Natural history, endocrine complications and testicular dysfunction in Klinefelter syndrome and high-grade sex chromosome aneuploidies

Despite celebrating 80 years since the original description of the first case of Klinefelter syndrome in 1942, several aspects relating to the morbidity of KS and HGAs are still unclear and are object of active research. In Chapter 2 we sought to describe the natural history of KS, from infancy into adulthood. We employed a semi-longitudinal approach in a large cohort of KS patients, describing how testicular dysfunction is not hormonally-detected in KS before Tanner stages 3-4. Specifically: a) INHB levels are maintained in the normal (adult) range until stage 4, b) the INHB/FSH ratio sharply declines between stages 3 and 4, c) T and cfT levels are highest between Tanner stage 5 and transition age and d) the T/LH ratio declines during Tanner stage 4. We further demonstrated the capability of quantitaty testicular ultrasound in revealing damage evolution in KS, and specifically how: a) the bitesticular volume gradually increases during puberty until Tanner stage 4, with subsequent regression, b) parenchymal echotexture progressively worsen throughout puberty, transition age, into adulthood and c) how the presence of hypoechoic testicular lesions, microlithiasis and higher entropy are independent predictors of worse testicular endocrine function. From our results we speculate that this combined approach could be used in the future as a clinical tool to more accurately identify the onset of testicular dysfunction, possibly guiding the start of TRT or the timing of (m)TESE procedures. In Chapter 3 we addressed the complex phenotypic spectrum of HGAs by comparison to KS, with regards to endocrine, metabolic and cardiac features. Specifically we assessed the hypothesis of a genotype-phenotype relationship dependent on the number of supernumerary X chromosomes in determining certain phenotypic traits, by adjusting for the number of supernumerary Y chromosomes. Indeed, the results of our study show that the increase in supernumerary X chromosomes determines several significant trends towards a progressive reduction in height, a worsening of testicular volume and function, a reduced overall steroidogenic activity, impaired glucose and lipid homeostasis, as well as cardiac structure and function. As no guidelines are currently available for the management of these complex syndromes; we believe that a better understanding of their intrinsic characteristics can guide researchers towards recognizing HGAs as indipendent clinical entities, and medical practitioners towards earlier identification of complications, and thus allow for a more tailored management and appropriate treatment of the individual HGA patient. In Chapter 4 we approached thyroid alterations specific to KS, revealing a more more complex and broader nature than previously proposed, characterized by reduced thyroid hormone levels, inappropriately normal/low TSH levels, altered pituitary deiodination mediated by reduced T, increased thyroid autoimmunity, and structural parenchymal changes. These findings are particularly relevant also in light of the recent evidence suggesting a stronger association of free thyroid hormones (and especially fT4), as compared to TSH levels, to clinical outcomes (e.g., atrial fibrillation, features of metabolic syndrome, osteoporosis, dementia, etc.). In this respect, it is noteworthy that several complications of KS are also seen in hypothyroidism, including metabolic syndrome, increased cardiovascular risk, cognitive dysfunction, reduced bone mass, and sexual dysfunction, and could thus possibly contribute to our understanding of the missing link between genoype and phenotype in KS morbidity. Whether the alterations in the HPT axis of KS contribute to the poorer outcome of the syndrome, or whether hypogonadism is involved the pathogenesis of thyroid structural and functional impairment warrant further studies. In Chapter 5 we took on the study of testicular vascularisation and microvascular dynamics by means of contrast-enhanced ultrasound (CEUS). We could show the presence of slower testicular perfusion kinetics in subjects with KS than in age-matched controls, and, specifically, the wash-in time, mean transit time, time to peak, and washout time were all prolonged. Most strikingly, faster testicular blood flow was associated with higher circulating total T levels. We then applied principal component analysis (PCA) and multiple linear regression analyses which supported a role for reduced venous blood flow as an independent predictor of testicular T peripheral release, consistent with previous reaserch highlighting increased intratesticular T levels in KS subjects. In Chapter 6 we explored for the first time the bone-testicular axis in the context of male hypogonadism, specifically by exploiting a model of hypergonadotropic hypogonadism represented by KS. We did so by a retrospective longitudinal approach at a single academic referral centre for KS, and evaluated subjects taking into account pubertal stage, gonadal status as well as TRT commencement. We could demonstrate the relationship between T and total osteocalcin (tOCN), and specifically the directionality of the association, which implicates OCN in regulating the HPG axis at a central (hypothalamic and/or pituitary) rather than peripheral (testicular) level, in the context of an LH-T feedback loop already stretched to its maximum capability. To conclude, this thesis aimed to provide further insight in the natural history, endocrine complications and testicular dysfunction in KS and HGAs. Although progress has been made in the understanding of these complex syndromes, some issues that were described above remain unsolved and warrant further study. Furthermore, the effects of T were studied. Despite the great clinical efficacy of TRT on some clinical endpoints, many other endocrine, metabolic, bone and cardiovascular complications of the syndromes generally remain unchanged. Whether timely hormonal therapy with T (and levothyroxine) can ultimately ameliorate the clinical burden of these conditions remains to be investigated.