That’s really super, supergirl (part 1)

28 11 2018

“What’s so bad about superbabies?”

It’s a question I ask my 300-level bioethics students after we’ve spent 12 or so weeks discussing eugenics, genetics, stem cell technology, somatic cell nuclear transfer, and the varieties of assisted reproductive technologies. They’ve also written their first set of research papers—some of which take up the question of just how humans could be enhanced (“should” is taken up in the second paper).

Most of them tend to recoil from the question, even if they can’t exactly say why, while a few take a Hell, yeah! Why not? approach to enhancement.

And, oh yes, we spend a fair amount of time trying to figure out just what counts as “enhancement.” Therapy or treatment, I note, is generally accepted by bioethicists as Good, but enhancement is looked upon with considerably more suspicion. Normal is good, well is good; better than well? Ehhhh.

Which is a long way of saying that there are a lot of us who are primed to react to the news that germline genetic engineering has not only been attempted, but accomplished, in human beings for the first time.

Germline genetic engineering (or transfer) is distinguished from somatic-cell gene transfer in that any genetic changes will be passed down to offspring—which is to say, the germ cells (sperm & eggs) are themselves changed. The most efficient way to achieve this is to alter the cells of an early embryo (a 2-3 day-old morula or 4-6 day-old blastocyst): at this stage the cells are undifferentiated, so if you are able to insert altered (“recombinant”) genetic material (rDNA or rRNA) into all of these cells, as they divide and specialize they will carry the altered material into every cell—including, of course, the germ cells.

One of the great challenges in gene transfer of whatever sort has been getting rDNA/rRNA into enough cells to affect function; that the morula and blastocyst have so few cells bypasses this problem. Another issue has been inserting rDNA/rRNA into the correct place in the genome—and only that one place in the genome—to affect genetic expression. (There are other issues, especially regarding the vectors , or delivery vehicles for recombined sequences, but I’ll skip over these for now.)

Okay, so you’ve heard of CRISPR, yes? Clustered regularly interspersed short palindromic repeats? Why, of course, Terri; who hasn’t filled their days wondering about clustered regularly interspersed short palindromic repeats? Skipping (almost) alllllll of the technical stuff, CRISPR is a huge advance over other forms of gene transfer and gene editing: by using a guide enzyme, eg, Cas9, researchers are able precisely to target a specific sequence. . . .

Well, shit, I’m losing you, aren’t I? I start with superbabies and now I’m talking about “targeting specific sequences.” So, some basics:

Almost all of our cells (exc: red blood cells) contain a nucleus, which in turn contains chromosomes, which are histones (a protein) wrapped in a very tight spool of DNA.

DNA comprises a sugar-phosphate double-helix connected by the nucleotide base-pairs (bp) adenine (A), which is always paired with thymine (T), and guanine (G), which is always paired with cytosine (C). Determining the exact order of these base pairs on a strand of DNA is “sequencing”; a sequence will look like this: ATTCCAAGGGGTAACAATTCGACCTGAT. . . .

A strand of DNA is about 6 feet long and is mostly noncoding: it has no direct role in protein synthesis. (I should note that the non/functionality of this noncoding or junk DNA is disputed, which is neither here nor there for this discussion but thought I should mention it anyway. If you don’t understand what this means, don’t sweat it.)

A very small portion of the DNA, less than 2%, comprises genes, or coding sections of DNA. Genes are the Action Jacksons of DNA, the functional units of heredity, involved in protein synthesis, and, unlike in the noncoding sections, if something is messed up with your genes it can affect how you function. The average gene size anywhere from 10-27K bp (yes, disputed), with the largest, dystrophin, over 2.3M bp.

So: DNA is the set, genes are the (very small) subset. Most members of our species have about 19,000 genes scattered amongst 3.2 billion base-pairs and across 46 chromosomes. (Some of us have more chromosomes, some have fewer, but most of us have 46.) Some chromosomes have thousands of genes, some (well, the Y chromosome) has fewer than 100.

This sounds like I’m going off track, but, really, there’s a point behind all of this.

I like to compare DNA to a road, genes to cities and towns, and chromosomes to different regions of the US: some chromosomes are like the northeast, packed with many genes, and others (the Y chromosome) are like South Dakota, with those city-genes few and far between.

To change gene NYC, say, you need to be able to get a package of rDNA or rRNA to the right place on the sequence, and then (to continue the analogy), you have to get that package not just to the right city (gene), but to the right borough, to the right street, the right address, the right apartment, and to the right room in that apartment.

This is “targeting.”

The other issue is to get the right package to the right city in the right borough, etc., in a sufficient number of cells, with “sufficient” often meaning “millions and millions.” (Again, with an embryo, this would appear to be less of an issue.) Oh, and once the package is delivered, it needs to be opened and turned on, i.e., integrated into the gene and made functional.

So, targeting, sufficiency, and functionality have all been obstacles to successful gene transfer. In fact, only one gene transfer product, involving immunotherapy, has been approved for use in the US, and that only happened last year.

Now CRISPR, CRISPR is highly efficient at targeting and integration, and is often referred to as a “cut-and-paste method” of gene editing: it uses an enzyme to guide the package to the targeted sequence, snip out the old sequence and insert the new. Another analogy: think of the “find and replace” function in a word processing document. You type the old word (say, “Chris”) into find, the new word (“Kris”) into replace, and hit go.

Much, much MUCH more efficient than manually looking for “Chris” and replacing it with “Kris.”

Great, yeah? Wellllll, not always. One of the problems with CRISPR is off-targeted mutations. Say you only want to replace Chris with Kris on page 1, but it find-and-replaces throughout the entire 20-page document. Or maybe it replaces “chris” in words like “Christmas” or “christen” or “christallmighty!” with “Kris”.

That would be bad for your document; it would be even worse for your genes.

Also—and here’s where the analogy to find-and-replace breaks down—the cut-and-paste doesn’t work every time. CRISPR efficiency is much higher than that for other methods, but it’s not 100 percent. This means that some cells will take up the changes and other cells won’t, resulting in mosaicism, or an organism which contains more than one genome.

Mosaicism is also a natural phenomenon, and not necessarily a dangerous one, but it could at the very least affect the efficacy of the gene transfer and the function (aka health) of the organism.

Okay, that’s enough for tonight. Next: whatsamatta with what He did?

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2 responses

29 11 2018
dmf

indeed, even when it goes as planned may not have the desired results and could have unintended consequences as we haven’t totally nailed down systemic factors, are you going to:

30 11 2018
dmf

this host is a twit but he gets some good guests
http://radioopensource.org/a-splice-of-life/

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