Discovering Human Genes & Cystic Fibrosis

Genes and DNA

• A gene is a DNA stretch determining a specific molecule's structure/function, usually a protein.
• DNA is transcribed into mRNA, then translated into protein.
• Codons (3 base pairs) form "words" in the gene "sentence."

Transcription and Translation

• Transcription: RNA is transcribed from DNA.
• Translation: mRNA is translated into a protein sequence (amino acids).

Gene Summary

• A gene is a DNA length with instructions for making a specific protein.
• Genes are arranged along 23 chromosome pairs in the cell nucleus.
• Genes specify a protein's amino acid sequence.

Mendel's Laws

• Law of Segregation: Somatic cells have two alleles for a trait; gametes have one allele chosen randomly.
• Law of Independent Assortment.

Human Disease Overview

• Four disease categories: monogenic, complex, genomic, environmental.
• Disease databases: OMIM and locus-specific databases.
• Four approaches to identifying disease genes: linkage, GWAS, chromosome analysis, sequencing.

Categories of Disease

• Single gene disorders: rare, caused primarily by mutation(s) in a single gene.
• Complex disorders: common, involve multiple genes.
• Genomic disorders: very common, caused by deletions, duplications, or rearrangements of chromosomal DNA.
• Environmental diseases: common, genes affect susceptibility to environmental insults.

Monogenic Disorders & Inheritance

• Autosomal Dominant: Males and females affected; male-to-male transmission; 50% recurrence risk.
• Autosomal Recessive: Males and females affected; recurrence within a sibship; often negative family history; 25% recurrence risk.
• X-linked Dominant: Females with one copy are affected; some disorders are lethal in males; all daughters and no sons of affected males are affected.
• X-linked Recessive: Males affected, related through mothers; females unaffected or mildly affected; 25% recurrence risk.

Cystic Fibrosis (CF)

• A monogenic recessive disorder caused by mutations in the CFTR gene.
• Classic CF symptoms: chronic bacterial infection, fat maldigestion, male infertility, elevated sweat chloride.
• Nonclassic CF: partial CFTR function, pancreatic function preserved.

CFTR Gene and Protein

• Locus: 7q31.2 on chromosome 7.
• Encodes a protein in exocrine epithelial cell membranes.
• Protein Function: Chloride channel protein in lungs, liver, pancreas, intestines, reproductive tract, and skin; regulates other transport pathways.
• Associated Disorders: Cystic fibrosis (CF) and congenital bilateral aplasia of the vas deferens (CBAVD).

CFTR Protein Structure

• CFTR transports chloride ions across cell membranes.
• Composed of:
  • Two membrane-spanning domains (MSD1 and MSD2).
  • Two nucleotide-binding domains (NBD1 and NBD2) that bind and hydrolyze ATP.
  • A regulatory (R) domain.
• Delta F508: Common CF-causing mutation in NBD1.

CFTR Mutations

• ΔF508: Most common mutation; protein misfolding and degradation.
• Homozygous for ΔF508: Severe CF symptoms due to chloride ion transport loss.
• Leads to thick, sticky mucus, chronic infections.

Diagnosis of CF

• Sweat test: Measures chloride concentration in sweat.
  • Cl^- ≥ 60 mEq/L indicates CF.
• Other tests: blood tests, chest x-rays, sputum cultures, pulmonary function tests.

Complex Disorders

• Involve multiple genes; non-Mendelian inheritance.
• Familial aggregation, but not segregation.
• Susceptibility alleles have high population frequency.

Genomic (Chromosomal) Disorders

• Caused by deletions, duplications, or rearrangements of chromosomal DNA.
• Aneuploidy: Abnormal number of chromosomes

Identifying Disease Genes: Approaches

• Linkage Analysis: Genes transmitted together from one generation to the next.
  • Recombination frequency (R.F.) of 50% or more indicates independent assortment.
  • R.F. = \frac{Total \ number \ of \ recombinant \ progeny}{Total \ progeny \ (parental + recombinant)} \ X \ 100
• Genome-Wide Association Studies (GWAS).
• Identification of Chromosomal Abnormalities.
• Genomic DNA Sequencing.