for genetic counseling of twin pairs discordant for clefting.
doi:10.1002/ajmg.a.33608
PMCID: PMC2946514  PMID: 20799319
recurrence risk; oral cleft; cleft lip and palate; twins; genetics

Results 1-3 (3)