BackClinical Cytogenetics: The Chromosomal Basis of Human Disease
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Clinical Cytogenetics: The Chromosomal Basis of Human Disease
Indications for Performing Chromosome Analysis
Chromosome analysis is a critical diagnostic tool in clinical genetics, used to identify chromosomal abnormalities that underlie various human diseases and syndromes. The following are key indications for performing chromosome analysis:
Suspected recognizable chromosome syndrome: For example, Down syndrome.
Undiagnosed pattern of two or more malformations: Multiple congenital anomalies may suggest a chromosomal disorder.
Ambiguous genitalia: Unclear sexual development can be linked to chromosomal abnormalities.
Intellectual disability or developmental delay: Especially when accompanied by physical abnormalities.
Parents and children with chromosomal translocations, deletions, or duplications: Family history increases risk.
Stillborn infants with malformation or unexplained fetal death: Chromosomal analysis can reveal underlying causes.
Females with short stature and primary amenorrhea: Consider Turner syndrome.
Males with small testes or significant gynecomastia: Consider Klinefelter syndrome.
Chromosomal Translocations and Human Disease
Chromosomal translocations are a major cause of genetic disease and cancer. A translocation occurs when segments from two different chromosomes are exchanged. This can disrupt gene function and regulation, leading to disease.
Philadelphia Chromosome: A well-known example is the reciprocal translocation between chromosomes 9 and 22, producing the Philadelphia chromosome. This abnormality is consistently found in chronic myelogenous leukemia (CML).
Mechanism: The translocation moves the ABL gene from chromosome 9 to chromosome 22, where it fuses with the BCR gene. The resulting BCR-ABL fusion gene encodes a tyrosine kinase with increased activity, driving uncontrolled cell division in hematopoietic cells.
Clinical Impact: Identification of the BCR-ABL fusion has led to targeted therapies, such as tyrosine kinase inhibitors, which are highly effective for CML.
Other Translocations: Translocations involving other chromosomes can activate oncogenes or disrupt tumor suppressor genes, contributing to various cancers and genetic syndromes.
Specific Cytogenetic Changes in Leukemias and Solid Tumors
Cytogenetic analysis is essential for diagnosing and classifying leukemias and solid tumors. Certain chromosomal aberrations are characteristic of specific cancer types.
TYPE | MOST COMMON CHROMOSOME ABERRATION |
|---|---|
Chronic myelogenous leukemia | t(9;22)(q34;q11) |
Acute myeloblastic leukemia | t(8;21)(q22;q22) |
Acute promyelocytic leukemia | t(15;17)(q22;q12-11) |
Acute lymphocytic leukemia | t(12;21)(p13;q22) |
Burkitt lymphoma | t(8;14)(q24;q32) |
Ewing sarcoma | t(11;22)(q24;q12) |
Meningioma | Monosomy 22 |
Retinoblastoma | del(13)(q14) |
Wilms tumor | del(11)(p13) |
Neuroblastoma | MYNC amplification |
Breast cancer | HER2/NEU amplification |
Applications and Importance of Cytogenetic Analysis
Cytogenetic analysis provides valuable information for diagnosis, prognosis, and treatment planning in genetic diseases and cancers. It is routinely performed on bone marrow cells from leukemia patients and on solid tumor samples to identify characteristic chromosomal changes.
Diagnosis: Identifies specific chromosomal abnormalities associated with disease.
Prognosis: Certain aberrations are linked to disease outcome and response to therapy.
Treatment: Enables targeted therapies based on genetic changes (e.g., tyrosine kinase inhibitors for CML).
Key Terms:
Translocation: Exchange of segments between nonhomologous chromosomes.
Oncogene: A gene that can cause cancer when activated or overexpressed.
Proto-oncogene: A normal gene that can become an oncogene due to mutation or translocation.
Tyrosine kinase: An enzyme that transfers phosphate groups to proteins, often involved in signaling pathways controlling cell growth.
Example:
The Philadelphia chromosome is a classic example of a disease-causing chromosomal translocation, leading to chronic myelogenous leukemia through the formation of the BCR-ABL fusion gene.
Additional info:
Cytogenetic analysis is increasingly used in personalized medicine to guide therapy choices and predict patient outcomes.
