Baby teeth may one day help identify kids at risk for mental disorders later in life

The team analysed 70 primary teeth collected from 70 children enrolled in the Children of the 90s study (also known as the Avon Longitudinal Study of Parents and Children) based at the University of Bristol. Parents donated primary teeth (specifically, the pointed teeth on each side of the front of the mouth known as canines) that naturally fell out of the mouths of children aged 5 to 7.

The results of this study could one day lead to the development of a much-needed tool for identifying children who have been exposed to early-life adversity, which is a risk factor for psychological problems, allowing them to be monitored and guided towards preventive treatments, if necessary.

The origin of this study traces back several years, when senior author Erin C. Dunn, ScD, MPH, learned about work in the field of anthropology that could help solve a longstanding problem in her own research. Dunn is a social and psychiatric epidemiologist and an investigator in MGH's Psychiatric and Neurodevelopmental Genetics Unit. She studies the effects of childhood adversity, which research suggests is responsible for up to one-third of all mental health disorders. Dunn is particularly interested in the timing of these adverse events and in uncovering whether there are sensitive periods during child development when exposure to adversity is particularly harmful. Yet Dunn notes that she and other scientists lack effective tools for measuring exposure to childhood adversity. Asking people (or their parents) about painful experiences in their early years is one method, but that's vulnerable to poor recall or reluctance to share difficult memories. "That's a hindrance for this field," says Dunn.

However, Dunn was intrigued to learn that anthropologists have long studied the teeth of people from past eras to learn about their lives. "Teeth create a permanent record of different kinds of life experiences," she says. Exposure to sources of physical stress, such as poor nutrition or disease, can affect the formation of dental enamel and result in pronounced growth lines within teeth, called stress lines, which are similar to the rings in a tree that mark its age. Just as the thickness of tree growth rings can vary based on the climate surrounding the tree as it forms, tooth growth lines can also vary based on the environment and experiences a child has in utero and shortly thereafter, the time when teeth are forming. Thicker stress lines are thought to indicate more stressful life conditions.

Dunn developed a hypothesis that the width of one variety in particular, called the neonatal line (NNL), might serve as an indicator of whether an infant's mother experienced high levels of psychological stress during pregnancy (when teeth are already forming) and in the early period following birth.

To test this hypothesis, Dunn and two co-lead authors -- postdoctoral research fellow Rebecca V. Mountain, PhD, and data analyst Yiwen Zhu, MS, who were both in the Psychiatric and Neurodevelopmental Genetics Unit at the time of the study -- led a team that analysed the teeth. The width of the NNL was measured using microscopes. Mothers completed questionnaires during and shortly after pregnancy that asked about four factors that are known to affect child development: stressful events in the prenatal period, maternal history of psychological problems, neighbourhood quality (whether the poverty level was high or it was unsafe, for instance), and level of social support.

Several clear patterns emerged. Children whose mothers had lifetime histories of severe depression or other psychiatric problems, as well as mothers who experienced depression or anxiety at 32 weeks of pregnancy, were more likely than other kids to have thicker NNLs. Meanwhile, children of mothers who received significant social support shortly after pregnancy tended to have thinner NNLs. These trends remained intact after the researchers controlled for other factors that are known to influence NNL width, including iron supplementation during pregnancy, gestational age (the time between conception and birth) and maternal obesity.

No one is certain what causes the NNL to form, says Dunn, but it's possible that a mother experiencing anxiety or depression may produce more cortisol, the "stress hormone," which interferes with the cells that create enamel. Systemic inflammation is another candidate, says Dunn, who hopes to study how the NNL forms. And if the findings of this research can be replicated in a larger study, she believes that the NNL and other tooth growth marks could be used in the future to identify children who have been exposed to early life adversity. "Then we can connect those kids to interventions," says Dunn, "so we can prevent the onset of mental health disorders, and do that as early on in the lifespan as we possibly can."

Dunn is also an associate professor of Psychiatry at Harvard Medical School. Mountain is now a postdoctoral research fellow at Maine Medical Center Research Institute. Zhu is now a doctoral student at the Harvard T.H. Chan School of Public Health.

A link between childhood stress and early molars

Early in her career neuroscientist Allyson Mackey began thinking about molars. As a researcher who studies brain development, she wanted to know whether when these teeth arrived might indicate early maturation in children.

"I've long been concerned that if kids grow up too fast, their brains will mature too fast and will lose plasticity at an earlier age. Then they'll go into school and have trouble learning at the same rate as their peers," says Mackey, an assistant professor in the Department of Psychology at Penn. "Of course, not every kid who experiences stress or [is] low income will show this pattern of accelerated development."

In the Proceedings of the National Academy of Sciences, Mackey, with doctoral student Cassidy McDermott and colleagues from Penn's School of Dental Medicine and the University of Missouri-Kansas City, shows that children from lower-income backgrounds and those who go through greater adverse childhood experiences get their first permanent molars earlier. The findings, generated initially from a small study and replicated using a nationally representative dataset, align with a broader pattern of accelerated development often seen under conditions of early-life stress.

"It's really important for us to understand how to detect early maturation sooner," Mackey says. "Right now, we're relying on seeing when kids hit puberty, which might be too late for some meaningful interventions. If we can inexpensively see that a child is experiencing this maturation earlier, we might be able to direct more intervention resources toward them."

A novel rating system

Broadly speaking, Mackey's lab studies how the brain changes and grows as people learn. It's well-established that stress during childhood speeds up maturation and that children who hit puberty earlier are at greater risk for both physical and mental health problems in adulthood.

Beyond that, in studies across primate species, molar eruption has been used to measure childhood length and correlates with a number of other developmental events. Similarly, for humans, the timing of dental events often plays a role in estimating biological age.

"That all made molar eruption a compelling developmental indicator," says McDermott, who is training to be a clinical psychologist.

It helped that more than 100 children, ages 4 to 7, had been participating in two Penn brain development studies, which included structural and functional MRI scans. "There's one type of MRI scan called a T2 weighted scan where you can visualize the morphology of the tooth pretty well," McDermott says. These scans -- typically used to look at the brain -- showed the researchers just how close these molars were to breaking through the gum line.

Once Mackey and McDermott realized this, they partnered with Katherine Hilton, then a student in Penn Dental Medicine, and Muralidhar Mupparapu, a professor in the Department of Oral Medicine, who developed a novel scale to precisely rate each tooth's position.

"The scale ranges from 1 to 4," McDermott says. "At the low end of the scale is 1, which is before the tooth has really developed at all. As the tooth emerges, there are intermediate stages, and the highest rating, a 4, is when the tooth is fully in the mouth and parallel with the other teeth." Four molars each received a score, which then got averaged, leaving a single score per individual.

Controlling for factors like age and gender, the researchers then looked for associations between early environment and molar eruption. "What we found is that income and adverse childhood experiences are both individually associated with molar eruptions status," McDermott says.

Replicating the findings

Those findings derived from just 117 participants, so although the correlation was clear, Mackey and McDermott hoped to replicate what they'd seen.

Collaborators at the University of Missouri-Kansas City told them about a large population-representative dataset called the National Health and Nutrition Examination Survey (NHANES), which is publicly available and includes dental data, demographic data, and family income, among other measures.

"Because our sample is only from one city and is much smaller than a population-representative study like that," McDermott says, "we saw it as an opportunity to verify that the findings exist outside of what we had collected in Philadelphia."

Though some facets differed -- NHANES measures dental development a little differently, for example -- the models showed similar results, indicating a connection between lower family income and earlier first molars.

Whether this overall trend is new or just now coming to light is something Mackey wants to study further. She's also curious about when the rate of maturation gets set. "Is it as early as in utero or is it dynamically adjusted based on stressors in the world?" she says. "If it's the latter, that tells you there are more opportunities to intervene."

Present implications, future work

There are still significant unknowns, as well as findings that need further examination, Mackey says. For example, the research team found racial disparities in this timing, with first molars emerging in Black children sooner than in white children.

"These race differences in molar eruption have been known for a long time, but no one thought critically about where they came from," she says. "It's consistent with higher levels of stress due to structural racism. This is a clear indication that it's not just speculation that experiences with racism can cause stress and early aging. They are having an effect on kids that we can't ignore."

For all children, a year-plus of pandemic-driven grief and social isolation most certainly amplified stress levels, making it even more important to understand who is at greatest risk for early maturation, Mackey says.

Yet she and McDermott emphasize that molar timing shouldn't become another parental fear. "What I really don't want is for parents to either worry or feel complacent just based on when their kids got their molars," says Mackey. "We don't have those data yet."

The Penn researchers are working on it. In the future, they hope to collaborate with dental offices to recruit children into studies based on their molar-eruption status. The goal would be to follow them into adulthood, to get more information on what precisely early first molars may indicate. "If this is the meaningful discovery that I think it is," Mackey says, "I would love for many scientists to jump on board and test these hypotheses."

Funding for this research came from the Jacobs Foundation, National Institute on Drug Abuse (Grant 1R34DA050297-01), and National Science Foundation.What would help, she thought, was a scalable, objective way -- a physical manifestation, of sorts -- to indicate how children embodied and responded to stresses in their world. Eruption timing of the first permanent molars proved to be just that.

New drug to regenerate lost teeth

The tooth fairy is a welcome guest for any child who has lost a tooth. Not only will the fairy leave a small gift under the pillow, but the child can be assured of a new tooth in a few months. The same cannot be said of adults who have lost their teeth.

A new study by scientists at Kyoto University and the University of Fukui, however, may offer some hope. The team reports that an antibody for one gene -- uterine sensitization associated gene-1 or USAG-1 -- can stimulate tooth growth in mice suffering from tooth agenesis, a congenital condition. The paper was published in Science Advances.

Although the normal adult mouth has 32 teeth, about 1% of the population has more or fewer due to congenital conditions. Scientists have explored the genetic causes for cases having too many teeth as clues for regenerating teeth in adults.

According to Katsu Takahashi, one of the lead authors of the study and a senior lecturer at the Kyoto University Graduate School of Medicine, the fundamental molecules responsible for tooth development have already been identified.

"The morphogenesis of individual teeth depends on the interactions of several molecules including BMP, or bone morphogenetic protein, and Wnt signaling," says Takahashi.

BMP and Wnt are involved in much more than tooth development. They modulate the growth of multiple organs and tissues well before the human body is even the size of a raisin. Consequently, drugs that directly affect their activity are commonly avoided, since side effects could affect the entire body.

Guessing that targeting the factors that antagonize BMP and Wnt specifically in tooth development could be safer, the team considered the gene USAG-1.

"We knew that suppressing USAG-1 benefits tooth growth. What we did not know was whether it would be enough," adds Takahashi.

The scientists therefore investigated the effects of several monoclonal antibodies for USAG-1. Monoclonal antibodies are commonly used to treat cancers, arthritis, and vaccine development.

USAG-1 interacts with both BMP and Wnt. As a result, several of the antibodies led to poor birth and survival rates of the mice, affirming the importance of both BMP and Wnt on whole body growth. One promising antibody, however, disrupted the interaction of USAG-1 with BMP only.

Experiments with this antibody revealed that BMP signaling is essential for determining the number of teeth in mice. Moreover, a single administration was enough to generate a whole tooth. Subsequent experiments showed the same benefits in ferrets.

"Ferrets are diphyodont animals with similar dental patterns to humans. Our next plan is to test the antibodies on other animals such as pigs and dogs," explains Takahashi.

The study is the first to show the benefits of monoclonal antibodies on tooth regeneration and provides a new therapeutic framework for a clinical problem that can currently only be resolved with implants and other artificial measures.

"Conventional tissue engineering is not suitable for tooth regeneration. Our study shows that cell-free molecular therapy is effective for a wide range of congenital tooth agenesis," concludes Manabu Sugai of the University of Fukui, another author of the study.