In 1940, Harry F. Klinefelter, Jr., a native of Baltimore and a graduate of the Johns Hopkins Medical School, spent a year working as a "traveling fellow" with Dr. Fuller Albright in the Metabolic Ward of the Massachusetts General Hospital in Boston (3). At that time, Dr. Albright was at the height of his formidable academic career and, among many other subjects, was interested in the endocrinology of sexual development and the therapeutic applications of the recently available steroid hormones. Dr. Albright had identified a group of adult men with a previously unchar-acterized syndrome typified by gynecomastia, small testes, and varying degrees of eunuchoidism. Dr. Albright assigned Dr. Klinefelter the task of further describing this syndrome for publication.
Klinefelter, Albright, and their colleague Dr. Edward Reifenstein, Jr. published their description of the syndrome in a classic article in The Journal of Clinical Endocrinology in 1942 (4). In this article, they described a cohort of nine men "characterized by gynecomastia, aspermatogenesis without a-Leydigism, and increased excretion of follicle-stimulating hormone." The authors noted the presence of testosterone-producing Leydig cells in testicular biopsy specimens, as well as a marked increase in FSH production, comparable to that seen in castrated men. The authors suggested that the markedly elevated FSH levels were secondary to the absence of a testicular hormone other than testosterone. This postulated hormone, called inhibin by prior researchers, is, in fact, lacking in individuals with KS, but its identity and role in endocrine feedback in the male would not be fully elucidated for more than 50 yr (5).
Klinefelter and his colleagues were uncertain of the cause of the syndrome. They excluded testicular inflammation, infection, or obstruction of the vas deferens and noted on testicular biopsy specimens that the lesion involved the seminiferous tubules without dramatically affecting the histology of the Leydig cells, testicular interstitium, or epididymis. They recommended testosterone therapy (available since the late 1930s) for those men with signs or symptoms of hypogonadism but noted that this therapy did not improve the gynecomastia or infertility.
Although Klinefelter, Reifenstein, and Albright first described the clinical and endocrine manifestation of KS in 1942, it wasn't until 1956, after the ascertainment of DNA as the main instrument of heredity, that the additional X chromosomes, or "Barr bodies," were identified in tissue from patients with KS (6,7). Three years later, Jacobs and Strong confirmed the association between the extra X chromosome and KS by
Mechanism of Origin of Additional X Chromosome in 47, XXY Males (10)
Sex Responsible Frequency chromosomes cell division X-Chromosome pattern (%)
XMXPY Paternal meiosis Identical to father 53
XM1XM2Y Maternal meiosis-I Identical to mother 34
XM1XM1Y Maternal meiosis-II Single maternal chromosome with 9
XM1XM1Y Mitotic (postzygotic) Single homozygous maternal chromosome 3
cytogenetic analysis of metaphase chromosomes, establishing KS as a sex chromosome disorder with a 47, XXY karyotype (8). Further work has demonstrated that some individuals may be mosaic (47, XXY/46, XY) with the extra X chromosome detectable in only peripheral blood or the testes; rare individuals with 48, XXXY and 49, XXXXY karyotypes have also been described (9).
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