MITOCHONDRIAL INHERITANCE

•  Some of the estimated 20,000 genes in the human genome are located in small compartments in the cell called the mitochondria, rather than on chromosomes in the cell’s nucleus. Some cells contain many hundreds of mitochondria.


• The genes found within the mitochondria contain the information that codes for the production of many of the important enzymes that drive the biochemical reactions to produce the body’s source of energy. The cells in the body, especially in organs such as the brain, heart, muscle, kidneys and liver, cannot function normally unless they are receiving a constant supply of energy.


• Faulty mitochondrial genes can result in absence of these enzymes, or enzymes that are impaired and do not work properly.
This leads to a reduction in the supply of ATP, and may result in problems with the body’s functions

• The pattern of inheritance of conditions due to faulty mitochondrial genes is often called maternal inheritance. This is because
a child inherits the great majority of their mitochondria from their mother through the egg.

• Usually a mother will have a mixture of mitochondria containing the working gene copy and others containing the faulty gene.
For a condition to develop, the number of mitochondria with the faulty gene must be above a critical level (the threshold)

• The cells of different tissues and organs can have varying amounts of mitochondria with a faulty gene, and the number of cells
with the faulty mitochondria in each tissue or organ may vary

• Conditions due to having faulty mitochondrial genes create complexities when assessing the risk the mother has for passing on
the faulty mitochondrial genes to a child and if, or how severely, the child would be affected
• While it is possible to test for the presence or absence of some faulty mitochondrial genes and their products during pregnancy,
the result may be difficult to interpret
• Whether symptoms of the condition will occur or not depends on whether the numbers of mitochondria with the faulty genes
is above the critical threshold in enough cells to interfere with energy production
• Genetic counselling can provide the most current information on the availability and appropriateness of testing for mitochondrial conditions, either in an affected person or during pregnancy.


MNEMONIC- MITOCHONDRIAL DISORDERS :


KLMNOP


K-     KSS


L-     LHON
         Leigh's disease


M-    Mitochondrial myopathy
         Mitochondrial deletion
         MELAS
         MERRF
         MMC


N-    NARP
        Navajo neurohepatopathy.


O-    Chronic Progressive External Opthalmoplegia


P-     Pearson Syndrome