A new study provides compelling evidence that children can inherit mitochondrial DNA from both their parents.<\/h6>\n<\/div>\n
<\/p>\n <\/p>\n The first time\u00a0Taosheng Huang<\/a>\u00a0saw the test results, he was sure there\u2019d been a mistake. Even after a technician repeated the diagnostic, Huang didn\u2019t believe it. \u201cThat\u2019s impossible,\u201d he said.<\/p>\n Huang, a pediatrician and geneticist at\u00a0Cincinnati Children\u2019s Hospital Medical Center<\/a>, asked the patient to come back and provide fresh samples of blood, which Huang then split among several research labs to reduce any chance of error. It was a finding that Huang knew would break a central tenet of human genetics\u2014but time and time again, the outcome was the same.<\/p>\n Huang\u2019s patient, a four-year-old boy, was carrying two different sets of\u00a0mitochondrial DNA<\/a>: one from his mother, as expected\u2014and another, from his father.<\/p>\n This was only the beginning. Using modern DNA sequencing technology, Huang and his colleagues have conclusively verified paternally-inherited mitochondrial DNA in 17 individuals spanning three unrelated families. Their\u00a0work<\/a>\u00a0appears today in the journal\u00a0PNAS.<\/i><\/p>\n \u201cThis is a really groundbreaking discovery,\u201d says\u00a0Xinnan Wang<\/a>, a biologist at Stanford University who studies mitochondria but was not involved in the new findings. \u201cIt could open up an entirely new field… and change how we look for the cause of [certain] diseases.\u201d<\/p>\n<\/div>\n Broadly speaking, we\u2019re each a genetic mix of Mom and Dad. In the nucleus, which contains our chromosomes, this holds true. But the nucleus isn\u2019t the only part of the cell that contains DNA. Cells contain power centers called mitochondria that also carry their own sets of DNA\u2014and in nearly all known animals, mitochondrial DNA is\u00a0inherited exclusively<\/a>\u00a0from the mother. This lopsided acquisition is so ingrained that researchers often analyze mitochondrial DNA to trace maternal lineages back in time.<\/p>\n Scientists still aren\u2019t completely sure\u00a0why<\/i>\u00a0this process has evolved to be strictly matrilineal, but a few theories have come to the forefront. For one, the mitochondria of sperm are believed to experience\u00a0higher rates of mutation<\/a>\u00a0than those in eggs, making their input somewhat riskier. Additionally, having only one type of mitochondria makes it easier for the genomes in the nucleus and mitochondria to\u00a0coordinate<\/a>, as both generate the raw materials necessary for proper cellular function, explains\u00a0Sophie Breton<\/a>, a mitochondrial geneticist at the University of Montreal who did not participate in the study. The addition of another mitochondrial stakeholder, so the theory goes, could muddle this intimate two-way dialogue.<\/p>\n Whatever is driving maternal mitochondrial inheritance, it\u2019s clear our cells have taken quite a few precautions to keep it that way. A formidable bevy of molecular machines stands at the ready to ensure that no unwanted paternal contribution slips through in the process of fertilization. Even though a sperm\u2019s mitochondria help power its odyssey to the egg, its mitochondria aren\u2019t meant to last. Some mitochondria are thought to be lost as sperm develop; the few that make it through the fertilization process are tagged with a chemical marker that allows the egg to\u00a0easily recognize and annihilate<\/a>\u00a0any rogue paternal interlopers.<\/p>\n A handful of animals<\/a>, including flies and mice, are known to occasionally\u00a0\u201cleak\u201d paternal mitochondrial DNA<\/a>\u00a0into offspring alongside maternal input, but humans have historically been excluded from this club. Only a few reports of dad-derived mitochondrial DNA exist, and most have been chalked up to contamination or laboratory sample mix-ups.\u00a0One case<\/a>, described in 2002, received considerable attention when it was independently confirmed by another research group. But in the 16 years since, the search for another instance of this genetic anomaly has been unfruitful.<\/p>\n <\/p>\n<\/div>\n So even after Huang was convinced that his remarkable four-year-old patient had inherited mitochondrial DNA from both parents, he knew the work was far from done. When the researchers next traced this odd genetic signature back through the boy\u2019s family tree, they found that, across three generations, 10 individuals in the patient\u2019s family appeared to harbor mixed mitochondrial DNA. Now that the researchers knew where to look\u2014and what to look for\u2014as they expanded their search. Not long after, seven additional individuals from two other unrelated families were confirmed to harbor the same condition.<\/p>\n<\/div>\n<\/div>\n Amazingly, all three families seemed to show similar patterns of inheritance of this unusual trait. Not all members were afflicted, meaning that some individuals passed on their genes in the typical fashion, conceiving children that inherited only their mothers\u2019 mitochondrial DNA. But it was clear that the mitochondrial genomes of several men in these families were breaking the biological rules of fatherhood. Rather than being discarded, these paternal packages of genetic information somehow kept pace with their maternal counterparts during fertilization, leaving some of their kids with mixed mitochondria.<\/p>\n Daughters with these mixed genomes then went on bequeath their heterogeneous heirlooms onto their own children in carbon copy. This meant their kids inherited mixtures indirectly\u2014even though the children possessed none of their own father\u2019s mitochondrial DNA. Such was the case with Huang\u2019s original four-year-old patient.<\/p>\n Sons who acquired mixed mitochondrial genomes presented a slightly more complex picture. The ability to pass on paternal DNA seemed to be a dominant trait, or one that requires a gene from only one parent to show itself in offspring. This meant that some of the men who had inherited hybrid mitochondrial genomes were able to transfer them into their children, who displayed even more diversity as they mixed Dad\u2019s genes with Mom\u2019s. But not all men with mixtures retained this ability, leading to occasional dead-ends.<\/p>\n Exactly\u00a0how<\/i>\u00a0paternal mitochondrial DNA infiltrates the embryo still isn\u2019t entirely clear. It\u2019s definitely a genetic trait\u2014but not one, ironically, found in the mitochondrial genome itself. In fact, apart from the unusual mode of inheritance, there don\u2019t seem to be any defects present in any of these patients\u2019 mitochondria. Instead, whatever allows dads to deposit their mitochondrial DNA in their offspring is likely a mutation encoded in the nucleus\u2014where things are\u00a0supposed<\/i>\u00a0to come from both mom and dad.<\/p>\n Florence Marlow<\/a>, a developmental biologist at the Icahn School of Medicine at Mount Sinai who was not involved in the research, theorizes that the anomaly could be at the step where the mitochondria of sperm are normally tagged for destruction. If this labeling step never occurs, she explains, the fertilized egg has no way of identifying and eliminating these paternal interlopers\u2014meaning Dad\u2019s mitochondria get a clear pass to proceed alongside Mom\u2019s.<\/p>\n<\/div>\n Since putting together their initial findings, Huang and his team have already identified several more candidate families with mixed mitochondrial DNA. According to his preliminary estimates, this phenomenon may be present in as many as 1 in every 5,000 people. While Wang stresses that more research by other groups is needed to confirm these numbers, the work from Huang\u2019s team indicates that paternal transmission of mitochondria may be far more widespread than researchers once thought.<\/p>\n So how has this been overlooked for so long? \u201cMost people have taken for granted that mitochondrial inheritance is strictly maternal,\u201d Breton explains. \u201cBut sequencing techniques are so much more powerful than they were even just a few years ago.\u201d<\/p>\n The possibility of paternal inheritance could also expand our options for assisted reproductive technology in the distant future. Huang\u2019s group previously played a part in the successful conception and delivery of a\u00a0\u201cthree-parent\u201d baby<\/a>\u00a0in 2016. The child\u2019s mother suffered from a mitochondrial disease, so Huang and his colleagues transferred her nuclear DNA into a donor egg, which had been stripped of its own chromosomes but could supply healthy mitochondria. This hybrid egg, containing the genetic information from two mothers, was then fertilized with the father\u2019s sperm.<\/p>\n In theory, Breton says, if paternal mitochondria are viable, they could be added to\u2014or perhaps even replace\u2014defective maternal mitochondria, obviating the need for a third parent.<\/p>\n But that\u2019s a big if, especially considering that the mitochondrial DNA of sperm is more prone to mutation. We\u2019re far from understanding the full repercussions of inheriting bi-parental mitochondria under natural circumstances, much less through artificial medical procedures, many of which are likely to fuel\u00a0continued debates on ethics<\/a>.<\/p>\n For now, maternal inheritance of mitochondrial DNA is still the norm. But simply expanding our view of genetic inheritance opens up countless doors. \u201cThese results will change the way we describe mitochondrial inheritance,\u201d Breton says. \u201cI\u2019m 100 percent sure we\u2019ll find more cases like these in the future.\u201d<\/p>\n<\/div>\n<\/div>\n<\/div>\n <\/p>\n <\/p>\n \uc6d0\ubb38: \uc5ec\uae30<\/a>\ub97c \ud074\ub9ad\ud558\uc138\uc694~<\/p>\n <\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":"![\"\"](\"http:\/\/163.180.4.222\/lab\/wp-content\/uploads\/2018\/12\/baby-2436661_1920.width-2500-300x169.jpg\")
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