U01.01.058 X-linked recessive disorders

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Learning Objectives

  • Identify classic X-linked Recessive (XLR) disorders and their clinical features.
  • Explain the process of X-inactivation (Lyonization) and its clinical impact.
  • Understand why certain populations (e.g., Turner Syndrome) are at higher risk for XLR diseases.

1. Common X-linked Recessive Disorders

These diseases primarily affect males because they only have one X chromosome. Females are typically asymptomatic carriers unless skewed X-inactivation occurs.

Category Examples
Immunology Bruton agammaglobulinemia, Wiskott-Aldrich syndrome.
Musculoskeletal Duchenne and Becker muscular dystrophies.
Metabolic/Enzyme G6PD deficiency, Fabry disease, Hunter syndrome, OTC deficiency, and Lesch-Nyhan syndrome.
Hematologic Hemophilia A and B.

2. X-Inactivation (Lyonization)

In individuals with two X chromosomes (XX), one X is randomly deactivated and condensed into a Barr body during early development. This Barr body consists of highly methylated heterochromatin.

  • Random Process: In a typical female, roughly 50% of cells express the maternal X and 50% express the paternal X.
  • Skewed Lyonization: If deactivation is not 50/50, an XX individual may express symptoms of an XLR disease (e.g., G6PD deficiency).
  • Dominant Impact: Lyonization also explains why X-linked dominant diseases have variable severity in females.

3. High-Yield Clinical Correlation

  • Turner Syndrome (45, XO): Because these individuals have only one X chromosome, they face the same risk as males for manifesting X-linked recessive disorders.
  • Hunter Syndrome: Remember that while most mucopolysaccharidoses are autosomal recessive, Hunter is X-linked (“The hunter aims for the X”).

 

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