Medical Genetics - V. M. Zaporozhan 2005

Chromosomal Diseases
Pathogenesis of Chromosomal Diseases

The Pathogenesis of Chromosomal Disorders is highly complex. It stems from dysregulated expression of numerous genes involved in chromosomal or genomic Mutations. Symptoms are predominantly driven by the so-called Gene dosage effect—where mutations result in either an excess or a deficit of gene copies—and less frequently by structural gene alterations. The effects of chromosomal and genomic mutations begin to manifest during the Embryonic period. The earliest stages of zygote Cleavage are governed by maternal transcripts accumulated within the oocyte, followed by the activation of the zygote's own genome. Overall, approximately 1,000 genes orchestrating various stages of ontogenesis are active during Embryogenesis. Genomic and chromosomal mutations disrupt the delicate balance of numerous genes, including those governing embryonic development. Furthermore, changes in Gene Expression linked to Genomic Imprinting are observed. This inevitably impairs histogenesis and Organogenesis, which is why chromosomal disorders typically manifest as congenital birth defects.

Most commonly, these abnormalities prove incompatible with life, resulting in intrauterine embryonic or fetal demise. Less frequently, an affected infant is born. Clinically, chromosomal disorders present as syndromes characterized by multiple Congenital Malformations, virtually all of which are established before birth. The only exception involves numerical and structural aberrations of the sex Chromosomes. The final phenotype in patients with sex chromosome imbalances may continue to develop well into Puberty.

Geneticists often compare chromosomal disorders to the aftermath of a fire. The fire represents the destructive processes occurring during the embryonic period, while the final phenotype observed at birth is akin to smoldering ashes. Nothing can be undone; only cosmetic corrections or palliative surgeries for malformations are possible (provided the syndrome is compatible with life).

Because chromosomal disorders disrupt the earliest stages of embryonic development, multiple Organs and Organ Systems are affected simultaneously. This accounts for the overlapping clinical presentations seen across many chromosomal disorders. The greater the chromosomal imbalance, the more non-specific the clinical picture tends to be—a phenomenon especially evident in polyploidies. Conversely, microdeletions and microduplications often produce highly specific clinical phenotypes.

Polymorphism is a hallmark of any chromosomal disorder, as the individual genotype of each Organism significantly modulates gene expression.