molecular pathology seminars

• Mitochondria


  Can be produced in both nDNA and mDNA
• nDNA


  • Redundant genes
  • Mutations appear from exposure to harmful agents, often as a result of their own activity
  • The rate of mtDNA variability is ten times higher because of the ROS contained, but also, due to the lack of efficiency in the mtDNA repairing mutations machinery

  
• Apparition of random mutations leads to apparition of rare diseases
• Heteroplasmy


  Inside the same cell, some mitochondria are affected by certain mutations, and other mitochondria by others
• Mitochondrial diseases are rare but not insignificant, with a prevalence of 1:5,000

  
• Mitochondrial genetic bottleneck


  Mitochondria segregates randomly, resulting in unequal penetration among different descendants, leading to some individuals being more affected than others

  
• The penetrance expressed by the mother may not necessarily be the same as that expressed by her children
• Mutations in nuclear-DNA (nDNA)

  • nDNA genes may follow autosomal inheritance patterns or be linked to the X chromosome, and can be dominant or recessive
  • There have been identified 338 genes that are susceptible to suffer from mutations that may lead to a mitochondrial disease: 36 (11%) belong to mitochondrial DNA and 302 (89%) are encoded in nDNA
  • There have been detected 262 mutations being autosomal recessive, 36 being maternally inherited, 8 being autosomal dominant, 6 being X-linked dominant, 4 being X-linked recessive, and 22 being a combination of autosomal recessive and autosomal dominant inheritance

  
• Primary mitochondrial diseases (PMDs)


  • Proteins associated with the mitochondrial respiratory chain, encoded in mtDNA and nDNA
  • The most common causes of PMDs are point mutations or large deletion

  
• Secondary mitochondrial diseases (SMD)


  Do not directly involve affectations on the mitochondrial respiratory chain, not always attributed only to genetic causes, can be inherited or acquired over time due to exposure to reactive oxygen species (ROS)

  
• Mitochondrial diseases are very varied, with a wide variety of phenotypes related

  
• Symptoms of mitochondrial diseases

  • Myoclonus
  • Opthalmoplegia
  • Ataxia
  • Ptosis

  
• Pearson's Syndrome (PS)


  • Caused by single large-scale mtDNA deletions (SLSMDs) in different locations, affecting proteins directly involved in the respiratory chain and tRNAs
  • Severe pathological disease, can be inherited (low quantity of affected mitochondria) or de novo mutation
  • Rare disease, with a prevalence of approx. 1:1,000,000, with less than 150 reported cases worldwide

  
• The most common mutation in PS is 8470-13446del4977

  
• Genes that encode for proteins subunits directly involved in the respiratory chain process and tRNA are affected in PS, KSS, and PEO
• Molecular genetics of PS

  High levels (>50%) of mitochondrial DNA deletion cause severe symptoms like PS, while moderate levels (11–49%) lead to KSS-like disease later on. Low levels (10%) keep mice healthy

  
• Biochemistry of PS


  1. Dysfunction of the respiratory chain

  2. Accumulation of reducing agents like NADH and ROS

  3. Removal of tRNA-Gly, tRNA-Arg, tRNA-Ser, tRNA-His, and tRNA-Leu

  4. Increased fermentation process

  5. Defect in the synthesis of proteins

  6. No obtention of energy - No ATP synthesis

  
• Symptomatology of PS includes delayed growth and development, production of blast with cytoplasmic vacuolization, accumulation of organic compounds that can not be oxidated because of the inhibition of aerobic oxidation, and pancreatic dysfunction, found in 12.7% of PS cases
• Representation of the deletion of 4,997 bps (8,470-13,446 bps) in the Homo sapiens' mitochondrial DNA and all the genes it contains by using the NCBI's Genome Data Viewer

  
• Pearson's syndrome


  • SLSMD
• Electron transport chain


  1. Complex I

  2. Complex II

  3. Complex III

  4. Complex IV

  5. ATP synthase
• Colorectal cancer


  Transformation of the colon epithelium from normal cells to an adenomatous polyps ultimately invasive cancer in a multistep progression over time

  
• Stages of colorectal cancer

  • Stage 0: Cancer is only in the innermost layer of the lining of the intestine
  • Stage I: Cancer is in the inner layers of the intestines
  • Stage II: Cancer has spread through the muscular wall of the colon
  • Stage III: Cancer has spread to the lymph nodes
  • Stage IV: The cancer has spread to other organs, such as the liver and the lungs

  
• Polyps that may be cancerous


  • If a polyp larger than one centimeter is discovered
  • If it is discovered that there are more than three polyps
  • If after removal of a polyp the presence of dysplasia is discovered

  
• Common symptoms of colorectal cancer

  • Diarrhoea
  • Constipation
  • Blood in the feces
  • Abdominal pain
  • Unexplained weight loss and loss of appetite
  • Anemia due to blood loss, causing low iron levels
  • Fatigue due to low iron levels
• Many people will not have symptoms in the early stages of the colorectal cancer disease

  
• Colorectal cancer


  The third most common cancer around the world

  
• Colorectal cancer is caused by changes on the DNA which may activate oncogenes or deactivate tumour suppressor genes, resulting in an uncontrolled cell growth

  
• Inherited genetic mutations linked to colorectal cancer

  • FAP
  • AFAP & Gardner Syndrome (APC gene)
  • MYH associated poliposis (MUTYH gene)
  • Lynch syndrome (MLH1, MSH2, MSH6, PMS2 genes)
  • Cystic fibrosis (CFTR gene)
  • Peutz jeghers syndrome, etc

  
• Approximately 75% of colorectal cancers are sporadic and occur in people without genetic predisposition or family history of colorectal cancer

  
• Pathways for genomic instability in colorectal cancer

  • Chromosome instability pathway (CIN)
  • Microsatellite instability pathway (MIP)
  • CpG island methylator phenotype (CIMP)

  
• KRAS and BRAF


  Sporadic mutations in colorectal cancer

  
• p53 mutations


  Some of the most frequent mutations in the CIN pathway of colorectal cancer

  
• p53 mutations lead to unlimited cellular proliferation in absence on growth factors and immune escape
• FAP


  The major hereditary predisposition event leading to colorectal cancer development

  
• FAP


  Autosomal dominant
• Somatic APC mutations


  Found in 80% of sporadic colorectal tumours

  
• Loss of heterozygosity


  Occurs in 30-40% of sporadic colorectal tumours

  
• APC gene


  • The protein encoded has 15 exons, with exon 15 containing 75% of the coding sequence

  
• Types of APC gene mutations

  • Truncating mutations leading to loss of function and stable truncated product (60% in the mutation cluster region)
  • Missense mutations (10% of total APC mutations) leading to gain of function and dominant functions