Heart Diseases Clinical Trial
To examine the genetics of human susceptibility to Chagas' disease, a leading cause of heart disease throughout Latin America.
BACKGROUND:
Chagas' disease is a leading cause of heart disease throughout Latin America, affecting
between 16 and 18 million individuals. The disease, named after the Brazilian physician
Carlos Chagas who first described it in 1909, exists only on the American Continents. It is
caused by a flagellate protozoan parasite, Trypanosoma cruzi, transmitted to humans by
blood-sucking triatomine insects known popularly in the different countries as "vinchuca",
"barbeiro", "chipo" etc. The geographical distribution of the human T.cruzi infection
extends from Mexico to the south of Argentina. In Brazil alone, approximately 10 percent of
the population is seropositive for T. cruzi. Given the large pool of primary hosts for this
zoonotic disease, complete eradication of Chagas' disease through control of the arthropod
vector is unlikely. Research with humans and animal models indicates that there is variation
in susceptibility to infection, and disease outcome, and that this variation may be due to
genetic factors. Thus, this form of heart disease represents a complex phenotype with
potential genetic determinants to both susceptibility to infection and differential disease
pathogenesis.
DESIGN NARRATIVE:
The genetic epidemiology project studies an estimated 1,125 Black individuals age 18 years
or older from a small community in Posse, Brazil in which the study population may be drawn
from as few as four to five large pedigrees with varying blood relationships. Preliminary
pilot data appear very promising regarding the primary hypothesis that specific genetic
factors influence the susceptibility to this complex zoonotic disease. The investigation
will be divided into three parts involving phenotypic characterization, statistical genetic
analyses to determine the relative proportion of the phenotypic variance due to additive
genetic effects, and localization of genes influencing the development of Chagas' disease.
The phenotypic characterization will involve clinical history, infection status, immunologic
studies, and cardiac status. The chronic phase of T. cruzi infection results in Chagas'
disease manifest as a cardiomyopathy with heart block, conduction disturbance, ventricular
tachyarrhythmia, heart failure, and sudden arrhythmic death. The cardiac phenotyping will
involve electrocardiographic time intervals (PR interval, QRS duration, QT interval
duration) as well as abnormal patterns (left anterior fascicular block, left ventricular
hypertrophy, right bundle branch block), and possibly some measure of heart failure in terms
of New York Heart Association functional classification. Right bundle branch block is
probably the most frequent early manifestation of chronic cardiac involvement.
The genetic linkage studies will attempt to localize genes influencing susceptibility to T.
cruzi infection, ECG parameters (e.g., right bundle branch block), and immunologic factors
using markers placed approximately every 10 centimorgans (cM) throughout the genome.
About 375 individuals will be sampled per year. Each subject will donate two 10 ml samples
of blood which will be used for immunoassays for seropositivity for T. cruzi, and for
genotyping with 382 polymorphic short tandem repeats. During the initial visits, the
population subjects will also be interviewed to gather information on demographic, pedigree,
and socioeconomic makeup. Additionally, questions on pregnancy status, household ecology
(years in the house, rooms shared, pets, rodents, food storage, triatomid bug history, etc)
and residence history will be determined during the survey. In the fourth year, the
investigators will return to the collection site and administer ECGs on 375 individuals per
year.
The blood collected in Brazil will be sent to San Antonio for high-throughput genotyping
using 382 polymorphic short tandem repeats (STRs) spread throughout the genome. A population
specific 10cM map is expected to be useful for linkage analysis.
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