New Horizons in Klinefelter Syndrome: Current Evidence, Gaps, and Research Priorities.

Klinefelter syndrome (47,XXY) is the most common sex chromosome abnormality in men.

• The classical karyotype is 47,XXY, caused by a supernumerary X chromosome.
• Classical clinical features include tall stature, small testes, hypogonadism, gynecomastia, and neurodevelopmental deficits.
• The syndrome presents a broad phenotypic spectrum that appears to be evolving alongside environmental and general health changes.
• A proportion of men with KS are asymptomatic while others experience severe comorbidities.

The phenotypic spectrum of Klinefelter syndrome extends beyond classical features to include cardiovascular and autoimmune disorders.

• Comorbidities range from cardiovascular to autoimmune disorders.
• The broad phenotypic spectrum is described as 'evolving, along with environmental and general health changes.'
• Some men with KS are asymptomatic while others experience 'numerous severe comorbidities.'

Infertility in Klinefelter syndrome, once considered an absolute hallmark, can now be overcome with assisted reproductive technology.

• The inability to conceive was 'once considered a hallmark of the syndrome.'
• Assisted reproductive technology now provides a pathway to biological fatherhood for men with KS.
• Fertility remains identified as a priority area for future research.

Neuropsychological features of Klinefelter syndrome have been historically misrepresented, first overstated and then inadvertently dismissed.

• The neuropsychological stigmata were described as 'once overstated, thereafter inadvertently dismissed.'
• The authors call for 'a more balanced and objective approach' to neuropsychological features.
• Neurodevelopment is identified as a priority area for future research.

Significant advances have been made in understanding the molecular mechanisms of chromosomal disjunction leading to Klinefelter syndrome.

• Recent years have seen advances in understanding 'the molecular machinery involved in the chromosomal disjunction that gives rise to the syndrome.'
• This represents one of the key areas of recent scientific progress highlighted in the review.

Understanding of the risk-benefit profile of testosterone replacement therapy in Klinefelter syndrome has improved but significant knowledge gaps persist.

• The authors state that 'our understanding of the risk-benefit of testosterone replacement therapy has greatly improved.'
• Despite improvements, 'many gaps persist' regarding testosterone replacement therapy.
• This area is implicitly identified as requiring further prospective study.

Future research in Klinefelter syndrome should be prioritized according to patient needs across five key domains.

• The five identified research priority areas are: fertility, cardiovascular prevention, neurodevelopment, quality of life, and bone health.
• Research should be 'prioritized according to the needs of people with KS.'
• The authors emphasize that these priorities should be patient-centered.

Solid registries and extensive prospective longitudinal studies with international collaboration are needed to advance understanding of Klinefelter syndrome across the lifespan.

• The authors call for 'solid registries and extensive prospective longitudinal studies' to enroll people with KS.
• Such studies are needed 'to determine their evolving needs as they progress through their lifespan.'
• International collaboration is emphasized 'to ensure the results apply to all those with this condition worldwide.'