Frameshift

by Robert J. Sawyer

  “And if you’ve got IT15, you’ve got Huntington’s?”

  “It’s not as simple as that. Everybody has IT15. Like all genes, IT15’s job is to code for the synthesis of a protein molecule. The protein IT15 makes has recently been dubbed ‘huntingtin.’”

  “So if everyone has IT15,” said Molly, “and everybody’s body produces huntingtin, then what determines whether you have Huntington’s disease?”

  “People with Huntington’s have a mutant form of IT15, which causes them to produce too much huntingtin. Huntingtin is crucial to organizing the nervous system in the first few weeks of an embryo’s development. It should cease to be produced at a certain point, but in those with Huntington’s disease it isn’t, and that causes damage to the developing brain. In both the normal and mutant versions of IT15, there’s a run of repeating nucleotide triplets: cytosine-adenine-guanine, or CAG, over and over again. Well, in the genetic code, each nucleotide triplet specifies the production of one specific amino acid, and amino acids are the building blocks of proteins. CAG happens to be one of the codes for making an amino acid called glutamine. In healthy individuals, IT15 contains between eleven repeats and thirty-eight repeats of this CAG triplet. But those who have Huntington’s disease have between forty-two and a hundred or so CAG repeats.”

  “Okay,” said Molly, “so we look at each of your chromosome fours, find the beginning of the run of CAG triplets, then simply count the number of repeats of that triplet. Right?”

  “Right.”

  “You’re sure you want to go through with this?”

  Pierre nodded. “I’m sure.”

  “Then let’s do it.”

  And they did. It was painstaking work, carefully examining the autoradiograph film. Faint lines represented each nucleotide. Pierre used a felt-tipped marker to write in the letters beneath each triplet: CAG, CAG. Molly, meanwhile, tallied the number of repeats on a sheet of paper.

  Without blood samples from Élisabeth Tardivel and Henry Spade, there was no easy way to tell which of his chromosome fours had come from his father, so he had to check them both. On the first one, the string of CAG triplets ended after seventeen repeats.

  Pierre breathed a sigh of relief. “One down, one to go,” he said.

  He began checking the sequence on the second chromosome. No reaction when they reached the tally of eleven; that was the normal minimum. When they got to twenty-five, though, Pierre found his hand shaking.

  Molly touched his arm. “Don’t worry,” she said. “You said you could have as many as thirty-eight and still be normal.”

  Pierre nodded. “But what I didn’t say was that seventy percent of all normal people have twenty-four or fewer repeats.”

  Molly bit her lower lip.

  Pierre continued sequencing. Twenty-six, twenty-seven, twenty-eight.

  His eyes were blurring.

  Thirty-five. Thirty-six. Thirty-seven. Thirty-eight.

  Damn. Goddamn.

  Thirty-nine.

  God fucking damn it.

  “Still,” said Molly, trying to sound brave, “thirty-eight may be the normal limit, but you have to have at least forty-two…”

  Forty.

  Forty-one.

  Forty-two.

  “I’m sorry, honey,” said Molly. “I’m so sorry.”

  Pierre put down his marker. His whole body was shaking.

  “God, I am so sorry,” said Molly.

  A fifty-fifty shot.

  A flip of a coin.

  Heads or tails.

  Call it!

  Pierre said nothing. His heart was pounding.

  “Let’s go home,” said Molly, stroking the back of his hand.

  “No,” said Pierre. “Not yet.”

  “There’s nothing more to be done here.”

  “Yes, there is. I want to finish the sequencing. I want to know how many repeats I have.”

  “What difference does that make?”

  “It makes all the difference,” said Pierre, his voice shaking. “It makes all the difference in the world.”

  Molly looked perplexed.

  “I didn’t tell you everything. Merde. Merde. Merde. I didn’t tell you everything.”

  “What?”

  “There’s an inverse correlation between the number of repeats and the age of onset of the disease.”

  Molly didn’t seem to understand, or didn’t want to. “What?” she said again.

  “The more repeats, the sooner symptoms are likely to appear. Some patients get Huntington’s as children; others don’t get it until their eighties. I—I have to finish the sequencing; I have to know how many repeats I’ve got.”

  Molly looked at him. There was nothing to say.

  Pierre rubbed his eyes, blew his nose, and bent back to the autorad film. The tally kept growing. Forty-five.

  Fifty.

  Fifty-five.

  Sixty.

  Time continued to pass. Pierre felt faint, but he pressed on, marking letters over and over again on the film: CAG, CAG, CAG…

  Molly got up and walked across the room. She found a box of Kimwipes—expensive, lab-quality tissues. She used them to dry her eyes. She tried to hide from Pierre the fact that she was crying.

  Finally, Pierre hit a codon that wasn’t CAG. The total count: seventy-nine repeats.

  There was silence between them for a time. Somewhere in the distance, a fire-truck siren was wailing.

  “How long?” asked Molly at last.

  “Seventy-nine is a very high number,” said Pierre softly. “Very high.” He sucked in air, thinking. “I’m thirty-two now. The correlation is inexact. I can’t be sure. But…I don’t know, I guess I’d expect to see symptoms very soon. Certainly by the time I’m thirty-five or thirty-six.”

  “Well, then, you—”

  “At the outside.” He raised a hand. “The disease can take years or decades to run its course. First symptoms might just be a reduction in coordination, or facial tics. It might be years before things got serious. Or…”

  “Or?”

  Pierre shrugged. “Well,” he said, his voice full of sadness, “I guess that’s it.”

  Molly reached for his hand, but Pierre pulled it away. “Please,” he said. “It’s over.”

  “What’s over?” said Molly.

  “Please. Let’s not make this difficult.”

  “I love you,” said Molly softly.

  “Please don’t…”

  “And I know you love me.”

  “Molly, I’m dying.”

  Molly moved over to him, draped her arms around his neck, and rested her head against his chest. His thoughts were all in French.

  “I still want to marry you,” Molly said.

  “Molly, I only want what’s best for you. I don’t want to be a burden on you.”

  Molly held him tighter. “I want to marry you, and I want to have a child.”

  “No,” said Pierre. “No, I can’t become a father. The number of CAG repeats tends to increase from generation to generation—it’s a phenomenon called ‘anticipation.’ I have seventy-nine; any child of mine who got the gene from me might very likely have even more—meaning he or she might come down with the disease as a teenager, or even earlier.”

  “But—”

  “No buts. I’m sorry; this was crazy. It can never work.” He saw her face, saw the hurt, felt his own heart breaking. “Please, don’t make it harder for both of us. Just go home, would you? It’s over.”

  “Pierre—”

  “It’s over. I’ve wasted too much time on this already.”

  He could see that the words had cut her. She headed for the laboratory door, but looked back at him once more. He didn’t meet her eyes.

  She left the room. Pierre sat down on a lab stool, his hands still shaking.

  C h a p t e r

  14

  Pierre called Tiffany Feng and told her to go ahead and put in his health-insurance application at the first of the year. Condor might have disputed t
he informal testing if the result had been negative, but there was no conceivable advantage to lying about having Huntington’s. Tiffany said Pierre’s statement on Human Genome Center letterhead, notarized by the campus archivist, would be acceptable proof that the test had indeed been conducted.

  Pierre went back to spending his evenings in Doe Library. Periodically he’d look up, look around, look for a familiar face.

  She never appeared.

  He spent each evening reading, searching the literature for information on junk DNA—now, more than ever, he knew he was in a race against time. He was already seven years older than James D. Watson had been when he’d made his great breakthrough—and only two years younger than Watson had been when he’d accepted his Nobel Prize.

  A wall clock above Pierre’s chair was ticking audibly. He got up and moved to another table.

  He’d started with current material and was working his way backward. A reference in a magazine index caught his eye. “A Different Kind of Inheritance.”

  Different kind of inheritance…

  Could it be?

  He asked Pablo to dig up the June 1989 Scientific American.

  There it was—exactly what he’d been looking for. A whole different level of information potentially coded into DNA, and a plausible scheme for the reliable inheritance of that information from generation to generation.

  The genetic code consisted of four letters: A, C, G, and T. The C stood for cytosine, and cytosine’s chemical formula was C4H5N3O—four carbons, five hydrogens, three nitrogens, and an oxygen.

  But not all cytosine was the same. It had long been known that sometimes one of those five hydrogens could be replaced by a methyl group, CH3—a carbon atom attached to three hydrogens. The process was called, logically enough, cytosine methylation.

  So when one wrote out a genetic formula—say, the CAG that repeated on and on in Pierre’s own diseased genes—the C might be either regular cytosine or the methylated form, called 5-methylcytosine. Geneticists paid no attention to which one it was; both forms resulted in exactly the same proteins being synthesized.

  But this article in Scientific American, by Robin Holliday, described an intriguing finding: almost always when cytosine undergoes methylation, the base next to the cytosine on the DNA strand is guanine: a CG doublet.

  But C and G side by side on one side of a DNA strand meant that G and C would be found on the opposite side. After all, cytosine always bonds with guanine, and guanine with cytosine.

  In the article, Holliday proposed a hypothetical enzyme he dubbed “maintenance methylase.” It would bind a methyl group to a cytosine that was adjacent to a guanine if and only if the corresponding doublet on the other side was already methylated.

  It was all hypothetical. Maintenance methylase might not exist.

  But if it did—

  Pierre looked at his watch; it was almost closing time. He photocopied the article, returned the magazine to Pablo, and went home.

  That night he dreamed of Stockholm.

  “Good morning, Shari,” said Pierre, coming into the lab.

  Shari was dressed in a beige blouse under a wine-colored two-piece suit. She’d cut her long, dark hair recently and was now wearing it fashionably short, parted on the left, and curving in toward her neck at the bottom. Like Pierre, Shari was burying herself in her work, trying to get over the loss of Howard.

  “What’s this?” she said, holding up an autorad she’d found while tidying up. The lab would have been a pigsty if it weren’t for Shari’s periodic attempts to restore order.

  Pierre glanced at the piece of X-ray film. He tried to sound nonchalant. “Nothing. Just garbage.”

  “Whoever this DNA belongs to has Huntington’s disease,” said Shari matter-of-factly.

  “It’s just an old sheet.”

  “It’s yours, isn’t it?” asked Shari.

  Pierre thought about continuing to lie, but then shrugged. “I thought I’d thrown it out.”

  “I’m sorry, Pierre. I’m so sorry.”

  “Don’t tell anyone.”

  “No, of course not. How long have you known?”

  “Few weeks.”

  “How is Molly taking it?”

  “We—we’ve broken up.”

  Shari put the film in a Rubbermaid garbage pail. “Oh.”

  Pierre shrugged a little.

  They looked at each other for a moment. Pierre’s mind did what he supposed every male’s did in moments like these. He thought for an instant about him and Shari, about the possibilities there. Both of them carried diseased genes. He was thirty-two and she was twenty-six—not an outrageous difference. But—but there were other gulfs between them. And he saw on her face no indication, no suggestion, no inkling. The thought had not occurred to her.

  Some gulfs are not easily crossed.

  “Let’s not talk about it,” said Pierre. “I—I’ve got some research I want to share with you. Something I found in the library last night.”

  Shari looked as though she wanted to pursue the subject of Pierre’s Huntington’s further, but then she nodded and took a seat on a lab stool.

  Pierre told her about the article in Scientific American; about the two forms of cytosine, the regular one and the 5-methylcytosine variant; and about the hypothetical enzyme that could turn the former into the latter but would do so only if the cytosine in the CG doublet on the opposite side of the strand was already methylated.

  “Hypothetically,” said Shari, stressing the word. “If this enzyme exists.”

  “Right, right,” said Pierre. “But suppose it does. What happens when DNA reproduces? Well, of course, the ladder unzips down the middle, forming two strands. One strand contains all the left-hand components of the base pairs, maybe something like this…” He wrote on the blackboard that covered most of one wall:

  Left side: T-C-A-C-G-T

  “See that CG doublet? Okay, let’s say its cytosine is methylated.” He went over the pair again with his chalk, making it heavier:

  Left side: T-C-A-C-G-T

  “Now, in DNA reproduction, free-floating nucleotides are plugged into the appropriate spots on each strand, meaning the right-hand side of this one will end up looking like this…” His chalk flew across the blackboard, writing in the complementary sequence:

  Left side:

  T-C-A-C-G-T

  Right side:

  A-G-T-G-C-A

  “See? Directly opposite the left-hand CG pair is the right-hand GC pair.” He paused, waiting for Shari to nod acknowledgment of this. “Now the maintenance methylase comes along and sees that there isn’t parity between the two sides of the strand, so it adds a methyl group to the right-hand side.” He went over the GC pair, making it darker, too:

  Left side:

  T-C-A-C-G-T

  Right side:

  A-G-T-G-C-A

  “At the same time, the other half of the original strand is being filled in with free-floating nucleotides. But maintenance methylase would do exactly the same thing to it, duplicating cytosine methylation on both sides, if originally present on one side.”

  Pierre clapped his hands together to shake off chalk dust. “Voilà! By postulating that one enzyme, you end up with a mechanism for preserving cytosine-methylation state from cell generation to cell generation.”

  “And?”

  “And think about our work on codon synonyms.” He waved vaguely at the wall chart labeled “The Genetic Code.”

  “Yes?”

  “That’s one possible additional level of coding hidden in DNA, if the choice of which synonym used is significant. Now we’ve got a second possible type of additional coding in DNA: the code made by whether cytosine is methylated or not. I’m willing to bet that one or both of those additional codes is the key to what the so-called junk DNA is really for.”

  “So what do we do now?” asked Shari.

  “Well, as Einstein is supposed to have said, ‘God is subtle, but malicious he is not.’” He smiled at Shari.
“No matter how complex the codes are, we should be able to crack them.”

  Pierre went home. His apartment seemed vast. He sat on the living-room couch, pulling idly at an orange thread coming unraveled from one of the cushions.

  They were making progress, he and Shari. They were getting close to a breakthrough. Of that he felt sure.

  But he wasn’t elated. He wasn’t excited.

  God, what an idiot I am.

  He watched Letterman, watched Conan O’Brien.

  He didn’t laugh.

  He started getting ready for bed, dumping his socks and underwear on the living-room floor—there wasn’t any reason not to anymore.

  He’d been reading Camus again. His fat copy of the Collected Works was facedown on one of the couch’s orange-and-green cushions. Camus, who had taken the literature Nobel in ’57; Camus, who commented on the absurdity of the human condition. “I don’t want to be a genius,” he had said. “I have enough problems just trying to be a man.”

  Pierre sat down on the couch and exhaled into the darkness. The absurdity of the human condition. The absurdity of it all. The absurdity of being a man.

  Bertrand Russell ran through his mind, too—a Nobel laureate in 1950.

  “To fear love,” he’d said, “is to fear life—and those who fear life are already three parts dead.”

  Three parts dead—just about right for a Huntington’s sufferer at thirty-two.

  Pierre crawled into bed, lying in a fetal position.

  He slept hardly at all—but when he did, he dreamt not of Stockholm, but of Molly.

  C h a p t e r

  15

  “I can’t let you redo the exam,” said Molly to the male student sitting opposite her, “but if you undertake another research project, I can give you up to ten marks in extra credit for that. If you get eight or above, you’ll pass—just barely. It’s your choice.”

  The student was looking at his hands, which were resting in his lap. “I’ll do the project. Thank you, Professor Bond.”