What chromosome is affected in Turner syndrome?

Turner Syndrome Chromosome Abnormality

Turner syndrome is caused by complete or partial loss of one X chromosome (45,X karyotype), affecting the X chromosome. 1, 2

Genetic Basis of Turner Syndrome

Turner syndrome is characterized by the following chromosomal abnormalities:

• Primary karyotype: 45,X (monosomy X) accounts for approximately 50% of cases 3
• Other variants:
  • X-mosaicism
  • X-chromosomal structural abnormalities including:
    • Deletions
    • Duplications
    • Ring chromosomes
    • Isodicentric chromosomes
    • Inversions
    • Translocations 3

The condition results from the absence of all or part of the second sex chromosome, leading to a female with only one complete X chromosome. This chromosomal abnormality occurs in approximately 1 in 2,500 live female births 2, 4.

Clinical Manifestations Related to X Chromosome Loss

The loss of X chromosome material leads to various clinical manifestations:

• Short stature: Present in 72.4-80.1% of patients with non-mosaic X chromosome structural rearrangements 5
• Ovarian dysfunction: Occurs in 78.8% of patients with non-mosaic X chromosome structural rearrangements 5
• Cardiovascular abnormalities:
  • Bicuspid aortic valve (10-25%)
  • Aortic coarctation (~8%)
  • Aortic dilatation (~33%) 2
• Other features:
  • Congenital lymphedema
  • Renal malformations
  • Sensorineural hearing loss
  • Skeletal dysplasia
  • Facial abnormalities 3, 4

Genotype-Phenotype Correlation

The specific X chromosome breakpoints influence the clinical presentation:

• X short arm (Xp) deletions: Associated with higher prevalence of short stature (80.1%) and lower prevalence of ovarian dysfunction (72.5%) 5
• X long arm (Xq) deletions: Associated with lower prevalence of short stature (37.5%) but higher prevalence of ovarian dysfunction (94.6%) 5

Diagnostic Considerations

Diagnosis requires both clinical features and cytogenetic findings:

• Recommended testing:
  • Karyotype analysis
  • Fluorescence in situ hybridization (FISH)
  • Chromosomal microarray analysis (CMA) 2

Clinical Implications

The X chromosome abnormality in Turner syndrome has significant clinical implications:

• Increased mortality: Women with Turner syndrome have higher morbidity and mortality compared to the general population 6
• Multisystem involvement: Requires multidisciplinary care throughout life 7
• Pregnancy risks: High-risk period for aortic complications, requiring pre-pregnancy cardiovascular assessment 2
• Aortic dissection risk: 36:100,000 Turner syndrome years (1.4% lifetime risk) vs. 6:100,000 in general population 2

Key Considerations in Management

• Early diagnosis allows for timely intervention with growth hormone therapy for short stature
• Estrogen replacement therapy is needed for pubertal development and prevention of osteoporosis
• Regular cardiovascular surveillance is essential due to the high risk of cardiac abnormalities
• Recognition of specific X chromosome breakpoints helps predict and manage complications 5, 4

Turner syndrome represents a classic example of how chromosomal abnormalities can lead to specific phenotypic manifestations, with the X chromosome being the critical genetic element affected in this condition.