D115 Unit 2: Genetic Testing and Disorders – Cohort Notes

D115 Unit 2: Genetic Testing and Disorders – Cohort Notes

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Western Governors University

D115 Advanced Pathophysiology for the Advanced Practice Nurse

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Unit 2: Genetic Testing and Genetic Disorders

Genetic Testing Overview

Genetic testing refers to a collection of laboratory techniques designed to identify genetic variations associated with disease susceptibility, diagnosis, prognosis, and therapeutic response. Advances in molecular genetics have enabled the identification of individuals at increased risk for complex, multifactorial disorders as well as single-gene conditions. These tests play a critical role in preventive medicine, reproductive planning, and personalized healthcare.

Types of Genetic Testing

Carrier Screening

Carrier screening is performed to identify individuals who are heterozygous carriers of autosomal recessive disorders. Although carriers are typically asymptomatic, they may pass pathogenic variants to their offspring. This type of screening is particularly valuable for couples planning pregnancy, especially in populations with a higher prevalence of specific inherited conditions.

Common Disorders Identified Through Carrier Screening

Carrier screening supports informed reproductive decision-making and allows couples to assess genetic risks prior to conception.

Prenatal Diagnosis

Prenatal diagnostic testing evaluates the genetic health of a fetus during pregnancy. These methods are recommended for all pregnant individuals, particularly those with known risk factors such as advanced maternal age, family history of genetic disorders, or abnormal screening results.

Common Prenatal Diagnostic Methods

These techniques allow early detection of chromosomal abnormalities and inherited disorders, enabling timely counseling and medical planning.

Presymptomatic Genetic Testing

Presymptomatic testing identifies individuals who carry genetic mutations associated with adult-onset disorders before clinical symptoms appear. Conditions such as hereditary breast cancer (BRCA mutations) and familial colon cancer can be detected early, allowing for enhanced surveillance, preventive interventions, or lifestyle modifications.

Pharmacogenetic Testing

Pharmacogenetics examines genetic variations that influence individual responses to medications. Testing for drug sensitivity and efficacy helps clinicians tailor treatments, reduce adverse drug reactions, and improve therapeutic outcomes.

Reasons for Genetic Screening

Chromosomal Aneuploidy Disorders

Turner Syndrome

Overview and Epidemiology

Turner syndrome is a sex chromosome aneuploidy characterized by monosomy X (45,X), resulting in the presence of a single X chromosome. First described by Dr. Henry Turner in 1938, the condition affects approximately 1 in 2,500 live female births. Due to the absence of a Y chromosome, all affected individuals are phenotypically female.

Pathophysiology

In Turner syndrome, one X chromosome is either missing or structurally abnormal. Most affected individuals lack functional ovaries and instead possess gonadal streaks composed of fibrous connective tissue. These streaks fail to produce estrogen and progesterone, leading to impaired sexual development and infertility. Mosaicism, involving combinations of 45,X and normal cell lines, improves survival rates.

Clinical Manifestations

Diagnosis and Treatment

Karyotyping of peripheral blood remains the definitive diagnostic test. Management is individualized and focuses on hormone replacement therapy, growth hormone administration, fertility counseling, and lifelong monitoring by multidisciplinary specialists.

Klinefelter Syndrome

Pathophysiology and Epidemiology

Klinefelter syndrome results from the presence of one or more extra X chromosomes in males, most commonly 47,XXY. It occurs in approximately 1 in 1,000 male births. Additional X chromosomes correlate with increased physical and cognitive impairment.

Clinical Manifestations

Affected individuals present with small testes, infertility, gynecomastia, sparse body hair, tall stature, and mild to moderate cognitive impairment. Mosaic forms often produce milder symptoms.

Cri du Chat Syndrome (5p Deletion Syndrome)

Genetic Basis

Cri du Chat syndrome arises from a partial deletion of the short arm of chromosome 5. The extent of deletion varies among individuals, influencing disease severity. Most cases occur sporadically, with a small proportion inherited.

Clinical Features

Diagnosis and Management

Diagnosis involves cytogenetic analysis and prenatal testing. Treatment is supportive and includes physical, occupational, and speech therapy.

Down Syndrome (Trisomy 21)

Overview

Down syndrome is caused by trisomy of chromosome 21 and represents the most common autosomal aneuploidy compatible with survival. It occurs in approximately 1 in 800 live births.

Clinical Manifestations

Immunity and Genetic Influence

Innate Immunity

Innate immunity constitutes the body’s first line of defense and is present at birth. It includes physical barriers (skin, mucosa), mechanical processes (coughing, urination), biochemical agents (lysozyme, stomach acid), and cellular components such as neutrophils, macrophages, natural killer cells, and dendritic cells.

Inflammation represents the second line of innate defense and is characterized by redness, heat, swelling, and pain.

Adaptive Immunity

Adaptive immunity develops over time and provides antigen-specific responses. It includes humoral immunity mediated by B lymphocytes and cell-mediated immunity driven by T lymphocytes. Memory cells ensure long-term protection following infection or vaccination.

Inheritance Patterns of Genetic Disorders

Modes of Genetic Inheritance

Autosomal Dominant Disorders

Autosomal dominant disorders require only one copy of a mutated gene for disease expression. Each offspring of an affected parent has a 50% chance of inheriting the condition. Examples include Huntington’s disease.

Autosomal Recessive Disorders

Autosomal recessive conditions manifest only when an individual inherits two mutated alleles. Parents are often asymptomatic carriers. Examples include cystic fibrosis and sickle cell disease.

X-Linked Disorders

Case Study: Systemic Lupus Erythematosus (SLE)

Pathophysiology

Systemic lupus erythematosus is a chronic autoimmune disease characterized by the production of antinuclear antibodies. Genetic susceptibility combined with environmental triggers such as UV radiation leads to immune complex formation and type III hypersensitivity reactions, resulting in multisystem inflammation and tissue damage.

Clinical Manifestations and Diagnosis

Diagnosis requires fulfillment of at least four established clinical and immunologic criteria, including malar rash, arthritis, renal involvement, hematologic abnormalities, and autoantibody detection.

Treatment and Management

Management focuses on preventing flare-ups, minimizing organ damage, and suppressing immune activity through corticosteroids, immunosuppressive agents, and lifestyle modifications.

References

American College of Obstetricians and Gynecologists. (2020). Carrier screening in the age of genomic medicine. ACOG Practice Bulletin.

Centers for Disease Control and Prevention. (2023). Genetic testing and screening.

Kumar, V., Abbas, A. K., & Aster, J. C. (2021). Robbins and Cotran pathologic basis of disease (10th ed.). Elsevier.

D115 Comprehensive Study of Human Genetics, Immunology, and Pathophysiology

National Institutes of Health. (2022). Genetic and rare diseases information center.

Strachan, T., & Read, A. (2018). Human molecular genetics (5th ed.). Garland Science.