I was given a challenge on Twitter, and some people dismissed me as a failure because I didn’t have the academic background to come back with a quick answer. (I also discovered that I knew the answer, but forgot the words because of post-surgical anomia. I digress.) I find that this is a problem with a lot of people with certain types of expertise. They forget what it was like back when they were first learning, and no longer have the patience to explain. I don’t think it helps that there is a shit-ton of people on the internet spouting nonsense and being taken seriously. Naturally, some of them will assume that I’m doing the same, but I really don’t want to be lumped in with them, so I’m going to show them the process I go through, and how seriously I take learning new things and separating fact from fiction.
As I said in my previous post, you guys are wicked smart, and I am very often in awe of how much you know. But one thing you’re not so good at is communicating to people outside your fields of expertise. This is why we have bad science journalism. Ask Ed Yong. However, if you want to stop all your discoveries from degenerating into misrepresentation or woo, then you need people who can translate Science into English.
I was given a long, information-dense study, Fidelity of the Methylation Pattern and Its Variation in the Genome by Malcolm M. Campbell, so it’s going to take several posts to dissect, research, learn the background information, and try to explain it in an accessible way. I fully expect to be wrong several times, and encourage people to correct me – in such a way that ordinary people can “get it.” So here goes:
The methylated or unmethylated status of a CpG site is copied faithfully from parental DNA to daughter DNA, and functions as a cellular memory. However, no information is available for the fidelity of methylation pattern in unmethylated CpG islands (CGIs) or its variation in the genome. Here, we determined the methylation status of each CpG site on each DNA molecule obtained from clonal populations of normal human mammary epithelial cells.
Methylation turns genes or pieces of genes “on” or “off”. There’s a detailed explanation of various ways it does this in the components of the whole process from DNA to cell, but it’s kind of hard to understand if you haven’t done a lot of reading beforehand. I’ll give you the link anyway.
CpG sites – the quick and dirty Wikipedia definition is this: The CpG sites or CG sites are regions of DNA where a cytosine nucleotide occurs next to a guanine nucleotide in the linear sequence of bases along its length. If you don’t remember from your Biology classes, or your biology classes never taught you, your entire DNA strand consists of combinations of four nucleotides – Cytosine, Guanine, Taurine, and Adenosine. I’m not going to get into that right now, because it’s just going to confound this with too much information, but if you think about the movie “Gattaca,” you’ll notice those four letters. In a movie about genetic engineering. Because those are the four letters you see in an illustration of a piece of DNA. The researchers were looking at the parts where the cytosine and guanine were next to each other.
Specifically, they were looking at epithelial cells from normal breast tissue. The link may be a little difficult to understand, but I think if you read all the way through, you’ll at least understand some of the reasons these cells were chosen. They have a lot of unique characteristics, and they’re pretty tough.
So the idea here is that we already know that if the cytosine and guanine pair are methylated in the on position or the off position in the DNA, that they’re going to stay that way in the cells that are produced by those instructions from the DNA. What we don’t know is that if that pair is unmethylated, will the cells made from the DNA instructions also be unmethylated? IOW, if they’re not already told to be switched on or told to be switched off, will they still be in that “neutral” position? In order to test that, they took a bunch of those epithelial cells and tested each one to see if it was methylated or unmethylated so they could get them to reproduce and see what happened.
This illustration is not specific to this piece of research, but keep reading, and you’ll see how it relates.. I wanted to give you a visual aid in case you learn better that way.
Methylation pattern error rates (MPERs) were calculated based upon the deviation from the methylation patterns that should be obtained if the cells had 100% fidelity in replicating the methylation pattern. Unmethylated CGIs in the promoter regions of five genes showed MPERs of 0.018–0.032 errors/site/21.6 generations, and the fidelity of methylation pattern was calculated as 99.85%–99.92%/site/generation. In contrast, unmethylated CGIs outside the promoter regions showed MPERs more than twice as high (P < 0.01). Methylated regions, including a CGI in theMAGE-A3 promoter and DMR of the H19 gene, showed much lower MPERs than unmethylated CGIs. These showed that errors in methylation pattern were mainly due to de novo methylations in unmethylated regions. The differential MPERs even among unmethylated CGIs indicated that a promoter-specific protection mechanism(s) from de novo methylation was present.
This explains how they figured a reasonable range of variation. The “islands” of unmethylated cytosine/guanine pairs in five genes over 21.6 generations (this is statistics, not absolute numbers. You clone enough cells, you sure as heck can get six tenths of a generation.) stayed unmethylated most of the time. This came from promoter regions, which are the areas in DNA that call the shots. It’s more likely that instructions from promoter regions are going to be followed.
The unmethylated cells that didn’t come from promoter regions showed more deviations – the cells after several generations were twice as likely to be different from the originals than the ones that came from the promoter regions. The methylated cells, which, as I mentioned, already have the specific instructions to turn a gene on or off, were more likely to maintain their integrity even if they weren’t from promoter regions. The unmethylated cells didn’t’ have that instruction, and hadn’t been told to stay unmethylated (because they weren’t from promoter regions) and so they just did whatever seemed right at the time and, well, mistakes were made.
CpG methylation is known to serve as cellular memory, and is involved in various biological processes, such as tissue-specific gene expression, genomic imprinting, and X chromosome inactivation (Jones and Takai 2001; Bird 2002; Futscher et al. 2002;Strichman-Almashanu et al. 2002). These important functions of methylations are based upon the fact that the methylated or unmethylated status of a CpG site is faithfully inherited. The methylated status of a CpG site is inherited upon DNA replication by the function of maintenance methylase, represented by DNA methyltransferase 1, which is located at replication forks and methylates hemimethylated CpG sites into fully methylated CpG sites (Leonhardt et al. 1992; Araujo et al. 1998; Hsu et al. 1999). The unmethylated status of a CpG site is inherited by not being methylated upon DNA replication or any other occasions. Unmethylated CpG sites generally cluster to form a CpG island (CGI), and most CGIs are kept unmethylated (Gardiner-Garden and Frommer 1987; Bird 2002). Methylations of CGIs in promoter regions are known to cause transcriptional silencing of their downstream genes by changing chromatin structures and blocking transcription initiation (Bird 2002;Richards and Elgin 2002). There are limited numbers of CGIs that are normally methylated (normally methylated CpG islands; NM-CGIs) (De Smet et al. 1999; Futscher et al. 2002). CpG sites outside CGIs, especially those in repetitive sequences, are also normally methylated (Bird 2002).
CpG methylation is important. It is carried on pretty faithfully when cells reproduce. It’s also important that unmethylated CpG remains unmethylated, and that’s usually passed on to new cells as well. Most of the unmethylated sites form a cluster called a CpG Island, or CGI. If these unmethylated CGIs become methylated, then it changes what genetic instructions get turned on or off in future generations of cells, if they’re in promoter regions. But it’s not always bad for CGIs to be methylated, because sometimes that’s on purpose.
I’m going to hold off on the transcription and chromatin stuff for later, because I think it’ll stick better when the paper goes into more detail.
To keep the methylation pattern, maintenance of both methylated and unmethylated statuses of CpG sites during DNA replication is necessary. However, the fidelity of the methylation pattern has been analyzed only for the maintenance of the methylated status (Wigler et al. 1981; Otto and Walbot 1990; Pfeifer et al. 1990). The fidelity in maintaining the methylated status of an exogenously introduced DNA was shown to be 94% per generation per site by Southern blot analysis (Wigler et al. 1981). The fidelity in maintaining the methylated status of a CGI in the 5′ region of the PGK1 gene, which was derived from the inactive X chromosome, was estimated to be 98.8%–99.9% per site per generation by the ligation-mediated PCR method after chemical cleavage of DNA (Pfeifer et al. 1990).
We’ve already studied methylated CpG sites and found that it’s pretty consistent. Some studies attesting to that are cited. We know that keeping them unmethylated is also important, but that hasn’t been investigated to our satisfaction.
Normally unmethylated regions might show different fidelities from normally methylated regions. Even among the unmethylated CGIs, the fidelities of their methylation pattern have been suggested to be different according to their location against a gene promoter. Methylation of CGIs in promoter regions almost always leads to transcriptional silencing while that of CGIs outside promoter regions does not (Gonzalgo et al. 1998; Jones 1999). Considering the cellular expense in maintaining methylation pattern, a cell could sacrifice the fidelity of methylation pattern for CGIs outside promoter regions. In addition, by recent genomic scanning techniques for methylation changes (Ushijima et al. 1997; Toyota et al. 1999; Costello et al. 2000; Jones and Baylin 2002), aberrant methylations of CGIs in cancers are observed in a nonrandom manner (Toyota et al. 1999; Costello et al. 2000; Kaneda et al. 2002a; Kaneda et al. 2002b). It is indicated that CGIs outside promoter regions were more frequently methylated than those in promoter regions (Nguyen et al. 2001; Takai et al. 2001; Kaneda et al. 2002a; Asada et al. 2003).
Unmethylated CGIs are more likely to change than methylated ones. Unmethylated CGIs from promoter regions of the DNA pretty consistently shut down the things they’re supposed to shut down, exactly as planned. Unmethylated CGIs from outside promoter regions of the DNA are not so good at that – they’re more likely to become methylated when they’re supposed to stay unmethylated. Some of this methylation of unmethylated CGIs has been seen in cancer. So that’s one example of why we don’t want this to happen.
Here, we analyzed the methylation status of each CpG site on each DNA molecule by the bisulfite sequencing technique (Clark et al. 1994) in six clonal populations of normal human mammary epithelial cells (HMECs), for CGIs in the promoter regions, CGIs outside the promoter regions, and CpG sites outside CGIs. By analyzing the deviation from the most common two patterns, MPERs, which reflected the fidelity in replicating both methylated and unmethylated statuses, were measured.
Like a five-paragraph essay here. Restating what they’re going to do and how they’re going to do it. Remember the illustration? Bisulfite sequencing technique. (Really detailed explanation, Wikipedia explanation).
And now my brain is very, very tired. I am going to watch “Besharam” because I’m also trying to learn Hindi, and I might as well be looking at Ranbir Kapoor while I’m doing it. Heh. I will continue this in a later post. Feedback is welcome and encouraged.