Sex Chromosome Aneuploidy

Sex chromosome aneuploidy is the presence of an abnormal number of sex chromosomes, most often the gain of an extra X or Y (the trisomies 47,XXY, 47,XXX and 47,XYY) or the loss of an X (45,X). These are the most frequent aneuploidies in liveborn humans, yet many cases, particularly the trisomies, go undiagnosed because their features are often mild and non-specific.

Definition

Sex chromosome aneuploidy is a constitutional abnormality in the number of gonosomes, producing a chromosome complement other than 46,XX or 46,XY through the gain or loss of one or more X or Y chromosomes, either in all cells or in a mosaic pattern.

Scope

The entry covers the numerical sex chromosome abnormalities as a group: how they arise, their broad phenotypic and neurocognitive patterns, and why they are commonly underascertained. The two best-defined single conditions, Turner syndrome (45,X) and Klinefelter syndrome (47,XXY), are treated in their own entries. The material is reference-educational and does not provide individual diagnostic or treatment guidance.

Key concepts

• Nondisjunction in meiosis
• Trisomy of the sex chromosomes (47,XXY; 47,XXX; 47,XYY)
• Monosomy X (45,X)
• Mosaicism
• X-chromosome inactivation buffering of dosage
• Phenotypic variability and underdiagnosis
• Karyotype and chromosomal microarray ascertainment

Mechanisms

Sex chromosome trisomies most often result from nondisjunction during parental meiosis, while 45,X frequently arises from loss of a sex chromosome in an early postzygotic division, which also explains the high frequency of mosaicism in monosomy X. Because supernumerary X chromosomes are largely subject to X-chromosome inactivation, the gene-dosage effect of an extra X is partly buffered, which contributes to the relatively mild and variable phenotypes of the trisomies; genes that escape inactivation account for residual dosage effects. The neurodevelopmental and behavioural profiles reported across the trisomies are generally subtle and group-level rather than uniform, and individual outcomes vary widely.

Clinical relevance

Sex chromosome aneuploidies are increasingly detected through prenatal screening and microarray testing, often incidentally, which raises questions of interpretation and counselling. Awareness of the wide and frequently mild phenotypic range is important for appraising the literature and understanding cytogenetic reports; this entry describes the conditions as a category and is not a basis for individual prognosis, diagnosis, or management.

Epidemiology

Numerical sex chromosome abnormalities are collectively the most common aneuploidies among liveborns. Newborn cytogenetic surveys, including Hamerton and colleagues' study of 14,069 infants, place the combined frequency at roughly 1 in 400 to 1 in 500 births, with 47,XXY, 47,XXX and 47,XYY each occurring on the order of 1 in 1,000 births of the relevant sex and 45,X being less common at birth. Comparative cohort reviews indicate increased long-term morbidity for some groups, although ascertainment bias complicates these estimates.

History

The conditions were defined as the field of clinical cytogenetics matured around 1959, when karyotyping linked specific aneuploidies to clinical syndromes: 45,X to Turner syndrome and 47,XXY to Klinefelter syndrome. The XYY (47,XYY) and triple-X (47,XXX) karyotypes were described shortly afterwards. Newborn surveys in the following decades established their population frequencies and revealed how often the trisomies escape clinical detection.

Debates

How should incidentally detected aneuploidies be counselled?

With more prenatal and microarray testing, many sex chromosome aneuploidies are now found incidentally in people without obvious features, raising debate over how to convey an often mild and variable prognosis without overstating risk.

Seminal works

• hamerton-1975
• rogol-2023
• vanrijn-2019

Frequently asked questions

What is the difference between sex chromosome aneuploidy and Klinefelter or Turner syndrome?

Klinefelter syndrome (47,XXY) and Turner syndrome (45,X) are specific, named sex chromosome aneuploidies; the term sex chromosome aneuploidy is the broader category that also includes 47,XXX, 47,XYY, and other gains or losses of the X or Y chromosome.

Why are many sex chromosome aneuploidies never diagnosed?

Several, especially the trisomies 47,XXX and 47,XYY, often produce mild or non-specific features, so without prenatal or other incidental testing they may never come to clinical attention.