Background. The clinical diagnosis of familial hypertrophic cardiomyopathy is usually made on the basis of the physical examination, electrocardiogram, and echocardiogram. Making an accurate diagnosis can be particularly difficult in children, who may not have cardiac hypertrophy until adulthood. Recently, we demonstrated that mutations in the cardiac myosin heavy-chain genes cause familial hypertrophic cardiomyopathy In some families. We report a diagnostic test for familial hypertrophic cardiomyopathy that relies on the detection of mutations In the β myosin heavy-chain gene in circulating lymphocytes that we used to evaluate three generations of a family, including the children. Methods and Results. Using the polymerase chain reaction, we found that normal and mutant β cardiac myosin heavy-chain genes are transcribed in circulating lymphocytes. This allowed us to examine β cardiac myosin heavy-chain messenger RNA from blood lymphocytes, even though ordinary expression of the gene is virtually restricted to the heart. Base sequences amplified from this messenger RNA were analyzed with a ribonuclease protection assay to identify small deletions, abnormal splicing, or missense mutations. Using this technique we identified a novel missense mutation in a patient with familial hypertrophic cardiomyopathy. We evaluated 15 of the patient's adult relatives and found perfect agreement with the clinical diagnosis (8 affected and 7 not affected). Clinical analysis of 14 of the children (age, 1 to 20 years) of these affected family members revealed 1 child with echocardiographic findings diagnostic of familial hypertrophic cardiomyopathy. However, genetic analyses showed that six other children had also inherited the missense mutation and might later manifest the disease. Conclusions. Transcripts of β cardiac myosin heavy-chain gene can be detected in blood lymphocytes and used to screen for mutations that cause familial hypertrophic cardiomyopathy. This approach makes practical the identification of mutations responsible for this disorder and may be applicable to other diseases in which direct analysis is difficult because the mutated gene is expressed only in certain tissues. Preclinical or prenatal screening in an affected family will make it possible to study the disease longitudinally and to develop preventive interventions. (N Engl J Med 1991;325:1753–60.).
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