In this bio/biochem passage, we focus on tumors, markers, and more. We also talked about the importance of reviewing and challenging the answers as you review.
Listen to this podcast episode with the player above, or keep reading for the highlights and takeaway points.
[32:42] General Tips'Just absorb the material in the passage. Get through what's on the page. And then I can figure out what I'm supposed to know.'Click To Tweet
When reading CARS, look for the main idea, look for comparing contrast, or cause and effect. Look for all the hallmarks of a passage that have meaning. Just try to see what they’re giving you because sometimes they give you more information than you think they’re going to. Sometimes the MCAT passage actually gives you knowledge that you should technically know already. But they’ll give it to you for free.
[04:44] Passage 3 (Questions 10 – 13)
Ovarian cancer is a highly lethal gynecologic malignancy affecting more than 22,000 women annually in the United States alone. Although most patients initially show a complete clinical response to surgery and chemotherapy, recurrence and progressive disease eventually occur in the majority of patients. The sensitivity of tumors to platinum-based chemotherapeutic agents is a major predictor of patients’ prognoses; platinum-resistant tumors are associated with poorer outcomes. This underscores the need to identify, and eventually target, pathways involved in platinum resistance. This task is complicated by the heterogeneity of cells within malignancies. The cancer stem cell (CSC) theory proposes that resistant cells encompass only a minority of cells within a cancer, but are solely responsible for long-term recurrence. Therefore, chemotherapy must target these CSCs in order to show long-term effectiveness.
Note: This is a point in the passage where you could stop and take a second to highlight some things meaningful to you. (Highlighting in bold those words/phrases in the paragraph above.)
It tells us that it’s not everyone, but recurrence and progressive disease is a thing. And then they talk about the association of platinum-resistant tumors with poor outcomes.
Always look for those in science passages. – what’s associated with what indicates what causes what. Those are keywords that have different meanings, but those are the things that can give us a lot of meaning from a passage.
Notice “cancer stem cell (CSC) theory proposes… responsible for long-term recurrence.” This tells us what the phenomenon is which is recurrent in ovarian cancer. It tells more specifically, what kind of cancers are really associated with that, and that’s platinum-resistant tumors.
And then it tells us what type of cell we need to target, which is the CSC or the cancer stem cell because they’re responsible for the recurrence. This can look really intimidating but reading through and going over it gives us the confidence to move on to the next paragraph.
[07:35] Paragraph 2
Aldehyde dehydrogenase (ALDH) activity has been shown to be a promising CSC marker in several types of cancers. Researchers investigated whether ALDH expression in the platinum-resistant ovarian cancer cell line A2780/CP70, was associated with aggressive tumor behavior. Cells were sorted by whether they expressed high levels of ALDH (ALDH+) or low levels of ALDH (ALDH−), and researchers measured invasiveness and response to carboplatin, a platinum-containing therapeutic.
Again, we’re highlighting in bold what may be important in this paragraph. So now we know ALDH is a CSC marker. One of the things you should consider is when you see “researchers investigated” because it usually tells you what their study was. In this case, they’re talking about ALDH expression and they’re investigating whether it was associated with aggressive tumor behavior. Another thing to note here is that they measured two things: invasiveness and response to carboplatin. And based on what we’ve read in the first paragraph, platinum resistance is associated with such poor outcomes. So that makes sense.
[09:30] Analyzing Figure 1
Figure 1 Invasiveness and platinum resistance in ALDH+ and ALDH− cells. **p<0.01.
Note: Here, we know they’re measuring invasiveness. We can also see that the ALDH+ subsets are more invasive. This is what we get from Part A is that more cells are invaded if there’s ALDH+. (If you forgot what ALDH+ is, you could always go back to the passage and see that ALDH+ would be high levels of ALDH expression.)
Now, we can move on to B. It’s looking at ALDH+ and ALDH- this time. There are two things here. One is being treated with a sham, which is the control. The other is being treated with carboplatin and now we’re seeing their response to carboplatin.
And so here, we could see there is a statistically significant difference between ALDH+ and ALDH- in both groups. CP is colonies per field so there is more proliferation with the ALDH+ in general. We also see it’s more resistant to carboplatin.
Hence, when we treat ALDH- cells with carboplatin, they go down to almost zero proliferation. But when we treat ALDH+ cells with carboplatin, there is still some proliferation. It’s less so the carboplatin is definitely doing something. But there is a resistance to carboplatin in ALDH+ cells. So that’s what we’re getting from these figures.
[11:49] Paragraph 2
In a subsequent experiment, targeted short hairpin RNA (shRNA) was used to silence ALDH expression. Invasiveness and percent survival in response to carboplatin treatment were measured in control (shControl) and ALDH-knockdown (shALDH) A2780/CP70 cells.
Figure 2 Invasiveness and platinum resistance in ALDH-knockdown cells. **p<0.01.
Note: We’re still looking in the same ovarian cancer line. For part A, we’re looking at the invasiveness. The error bars are overlapping and there are **p<0.01. So the ALDH knockdown does not affect how invasive the cells are. It will invade approximately the same number, whether or not we have that gene.
Then over on Part B, you could see what happens when you treat it with carboplatin. So again, we’re looking at that resistance here. If you look at the control, which is the darker bar, and this is always hard on the MCAT because they give you all these grayscale images. The darker bar always has a higher rate of survival than the lighter gray bar, which is the knockdown gene.
Now we see that knocking out the ALDH does reduce the carboplatin resistance. This makes logical sense earlier in the passage, we read that the more ALDH expression meant more aggressive tumors and more resistance. So taking out the ALDH gene should make the resistance go down. And that’s exactly what we saw here.
[13:36] Paragraph 3
The expression levels of genes in the ALDH-knockdown cells were analyzed using polymerase chain reaction (PCR). A significant increase was found in the expression of BAX, a pro-apoptotic gene, and a significant decrease in CDK4, which encodes a protein that promotes progression from G1 into S.
Note: This is very cause-and-effect heavy. This tells me what things do what. And so that’s something we should care about. First, we know that this analysis was done specifically in the ALDH knockdown cells.
We see an increase in BAX and pro-apoptosis. This is where your external knowledge comes in where apoptosis is programmed cell death. So a pro-apoptotic gene means it makes them die. And then we see a decrease in CDK4, which is responsible for promoting a protein that promotes progression from G1 into S. And this requires you to go back to your cell cycle knowledge. G1 is the growth phase. S is the DNA synthesis where the actual chromosomes get duplicated before cell division.
If CDK4 promotes that and we decrease CDK4, we’re probably going to have a decrease in the number of cells progressing from G1 into S. So this is all that’s happening in our ALDH knockdown cells.
[15:10] Question 10
Which of the following is most likely to be a trait of cancer stem cells?
- Ability to differentiate into any cell type in the body
- Greater reliance on aerobic glucose metabolism
- Elevated levels of BAX expression
B – They told us in the very first paragraph that the cancer stem cell theory proposes that resistance cells are solely responsible for long-term recurrence. And recurrence means that you got treated with chemo or surgery or whatever. And then it came back. And that sounds like the definition of self-renewal. So we’re leaning way more towards B because the passage plus our knowledge of cancer tells us that cells can come back. We use chemotherapy to eliminate the stem cells and then they’re able to come back and they’re solely responsible. So this is the correct answer because the passage is giving us that information that it’s just them.
A & C – The trait of cancer cells and stem cells, or cancer, in general, is that they do division like crazy. So they do it more, but they don’t necessarily do it differently. They’re not like having a different mechanism. They will still just do it and cellular respiration is eating up glucose, like the rest of us. They’re just doing it out of control. So the impulse is actually to cross this out.
Correct Answer: B
[19:33] Question 11
Which of the following conclusions is most strongly supported by the data presented in the passage?
- ALDH expression is associated with additional factors that promote invasiveness distinct from platinum resistance.
- Platinum resistance is the mechanism through which ALDH promotes invasiveness.
- ALDH expression is exclusively responsible for the platinum-resistant phenotype.
- BAX expression is promoted by ALDH.
A – The word “distinct” is the one that sticks here. So you want to look if there is a separation between invasiveness and platinum-resistance. And we see in Figure 2 that ALDH knockdown alone doesn’t actually change invasiveness. But it does change carboplatin resistance. And so that tells us that whatever those two things are, they’re both associated with ALDH. But they’re not inherently linked. So there are some other factors that are happening aside from those two things. They’re not 100% happening all the time. So we know that ALDH expression is associated with invasiveness and platinum resistance, but that they don’t always go together in a one-to-one way.
B – Figure 2 is the one where we knocked out the ALDH and we still looked at invasiveness. carboplatin resistance. But just the ALDH and platinum resistance doesn’t necessarily tell us anything about invasiveness. So B is out.
C – The use of the word “exclusively’ is too aggressive.
D – If we quickly scan down to the bottom, we know that if we knock out ALDH, there’s more about BAX expression, which seems to go against answer choice D.
Correct Answer: A
[23:59] Question 12
Which of the following would be expected as outcomes of ALDH knockdown gene therapy?
- Slower proliferation
- Reduced invasiveness
III. Increased platinum sensitivity
- II only
- I and III only
- II and III only
- I, II, and III
I – In Figure 1, look at the number of colonies per field, which is proliferation. But this also means low levels of ALDH versus high levels of ALDH. Even though it’s not a knockdown, showing that lower expression of something does lead to a difference in the number of colonies per field. And this is associated with slow proliferation. So Roman numeral I is true.
II – In Figure 2, we see that invasiveness wasn’t affected by the knockdown. So that makes us say that reduced invasiveness isn’t the thing that happens if you get ALDH knocked out on gene therapy. Your invasiveness probably stays the same. Instead, what gets affected are your carboplatin sensitivity and your proliferation. So for that reason, we’re knocking out Roman numeral number II and go with answer choice B.
When you see the colonies per field number and ALDH- which is the dark gray bar, that’s so much lower than the ALDH+ bar. Even in the Sham, where we didn’t even treat it with carboplatin, just the fact lower ALDH expression leads to fewer colonies per field. And so that tells you that it’s just proliferating.
So there’s an effect on the proliferation just by virtue of having less ALDH expression, which is the result of knockdown therapy.
This is a bit tricky because they almost wanted us to only look at Figure 2. But if we remember what the effect of a knockdown is, which is that the thing gets expressed less if it’s been knocked down. That means that Figure 1 is still appropriate for us to draw conclusions from because it’s giving us the outcome of an ADHD knockdown without calling it that.
Correct Answer: B
[29:32] Question 13
If true, which of the following findings would most directly challenge the results of this study?
- ALDH expression correlates with the expression of other determinants of tumor invasiveness.
- Targeting a different pathway, instead of ALDH, yields better improvements in platinum-sensitivity.
- No significant relationship is found between ALDH expression levels and clinical behavior in non-carboplatin-resistant ovarian cancer cell lines.
- ALDH overexpression results in no significant differences in platinum resistance compared to control cells.
A – If ALDH expression correlates with expression of other determinants of tumor invasiveness, that doesn’t change the results of what we’re studying here. So this is beyond the scope.
B – Again, this is out of scope.
C – We’re looking at carboplatin-resistant cells. That’s what this whole study is about. So that’s out of scope.
D – Looking at Figures 1 and 2, you can tell that having ALDH present means that you’re more platinum-resistant. We saw both a knockdown and a decreased expression. And so in general, having more ALDH means you have more resistance. That’s the theme we’ve seen. So if we saw ALDH more, whether it’s overexpression or expression, if we got a finding that said the presence of ALDH is not going to have a difference in platinum-resistance that like flies in the face of what we see in the tables.
So it’s understandable that the word “overexpression” can give you pause. For this one, we could probably use the process of elimination because the other ones are so much more clearly wrong.
Correct Answer: D
[38:01] Final Thoughts“When students are going over passages, it's super important to challenge.”Click To Tweet
If you don’t like the answer, think about it, write out your argument with it, and then try to find why it’s right or literally challenge it that exists. You have to engage all the answer options and look at all the explanations for all the questions when you’re reviewing the full length.
Ideally, you should be having this argument with yourself or with a friend. Study groups are great. And also talking to a cat or talking to a wall, you can explain that so much because walls and stuffed animals are great listeners.
A very important part of reviewing a full-length exam is the active aspect. This is why podcasts like this are really great where you get to watch other people do it. But when you’re on your own, you’ve got to do this part. You have to grapple with the stuff.
Especially if you don’t totally believe that you were wrong, that’s all the more reason to dig into it. Parse through the passage again, the answer options, and the explanations because that’s how you learn. That way, you’re not going to forget to read the whole answer or look at what was being compared the next time you see it.
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