Thank your inner retrovirus for your existence

When the human genome project was first rolled out 20 years ago it came as a shock to find out that 8% of our 3,200,000 nucleotide genome was made of retrovirus relics.  They are the perfect example of selfish DNA — they don’t do anything other than insure their transmission to the next generation.   They are the perfect parasite infecting the host without killing it.  Since they don’t have to do anything, mutations rapidly accumulate in them and none of them can make a functioning virus.

As most know, retroviruses have genomes made of RNA, which is reverse transcribed into by an enzyme they contain into a DNA copy (cDNA) which then is inserted into the genome of the host.  HIV1, the virus of AIDs is one such retrovirus.   Fortunately HIV1 hasn’t entered the genome of eggs or sperm, so it hasn’t become an endogenous retrovirus, but it is all over the DNA or immune cells of those infected.

What is even more interesting (and totally unexpected) is that the host can repurpose these retroviral relics to do something useful.

In fact they’ve become so useful that we couldn’t reproduce without them.  The syncytiotrophoblast layer of the placenta is at the maternal fetal interface.  It is a continuous structure, one cell deep formed by fusion of the constituent trophoblast cells.  The layer has microvillar surfaces which facilitate exchanges of nutrients and waste products between mother and fetus.

Syncytin1 is a protein expressed here.  It is produced from the env gene, of a Human Endogenous RetroVirus (HERV) called HERV-W.  Adding the protein to culture systems leads to syncytium formation.  Mice in which the gene has been knocked out die in utero, due to failure of trophoblast cells to fuse.

Well that’s pretty spectacular and not much commented on although it’s been known for 20 years.

It shows that the envelope protein from another retrovirus (HERV-K subtype HML-2 is expressed at high levels on human pluripotent stem cells.  Not only that it keeps them from differentiating — something important for our longevity — so we always have a few pluripotent stem cells around.

As a neurologist I find it fascinating that knocking down the env protein causes the stem cells to differentiate into neurons.  Don’t get too excited that we’ve found the fount of neuronal youth, as forced expression of the env protein in terminally differentiated neurons kills them.

Bad news on the AIDs front

Bad news for those hoping for an AIDs cure. As you know, the active virus (HIV1) has a genome made of RNA. However, thanks to an enzyme it possesses called reverse transcriptase (which has led to Nobel prizes), it copies itself into DNA and integrates into the genome of lymphocytes. There it sits presumably doing nothing, but it’s always capable of activating and producing more infectious virus.

We seem to have fought the virus to a draw, using a cocktail of drugs which attack different aspects — HAART (Highly Active Antiretroviral Therapy). Success is usually considered being unable to detect viral RNA in the blood (see later). However blood cells are short-lived. What about the longer living lymphocytes found in the lymph nodes and spleen.

That’s what was studied in a current paper [ Nature vol. 530 pp. 5` – 45 ’16 ] but in only 3 people. All had no detectable virus in the blood (under 48 copies/milliLiter — an incredibly tiny amount — see later). What they did was to biopsy lymph nodes in the groin on study entry and at 3 and 6 months.

Then they sequenced the genomes of the lymphocytes from the nodes, to study the HIV1 DNA integrated into the genome. They found that the genome changed with time. This is very bad. Why?

Because it implies that, even though you the virus in the blood, the virus was not staying latent in the lymph nodes, but coming out of the lymphocytes and forming infectious virus which then mutated. Subsequently the mutated virus integrated into the genome of another lymphocyte. So even with what we consider excellent control, the virus is not purely latent. Drug resistance could arise from mutations (although they didn’t see it in this study).

Clearly, more people need to be studied this way (but serial biopsies? It will probably be done in prisoners, if such things are still done).

It’s worthwhile thinking about how incredibly selective and accurate our methods of analysis are. 48 copies of the viral RNA per milliLiter of blood is the lower limit of detection. Remember that water has a molecular weight of 18, so a liter of distilled water is 1000 grams / 18 grams = 55.5 Molar. A mole has 6 x 10^23 molecules. A milliLiter is 10^-3 liters. So 1 milliLiter of distilled water has 55 * 6 * 10^23 * 10^-3 == 3 * 10^22 molecules of water in it so the assay is finding 48 or more molecules of HIV1 RNA in the water haystack. Even figuring that the concentration of water in blood is 1/10 that of distilled water, this is still impressive.

Scary stuff

While you were in your mother’s womb, endogenous viruses were moving around the genome in your developing developing brain according to [ Neuron vol. 85 pp. 49 – 59 ’15 ].

The evidence is pretty good. For a while half our genome was called ‘junk’ by those who thought they had molecular biology pretty well figured out. For instance 17% of our 3.2 gigaBase DNA genome is made of LINE1 elements. These are ‘up to’ 6 kiloBases long. Most are defective in the sense that they stay where they are in the genome. However some are able to be transcribed into RNA, the RNA translated into proteins, among which is a reverse transcriptase (just like the AIDS virus) and an integrase. The reverse transcriptase makes a DNA copy of the RNA, and the integrates puts it back into the genome in a different place.

Most LINE1 DNA transcribed into RNA has a ‘tail’ of polyAdenine (polyA) tacked onto the 3′ end. The numbers of A’s tacked on isn’t coded in the genome, so it’s variable. This allows the active LINE1’s (under 1/1,000 of the total) to be recognized when they move to a new place in the genome.

It’s unbelievable how far we’ve come since the Human Genome Project which took over a decade and over a billion dollars to sequence a single human genome (still being completed by the way filling in gaps etc. etc [ Nature vol. 517 pp. 608 – 611 ’15 ] using a haploid human tumor called a hydatidiform mole ). The Neuron paper sequenced the DNA of 16 single neurons. They found LINE1 movement in 4

Once a LINE1 element has moved (something very improbable) it stays put, but all cells derived from it have the LINE1 element in the new position.

They found multiple lineages and sublineages of cells marked by different LINE1 retrotransposition events and subsequent mutation of polyA microsatellites within L1. One clone contained thousands of cells limited to the left middle frontal gyrus, while a second clone contained millions of cells distributed over the whole left hemisphere (did they do whole genome on millions of cells).

There is one fly in the ointment. All 16 neurons were from the same ‘neurologically normal’ individual.

Mosaicism is a term used to mean that different cells in a given individual have different genomes. This is certainly true in everyone’s immune system, but we’re talking brain here.

Is there other evidence for mosaicism in the brain? Yes. Here it is

[ Science vol. 345 pp. 1438 – 1439 ’14 ] 8/158 kids with brain malformations with no genetic cause (as found by previous techniques) had disease causing mutations in only a fraction of their cells (hopefully not brain cells produced by biopsy). Some mosaicism is obvious — the cafe au lait spots of McCune Albright syndrome for example. DNA sequencing takes the average of multiple reads (of the DNA from multiple cells?). Mutations foudn in only a few reads are interpreted as part of the machine’s inherent error rate. The trick was to use sequencing of candidate gene regions to a depth of 300 (rather than the usual 50 – 60).

It is possible that some genetically ‘normal’ parents who have abnormal kids are mosaics for the genetic abnormality.

[ Science vol. 342 pp. 564 – 565, 632 -637 ’13 ] Our genomes aren’t perfect. Each human genome contains 120 protein gene inactivating variants, with 20/120 being inactivated in both copies.

The blood of ‘many’ individuals becomes increasingly clonal with age, and the expanded clones often contain large deletions and duplications, a risk factor for cancer.

Some cases of hemimegalencephaly are due to somatic mutations in AKT3.

30% of skin fibroblasts ‘may’ have somatic copy number variations in their genomes.

The genomes of 110 individual neurons from the frontal cortex of 3 people were sequenced. 45/110 of the neurons had copy number variations (CNVs) — ranging in size from 3 megaBases to a whole chromosome. 15% of the neurons accounted for 73% of of the CNVs. However, 59% of neurons showed no CNVs, while 25% showed only 1 or 2.