Genetic and Genomic Principles
Genetic and Genomic Principles
This domain covers the genetic basis of metabolic disorders: inheritance patterns (autosomal recessive, X-linked, mitochondrial), mutation types, and the use of genetic testing (e.g., targeted mutation analysis, gene panels, exome sequencing). You must understand concepts such as penetrance, variable expressivity, genetic heterogeneity, and genotype-phenotype correlations. Also, know the principles of mitochondrial inheritance (maternal transmission, heteroplasmy, threshold effect) and X-linked disorders (e.g., OTC deficiency, Fabry disease).
Traps include assuming all metabolic disorders are autosomal recessive, overlooking X-linked disorders with skewed X-inactivation in females, and misinterpreting variants of uncertain significance (VUS). Be cautious with disorders caused by trinucleotide repeats (e.g., some mitochondrial disorders) and imprinting (not common in metabolic disorders). Also, remember that enzyme activity may be normal in carrier states, and that genetic testing may not always be necessary for diagnosis if biochemical testing is definitive.
A rule of thumb: when a metabolic disorder is suspected, consider the inheritance pattern to guide family testing and counseling. For example, OTC deficiency is X-linked; affected males are severely affected, while heterozygous females may have variable symptoms due to lyonization. This has implications for family screening and prenatal diagnosis.
Key Points
- Most metabolic disorders are autosomal recessive; exceptions include X-linked (OTC deficiency, Fabry) and mitochondrial.
- Mitochondrial disorders: maternal inheritance, heteroplasmy, threshold effect; genetic testing may require testing multiple tissues.
- Genetic heterogeneity: mutations in different genes can cause similar biochemical phenotypes.
- Variants of uncertain significance require careful interpretation and may not be diagnostic.
- Genetic counseling should include discussion of recurrence risk and options for prenatal/preimplantation testing.