USMLE Guide: Genetics of Turner Syndrome

Introduction

Turner Syndrome (TS) is a genetic disorder that affects females, resulting from complete or partial absence of one of the X chromosomes. It is a common cause of primary ovarian failure and short stature in women. This USMLE guide aims to provide an overview of the genetics of Turner Syndrome, including its etiology, clinical manifestations, diagnostic criteria, and management.

Genetics of Turner Syndrome

Turner Syndrome is typically caused by one of two genetic abnormalities:

1. Monosomy X (45,X): This is the most common chromosomal abnormality in Turner Syndrome, occurring in approximately 50% of cases. It results from the complete absence of one X chromosome in all or a majority of cells.

2. Structural abnormalities of the X chromosome: In the remaining cases, individuals may have a structurally abnormal X chromosome, such as a deletion or isochromosome formation.

Etiology

The etiology of Turner Syndrome can be attributed to different genetic mechanisms:

1. Non-disjunction during paternal meiosis: Approximately 50% of Turner Syndrome cases result from failure of the paternal X chromosome to segregate during meiosis. The resulting embryo lacks an X chromosome in all cells and manifests as 45,X karyotype.

2. Mosaicism: In some instances, there may be post-zygotic loss of one X chromosome during early embryogenesis, leading to mosaicism. Mosaic Turner Syndrome is characterized by a mixture of 45,X and 46,XX or 46,XY cell lines.

3. Structural abnormalities: Structural abnormalities of the X chromosome, such as deletions, duplications, or isochromosomes, can also contribute to the development of Turner Syndrome.

Clinical Manifestations

The clinical features of Turner Syndrome can vary widely among affected individuals. Common clinical manifestations include:

• Short stature: Girls with Turner Syndrome often have impaired growth, resulting in short stature. Growth hormone therapy can be beneficial in improving final adult height.

• Primary ovarian failure: The absence of one X chromosome leads to ovarian dysgenesis and subsequent infertility. Turner Syndrome is the most common genetic cause of premature ovarian failure.

• Webbed neck: Many individuals with Turner Syndrome have a low posterior hairline and excess skin on the neck, known as a "webbed neck."

• Cardiovascular abnormalities: Congenital heart defects, particularly coarctation of the aorta and bicuspid aortic valve, are frequently observed in Turner Syndrome.

• Renal abnormalities: Kidney abnormalities, such as horseshoe kidneys or renal malformations, are more prevalent in Turner Syndrome.

Diagnostic Criteria

The diagnosis of Turner Syndrome is primarily based on karyotype analysis, which involves examining the number and structure of chromosomes. Diagnostic criteria include:

1. Cytogenetic analysis: This involves analyzing a karyotype from peripheral blood lymphocytes. The presence of 45,X karyotype or mosaic cell lines confirms the diagnosis.

2. Clinical features: The clinical manifestations mentioned earlier, such as short stature and ovarian failure, aid in the diagnosis but are not specific.

Management

The management of Turner Syndrome is multidisciplinary, involving various specialties. Key aspects of management include:

• Growth hormone therapy: Administering growth hormone to improve height potential in girls with Turner Syndrome.

• Estrogen replacement therapy: Initiation of estrogen replacement therapy at the appropriate age to induce puberty and prevent osteoporosis.

• Cardiovascular surveillance: Regular monitoring and management of cardiovascular anomalies, such as coarctation of the aorta and bicuspid aortic valve.

• Psychosocial support: Providing psychological support and counseling to individuals with Turner Syndrome and their families to address potential emotional and social challenges.

Conclusion

Understanding the genetics of Turner Syndrome is essential for medical professionals. This USMLE guide has provided an overview covering the etiology, clinical manifestations, diagnostic criteria, and management of Turner Syndrome. By familiarizing oneself with this information, healthcare providers can effectively diagnose, treat, and support individuals with Turner Syndrome.