Segregation analysis of asthma: Recessive major gene component for asthma in relation to history of atopic diseases

Introduction to genetics and genomics in asthma: genetics of asthma

While asthma is a heterogeneous disease, a strong genetic basis has been firmly established. Rather than being a single disease entity, asthma consists of related, overlapping syndromes [Barnes (Proc Am Thor Soc 8:143-148, 2011)] including three general domains: variable airway obstruction, airway hyper-responsiveness, and airway inflammation with a considerable proportion, but not all, of asthma being IgE-mediated further adding to its heterogeneity. This chapter reviews the approaches to the elucidation of genetics of asthma from the early evidence of familial clustering to the current state of knowledge with genome-wide approaches. The conclusion is that research efforts have led to a tremendous repository of genetic determinants of asthma, most of which fall into the above phenotypic domains of the syndrome. We now look to future integrative approaches of genetics, genomics (Chap. 10), and epigenetics (Chap. 11) to better understand the causal mechanism through which, these genetic loci act in manifesting asthma.

Major gene effect and additive familial pattern of inheritance of asthma exist among families of probands with sickle cell anemia and asthma

In the United States, sickle cell anemia (SCA) affects approximately 1 in 400 African-American newborns. Acute episodes of pain and acute chest syndrome (ACS) are the two leading causes of hospitalization. A relationship between the diagnosis of asthma and the incidence of pain and ACS has been established. We tested the hypothesis that a familial pattern of inheritance of asthma exists among first degree relatives of probands with SCA and asthma. Segregation analysis was performed in 104 families ascertained through affected probands. Of these, 19.7% (41/208) of the parents and 31.8% (28/88) of siblings of affected probands reported having been told by a doctor he or she had asthma at any age. Modes of inheritance were tested, using the Pedigree Analysis Package parameterized for the discrete trait of asthma affection status. A major effect was present and significant. Further tests were performed to determine whether transmission probabilities of the major effect followed Mendelian expectations. The additive mode of inheritance was the most parsimonious, while the residual heritability was found negligible. Our results support the hypothesis that a familial pattern of inheritance of asthma exists among first degree relatives of probands with SCA and asthma, suggesting that asthma is a co-morbid condition with SCA rather than a lung disease phenotype mimicking asthma.

Association of a lymphotoxin-alpha gene polymorphism and atopic asthma in Taiwanese children

BACKGROUND

The lymphotoxin-alpha (LT-alpha) gene is located on chromosome 6 (6p21.1-6p21.3) and it may regulate tumor necrosis factor (TNF) production. TNF is a potent cytokine in the airway inflammatory response. Polymorphisms of TNF-associated genes have been related to asthma. This study investigated an LT-alpha-Ncol polymorphism in the first intron of the LT-alpha gene (LT-alpha-Ncol*1 allele, as a variant type; and LT-alpha-Ncol*2 allele), which may predispose individuals to asthma and atopy.

METHODS

Polymerase chain reaction-based assays were performed to determine LT-alpha-Ncol genotypes among our subjects. A genetic case control analysis was then performed on 114 atopic asthmatic and 155 non-asthmatic unrelated children.

RESULTS

There was a statistically higher frequency of LT-alpha-Ncol*1 allele carriers (1/1+1/2) in the subjects with atopic asthma than in controls (OR=1.923; 95% CI = 1.061-3.484; p = 0.031).

CONCLUSION

The results indicate that LT-alpha-Ncol*1 may be a risk factor for atopic asthma in Taiwanese children.

Polymorphisms in the IL-4, IL-4 receptor alpha chain, TNF-alpha, and lymphotoxin-alpha genes and risk of asthma in Hong Kong Chinese adults

BACKGROUND

Susceptibility to the development of asthma and other atopic diseases is known to be associated with genetic components. However, association studies with interleukin-4 (IL-4), IL-4 receptor alpha subunit (IL-4R alpha), tumor necrosis factor-alpha (TNF-alpha) and lymphotoxin-alpha (LT-alpha) genes were inconclusive, as both positive and negative results were obtained in several populations studied. We aimed to investigate the association of the polymorphisms for IL-4 (C-589T), IL-4R alpha (Gln576Arg), TNF-alpha (G-308A) and LT-alpha (A252G) genes as candidates and asthma in adult Hong Kong Chinese population.

METHODS

The association study was conducted in an age- and smoking status-matched case-control design in asthma patients (n = 292) and healthy controls (n = 292) using polymerase chain reaction and restriction fragment length polymorphism.

RESULTS

No significant differences were found in allele and genotype frequencies of all four genes between patients and controls. After stratification by atopic status, the heterozygous AG genotype of LT-alpha (A252G) was found to increase risk of asthma in atopic population [odds ratio (OR) = 2.00, 95% CI 1.09-3.67, p = 0.024]. When stratified by smoking status, we found increased risk of asthma with subjects carrying the heterozygous AG and homozygous GG genotypes of LT-alpha in ever-smokers (OR = 2.73, 95% CI 1.11-6.69, p = 0.028 for heterozygotes; OR = 3.34, 95% CI 1.16-9.62, p = 0.026 for homozygotes).

CONCLUSION

Our results suggest that the variability of LT-alpha genotypes may have potential implications for individual susceptibility to asthma in atopic or in ever-smoking Chinese adults in Hong Kong.

Segregation analysis of bronchial hyperresponsiveness in a general population in north Italy

Bronchial hyperresponsiveness (BHR) is an intermediate phenotype of asthma, with a heritability component of 30-67% and possible linkage to regions on chromosome arms 5q, 11q, and 20p. Familial correlation analysis and segregation analysis for BHR, using the FCOR and REGC programs of the S.A.G.E package, were performed to examine inheritance patterns of BHR in a general population of 1167 subjects in 550 families from the Po River Delta. BHR was assessed using the log(10) of the slope of the methacholine dose-response curve (log slope) for each subject who met inclusion criteria. Using multiple linear regression analysis, the log slope values were adjusted for age, age(2), sex, and height, and used in the familial correlation and segregation analyses. Father-offspring correlations are statistically significant, due specifically to high father-son correlations (r = 0.296, P < 0.001, adjusted values). Segregation analysis of BHR in the overall population, with and without a smoking covariate (number of packyears smoked), indicates an apparent absence of genetic transmission within families. However, in a segregation analysis of BHR in smoking families only, after adjusting for number of packyears smoked, the Mendelian transmission models could not be rejected. This may be evidence of a gene by smoking effect, and suggests that in families of smokers, a single locus gene may in part explain the inheritance of a compound phenotype (BHR x packyears).

Adaptations of linkage and association methods for the study of asthma, a complex trait

Early studies that found significant linkage between markers on 5q and asthma and IgE have not been reproduced. In an attempt to improve the power of these studies we performed a variance components linkage analysis and transmission-disequilibrium tests (TDT) with haplotypes using markers on 5q, using the Southampton and Perth data sets supplied by GAW. The linkage analysis with covariates revealed a maximum lod of 1.57 in the Perth families. The addition of age and RAST significantly improved the fit of the null models but did not improve the lod scores. The TDT tests showed a marginally significant association with D5S393 and D5S399 and with three markers together (IL9, IL4, D5S393). We conclude that further studies are needed to delineate the environmental contribution to this disease so that the genetic factors can be more easily identified. In addition, haplotype analysis may help to identify specific genetic effects.