Mitochondria are powerhouses of our cells and until recently we were thought to inherit them only from our mothers. However, new research may be about to change the textbooks.
Mitochondria are small organelles (i.e. a sub-compartment in a cell) which are found in every cell (apart from red blood cells). They are capable of producing 90% of the energy a cell requires by using oxygen to burn our food, making chemical energy via oxidative phosphorylation. Alongside energy production, mitochondria help to break down waste products, produce vital chemicals, and recycle waste products to produce energy. They’re pretty important. Mitochondria also have a role in cell death (i.e. apoptosis) when cells have aged, and a disturbance in this can lead to cancer.
When mitochondria don’t work or are faulty, numerous diseases can occur. Mitochondrial diseases are genetic and effect around 1 in 5,000 people. Such diseases can be present from birth or present themselves later in life, affecting almost any part of the body. Some symptoms can include poor growth, muscle weakness, vision and hearing problems, seizures, heart diseases and much more. There are currently no cures for mitochondrial diseases but instead, treatments to reduce symptoms and a slow decline in health. Some examples of mitochondrial disease can be found here: LINK, one in particular is MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis and Stroke-like episodes) which affects my grandmother. Mitochondria have their own DNA (mtDNA) which codes for numerous proteins they use to produce energy and until recently, it was thought that we inherited this mtDNA from our mothers.
Inheritance happens through the transfer of mtDNA to the embryo from solely the egg, never sperm. Therefore, it was thought that if your mother had alterations in her mtDNA, you too would inherit the disease, but couldn’t inherit changes from your father. This is still stated on the NHS website. However, new research may change the textbooks. Taosheng Huang, a paediatrician and geneticist at Cincinnati Children’s hospital, had a 4 y/o patient who carried two sets of mtDNA: one from his mother, one from his father. Even Huang didn’t believe it. Since this finding, Huang has verified paternally-inherited mtDNA in 17 individuals, from 3 separate families. Until now, mitochondrial disease has only been tested down the maternal lineage; this finding will change research and genetic testing.
There are a few theories as to why we had only seen maternal mtDNA inheritance: (1) sperm undergo much higher rates of mutation and thus, evolution decided they were too risky to pass on mtDNA; or (2) we only get one set of mtDNA to streamline the coordination of DNA sets, ensuring good cellular function. It was thought that a secondary set of mtDNA could muddle this combination. Sperm do have mitochondria which help power them on their journey to an egg, but upon fertilisation they are marked so the egg can recognise and destroy rogue paternal mitochondria.
So how did paternally-inherited mtDNA happen? Strangely, in families where this had occurred, paternal mtDNA wasn’t passed to everyone. In cases where paternal mtDNA was sneaking through and a daughter inherited a mixture from both parents, her children all had this same mixture and thus, a mitochondrial disease. Alternatively, when a son had acquired a mixture of maternal and paternal mtDNA, he was able to pass this on and his children then have an even broader mixture when their mother’s mtDNA was added to the equation. However, men who obtained such mixtures were not always able to pass them on. Confusing right?
The reasons behind this inheritance are still unknown and strangely, the answers don’t appear to be in the mtDNA itself but are likely from mutations to the DNA in cell nuclei. Research into mitochondria has already yielded new procedures such as 3 person babies. The family reported had suffered 4 miscarriages and the death of two older children resulting from the inheritance of a maternal mitochondrial disease. The “3 person baby” takes the father’s sperm and mother’s egg, but replaces the mtDNA in the egg with a donor’s mtDNA, resulting in a baby without a mitochondrial disease. In the future, even techniques as advanced and life changing as this, will need to consider the effects of paternal mtDNA.
With this new research, many more investigations are underway as to why mtDNA can be inherited paternally, and testing for paternal mtDNA will need be included in mitochondrial disease testing. It is interesting that in 2019 we are still rewriting textbooks which have been trusted for decades. Go Science!