The discovery of clock genes in the fruit fly and the bread mold had been known for almost twenty years when Christopher discovered the unusual hamster.5 The excitement of the team about their new discovery was, therefore, not surprising. Because hamster clocks are usually very accurate and reproducible with a period close to twenty-four hours in constant darkness (hamsters very rarely live days shorter than 23.5 hours), the abnormal rhythm of hamster #31M18, which they called the tau hamster, suggested a mutation in a clock gene.6 When genes are involved in a biological function, we can apply classical genetics to help us understand more about the underlying genes. This always involves breeding, which is why the scientists in our story decided to do breed the tau hamster and its offspring.
The outcome of these experiments proved that the fast, free-running rhythm had a genetic basis. The combination of offspring suggested that hamster #31M18 carried only one gene with a semi-dominant mutation while the other gene appeared to be “normal.”7 If the newcomer carried one normal version of the gene (N) and one mutated version (M), and if his mate carried two normal copies of the gene (NN), the combination in their offspring was predictable. Since each of them can only inherit either M or N from their father and only N from their mother, the chances of inheriting an MN or NN are equal. Half of the offspring in the first generation should show the same period in their wheel-running activity as their “normal” mother and the other half should inherit the fast clock from their father. This was in fact the outcome of the first round of breeding experiments with hamster #31M18.
In the second generation, however, the dice are thrown again. This time it is possible to mate two MN animals—hamster #31M18’s offspring. So after several rounds of breeding, the team took two of the offspring with the fast, twenty-two-hour free-running rhythms of their father. Again the outcome is predictable. M and N now have equal chances, so that 25 percent of the next generation should have NN (the normal clock of their grandmother), and 50 percent should have MN or NM, showing the fast clock of their grandfather. The remaining 25 percent, however, should carry MM, and if one mutated gene could shorten the period of the rhythm by two hours, animals carrying two of these genes may have an even shorter period, which turned out to be true. The homozygous animals live days as short as twenty hours.8
Twelve years later, proof was provided that Martin Ralph, alias Christopher, had discovered the first known mammalian clock mutant. The fast clock of hamster #31M18 was due to a mutation in a single gene that was crucial for the clock to tick at its normal rate. Experiments with this mutant also underlined the important role of the suprachiasmatic nucleus or SCN in the mammalian body clock. The transplantation experiments described in the last chapter showed that the SCN was able to rescue the rhythmicity in arrhythmic animals. Martin Ralph performed similar transplantation experiments with the tau-mutant and the normal hamster. The recipients of an SCN removed from a tau-mutant hamster showed a fast activity rhythm in spite of the fact that their genome should have produced a normal activity rhythm. The opposite was also true: mutant hamsters lived normal circa twenty-four-hour days if they received an implant from a normal hamster (supporting the role of the SCN as the master clock in mammals).
Thus, the clock mechanism involves dedicated genes that determine the clock’s qualities, such as its free-running period. But the tau-mutations apparently also changed the relationship between internal (body clock) time and external (light–dark cycle) time. Under constant conditions, the shorter their period, the earlier the mutant hamsters rose to start their day.
8
Dawn at the Gym
It was 5 A.M. when the family entered their 24/7 workout facility in Utah. A gray day with a slight drizzle had just begun. The large, cheerful group was quite a sight. There was great-grandmother Sarah; her daughter and son, Alicia and Frederic; Alicia’s son and daughter, Phillip and Rebecca; Rebecca’s two teenage girls, Julia and Anna; and Frederic’s two youngsters, Peter and Jessica. It was almost the entire tribe. Only Sarah’s two sisters, Isabelle and Judith, were missing. They had arisen with the workout group but had stayed at home, responsible for the traditional family breakfast later on, which would last for hours. Alicia’s, Frederic’s, Rebecca’s, and Judith’s spouses and Judith’s children were still sleeping soundly—the “early shift” had left the house on tiptoe to avoid disturbing the “late shift” of the family.
Sarah was in seventh heaven. During the summer holidays the extended family always came together, and she thrived in the presence of her children, grandchildren, and great-grandchildren. She had had her first child, Alicia, at the age of nineteen and was only twenty-one when Frederic arrived. Early marriages ran in her family. When Alicia made her a grandmother she was only thirty-nine, and her first great-granddaughter was born shortly before her fifty-eighth birthday. Now, at seventy-five, she didn’t exercise with the rest of her family but refused to stay home when they all went on their traditional dawn gym outing. She sat on the long bench on one of the sides of the large workout room watching the various activities of her offspring. Alicia and Frederic, her children, were chattily jogging next to each other, occupying two of a long row of treadmills. Alicia was still in great shape at fifty-six and liked to gently mock her younger brother, who puffed quite a bit on his running machine. He could do with some exercise because he was already showing quite a belly. Granddaughter Rebecca was using the rowing gear without exhausting herself too much while watching her teenage girls, who spent more time giggling than hitting the punching balls at the other end of the gym. Rebecca had always taken life easy, unlike her brother Phillip, who was incessantly hard on himself. “If he hadn’t such ridiculously high standards, he might be married by now,” Sarah thought. Being single at the age of thirty-three came close to skipping a generation in her family—his much younger cousins already had had steady partners for some time now, and Sarah hopefully anticipated the arrival of more great-grandchildren.
By seven o’clock, they were all back in the large kitchen sitting down to the breakfast the great-great aunts had meanwhile prepared. Julia and Anna were still lively. They were probably making fun of their uncle Phillip again—it seemed to be their favorite pastime. “Quiet down a bit you two, daddy and the others are still sleeping,” Rebecca admonished them without much of an effect. Alicia’s husband, Terry, was the first of the “late shift” to arrive at around eight when they were already sitting in front of their empty breakfast plates. He was served coffee by his mother-in-law. Within the next hour, all members of the big family were sitting around the extensive kitchen table, happily planning the rest of the day.
The sequence of appearances or rather disappearances was reversed in the evening. After their usual six o’clock dinner, the two ends of the generation scale played games in the drawing room while the rest were reading or engaging in discussions on the porch. Sarah and her sisters were the first to retire at around 9 P.M., about an hour later than they normally did when the big house wasn’t filled with family. No matter how late they went to bed, they would still wake up around 4 A.M. without being able to go back to sleep. Her family had always been on the early side, as she remembered, right back to her grandparents. Before Sarah went to bed, she took the medication for her blood pressure problems that she had forgotten about earlier when she was concentrating on her card game. “Take it an hour before bedtime” were the doctor’s orders. She wondered whether he meant her usual bedtime or the actual time she went to bed.
Not much later, the other members of the “early shift” said good night one by one. Great-great aunt Judith and her son (his brother had died some years back) as well as her two grandchildren were the last of the “early shift” to hit the hay. The remaining spouses were left behind, smiling to each other about the strange family they had married into and enjoying their evening for another couple of hours in peace and quiet. Judith’s husband, James, and Frederic’s wife, Mildred, were the last to go up. They had spent the evening exchanging anecdotes abo
ut the early shift. James insisted that his part of the family was not quite as bad as the others. On her way up, Mildred made herself some chamomile tea. While she was waiting for the kettle to boil, her tired gaze was caught by a drawing, which Sarah had recently framed and hung on the kitchen wall. She looked at the various circles and squares, smiled, and thought that James was probably right.
You may have recognized that the “early shift” of the Utah family has some relationship with hamster #31M18 and its offspring in the previous chapter. The mutant hamsters with their fast clock started to run in their wheels in the afternoon—four hours earlier than their “normal” litter mates. The visit to the gym is somehow comparable to wheel running; the “early shift” in Sarah’s family starts its day before sunrise, much earlier than other humans. Thus, to be earlier or later than the other members of one’s species is independent of being night-active, like hamsters, or day-active, like humans. The “early shift” in Sarah’s family suffers from what is technically called Advanced Sleep Phase Syndrome and—as in the case of the tau-mutant hamster—the affected individuals of the Utah family have a gene that carries a mutation. It is not the same gene that is responsible for the short days of hamster #31M18 and his offspring, but the functions of these two genes are closely linked. Metaphorically, the hamster mutation produces a blunt screwdriver that cannot turn the screw efficiently whereas the mutation in Sarah’s family makes a blunt screw-head so that the screw cannot be turned efficiently, even by an intact screwdriver. In the tau-hamster, the affected gene encodes an enzyme that modifies other proteins, among them an important clock protein.1 With its mutation, the enzyme doesn’t do its job as efficiently as it normally does. The gene that carries a mutation in Sarah’s family, on the other hand, encodes the important clock protein so that it cannot be modified as efficiently.2 Thus, although the mutations affect different genes, they end up having the same effect.
I have gone into the molecular and genetic mechanisms of the body clock in some detail, although they may not be essential for understanding the phenomenon of internal time. I’ve done so because, in my experience, some people still think the body clock is something esoteric rather than a profoundly biological function. When I joined the medical faculty in Munich many years ago, I followed the custom of introducing myself to my colleagues. Several of them were directors of large clinical departments. Although all were extremely friendly, some of them didn’t hold back their opinion about the circadian clock. One of them even said, “My dear colleague, this is all fascinating, but surely the biological clock is only an issue for very sensitive people.” Yet the biological details, right down to the molecular and genetic levels, prove how much biology is behind our internal timing system and how well our field already understands its functioning.
The last chapter demonstrated that one can find dedicated genes which are essential for the body clock to tick properly and that their function within the clock appears to be the same (conserved) across very different species—from insects to humans. Some of the molecular geneticists who were instrumental in discovering the first clock genes were naively optimistic, hoping they had “cracked the clock” when the first understanding evolved of how a very limited set of genes and their protein products come together to generate an internal day—and so were the media.3 The concept of the molecular clock was ingenious and simple. At the Gordon Research Conference for Chronobiology in Irsee, Germany, in 1991, Michael Rosbash, who presented the results of his student, Paul Hardin, based on experiments in the fruit fly, opened his talk with the following joke, which I relate from memory.
A world-famous physicist traveled the country to give public speeches about his groundbreaking work. One evening, on the way from a city where he had just finished his presentation to the next town where he would be giving the same speech all over again, he remarked to his chauffeur how sick he was of telling his story for the umpteenth time. The chauffeur looked at him in the rearview mirror and smiled gently. “I can understand your frustration, Sir, because meanwhile even I know your speech by heart.” “Does that mean that you could actually GIVE my speech?” replied the professor, and the chauffeur simply said, “I believe so, Sir.” “Then why don’t we switch roles tomorrow evening to give both of us a break—it’s a small town and nobody knows my face.” So the next evening, the chauffeur, dressed in the professor’s suit, climbed onto the stage while the professor, wearing the chauffeur’s uniform, took a seat in the last row. The fake professor did a superb job. He really knew every word of the speech by heart and even produced the little jokes with perfect timing. He also mastered all the questions in the discussion that followed as if he had done the famous experiments himself. At the end of the discussion, however, a physics professor from the local college asked a difficult question, one that had never been asked before, so the chauffeur couldn’t know the answer. He paused for a fraction of a second and then replied with a broad smile. “The answer to this question is so simple that it can even be given by my chauffeur,” he said, pointing with a grin to the person in a chauffeur’s uniform sitting in the last row.
At the climax of his introductory joke, Michael Rosbash pointed at Paul Hardin, who was sitting in the back of the room, as if Michael was the chauffeur and Paul the professor. He then proceeded to explain their hypothesis of how molecules generate a circa twenty-four-hour rhythm. The hypothesis describes a simple negative feedback loop. Think of a chain-production process that produces the most delicious pralines as its final product. Worker number 1 is a very self-centered and vain person who is extremely proud to be one of the most important people in the production process. He is responsible for getting the secret recipe out of a safe. He then copies it onto a piece of paper, which he hands through a small opening in the wall of his room to another worker. Worker number 2 then produces the heart of the praline and diligently throws away the piece of paper so that the recipe cannot be stolen. The unfinished praline is now handed to worker number 3, who is responsible for its icing and other delicious decorations. Once a considerable number of the delicious pralines have been made, they are handed back to worker number 1 for inspection. However, as soon as our self-centered, vain recipe-copier sets eyes on the first final product, he stops everything and merely looks with pride at the praline. No more copies of the recipe are handed through the little hole in the wall. No more pralines are produced and refined with icing and decoration. Finally, a fourth worker collects the finished pralines to be shipped off to customers.
In this scenario, the production line would stop if worker number 4 were not to collect the finished products. But as soon as he has removed the last praline, worker number 1 wakes up from his spell and continues to copy the recipe on little pieces of paper, so that the production is reinitiated. As a consequence, the amount of pralines being produced oscillates in an endless rhythm.
So much for the metaphor—now to the scientific explanation of how cells produce circadian rhythms with the help of genes and proteins.4 In the nucleus, the DNA sequence of a clock gene is transcribed to mRNA; the resulting message is exported from the nucleus, translated into a clock protein, and is then modified.5 This clock protein is itself part of the molecular machinery that controls the transcription of its “own” gene. When enough clock proteins have been made, they are imported back into the nucleus, where they start to inhibit the transcription of their own mRNA. Once this inhibition is strong enough, no more mRNA molecules are transcribed, and the existing ones are gradually destroyed.6 As a consequence, no more proteins can be produced and the existing ones will also gradually be destroyed. When they are all gone, the transcriptional machinery is not suppressed anymore, and a new cycle can begin.
This negative feedback loop hypothesis was created at a time when only one clock gene was known in the fruit fly. A single gene and its protein were presumed to create the internal day simply by acting on each other in a negative feedback loop. The authors, however, predicted in their first paper that more players we
re bound to be found. Shortly after the birth of the negative feedback hypothesis, another important clock gene was identified in the fruit fly, and now we know more than twenty genes to be involved in building the body clock of animals: the picture of the molecular circadian machinery has become rather complex. The idea of negative feedback loops is still the basis of the current hypotheses that try to explain how cells generate daily rhythms, but many components are thought to form a network of feedback loops. Despite this complexity, the important take-home message is that daily rhythms are generated by a molecular mechanism that could potentially work in a single cell, for example a single neuron of the SCN.
The fact that many genes are involved in the biological clock predicts that many things can go wrong by mutation. There can, therefore, be many causes for somebody’s body clock ticking differently from other individuals, making him or her much earlier or much later than the rest of the population—far beyond the two genes we have encountered in the tau-hamster and in Sarah’s family. The picture Mildred looked at while waiting for the water to boil is similar to a family tree made by the scientists who investigated the phenomenon of the “early shift” and its genetic basis.7
In this family tree, you may notice the dotted square surrounding the descendants of Judith and James. Great-great aunt Judith and her descendants are also early birds, but not quite as early as Isabelle or Sarah, or Sarah’s children and grandchildren. Although the geneticists also identified some members of Judith’s branch of the family as early birds, these family members did not carry the mutations found in the others. The family tree that Sarah was given by the geneticists shows only part of the even bigger family tree, of which many branches show that not every individual in the family inherits the life-pattern of a lark from his or her respective parent—genetics is hardly ever a simple yes-or-no story.
Internal Time: Chronotypes, Social Jet Lag, and Why You’re So Tired Page 7
