Robertsonian chromosomes (ROB) are a type of structurally variant chromosome that is created when two chromosomes fuse together to form an unusual bond. Found commonly in nature, these chromosomes are present in about 1 in 800 humans, and while carriers may be asymptomatic, ROBs can underlie infertility, and contribute to trisomy.
Scientists at the Stowers Institute for Medical Research, led by postdoctoral research associate Leonardo Gomes de Lima, PhD, have now identified the precise location where human chromosomes break and recombine to form Robertsonian chromosomes. Their findings explain how these rearrangements form and remain stable—but also point to how repetitive DNA, once dismissed as “junk,” may play a central role in genome organization and evolution.
“This is the first time anyone has shown where this exact DNA breakpoint occurs,” said Jennifer Gerton, PhD, Stowers Institute Investigator and dean of the Graduate School. “It opens the door to understanding how chromosomes evolve in a way that we had no appreciation for before.” The revelation of exactly how chromosomes fuse together and make Robertsonian chromosomes will help scientists better understand how to identify them and how they operate. One day, we may be able to give carriers better genetic counseling and better options.”
Gerton is co-senior author of the team’s published paper in Nature, titled “The formation and propagation of human Robertsonian chromosomes.” In their paper the team stated, “In conclusion, we provide, to our knowledge, the first complete assemblies of common ROBs, precise mapping of their fusion sites, and insight into their formation and transmission mechanisms.”
The research was conducted in collaboration with Adam Philippy, PhD, at the National Human Genome Research Institute, and Erik Garrison, PhD, at the University of Tennessee Health Science Center. “One of the joys of this project was bringing together three labs with complementary expertise,” Gerton commented. “Adam’s team is a world leader in assembling complex genomes, Eric’s group excels at studying variation across human populations, and our team specializes in repetitive DNA and chromosome biology. “Together, we could tackle a question that none of us could have answered alone.
First discovered in 1916 in grasshoppers, Robertsonian chromosomes are found in many kinds of animals and plants, including humans. Of the 23 pairs of human chromosomes, five pairs—known as acrocentric chromosomes—have asymmetric arm lengths, one short and one long. Robertsonian chromosomes are created by the fusion of two acrocentric chromosomes to create a single metacentric chromosome. “
Carriers of Robertsonian chromosomes are generally healthy and often unaware they’re different, but may be infertile or may suffer miscarriages. There is an increased risk of Down syndrome when they do have children. “Robertsonian chromosomes form when two long arms fuse and the short arms are lost,” Gerton said. “That leaves 45 chromosomes instead of 46—and sometimes lining up 45 with 46 doesn’t work very well, which can result in infertility.”
The short arms are particularly difficult to sequence, and before 2022, they were missing from the complete sequence of the human genome. “In humans there is no immediate known impact of losing the short arms,” Gerton told GEN.
For their newly reported study Gerton and colleagues made use of long read sequencing technology to produce the first ever complete sequences of Robertsonian chromosomes. Long-reads revolutionized the study of the human genome in 2022 by allowing scientists to read repetitive DNA sequences that tripped up older sequencing technologies.
Gerton and team compared the sequences of three human Robertsonian chromosomes to normal chromosomes, finding a common break point, which lies in a specific repetitive DNA sequence called SST1. “Here we fully assemble three common ROBs: two chromosome 13–chromosome 14 (13;14) fusions and one 14;21 fusion,” they reported. “We identified a common breakpoint in SST1, a macrosatellite DNA located on chromosomes 13, 14, and 21, which commonly undergo Robertsonian translocation.”
“That’s never been shown before—in humans or in any other species,” said Gerton. The investigators showed that when SST1 sites come together inside the nucleolus of a cell, their proximity can cause a merger that results in a Robertsonian chromosome. These repetitive DNA sequences are located near chromosome centromeres—the regions at their centers from which two arms extend.
The research reveals that a key part of how Robertsonian chromosomes fuse is due to the orientation of the SST1 on chromosome 14. Its sequence is inverted or facing the opposite direction, allowing either chromosome 13 or 21 to attach. In both cases, the resulting Robertsonian chromosomes carry almost all the genetic material from both original chromosomes. There were some surprises in their findings, Gerton suggested to GEN. “One surprising result is that some individuals in the human population are missing the chromosomal region on chromosome 14 that allows the most common Robertsonian fusions to occur. The implication is that the chromosomes in these individuals would be much less likely to make a Robertsonian fusion.”
The team’s results also showed why these fused chromosomes can remain stable. Although they carry two centromeres—the anchor point where chromosomes are pulled apart during cell division—only one is active. This prevents the fused chromosome from being pulled in opposite directions.
“Now that we know how these chromosome form in humans, it gives us insight into how they occur broadly in nature,” Gerton further suggested. In some cases, the specific Robertsonian arrangement carried by an animal limits which other animals it can reproduce with. The common house mouse, for example, has the best chance of having offspring when it mates with others carrying the same arrangements.
When Gerton and her colleagues looked at the genomes of humans’ closest living relatives, chimpanzees and bonobo monkeys, they saw some distinct differences from humans. The great apes still had the SST1 sequences that drive Robertsonian chromosome formation, but their arrangement was slightly different. “Investigation of the assembled genomes of chimpanzee and bonobo highlights that the inversion on chromosome 14 is unique to the human genome,” they stated.
“Robertsonian chromosomes are a common occurrence in nature in animals and plants and are a type of evolutionary event,” Gerton told GEN. “In natural populations, Robertsonian chromosomes often underlie speciation. Knowing how these chromosomes form in the human genome helps us appreciate more general principles about how Robertsonian chromosomes form in other species.”
It seems possible that the arrangement of repetitive DNA sequences within genomes are a part of what makes each species distinct, Gomes de Lima said. “It really got us thinking about the role these repetitive DNA sequences play in shaping the genome and potentially creating new species.”
Gerton added, “It’s clear that there’s a story there, and that’s what we plan to study next.” Commenting further, Gerton said, “The research has helped us appreciate the importance of repetitive ‘junk’ DNA in how chromosomes evolve. We are exploring how different types of repetitive DNA create genome instability and structurally variant chromosomes.”
In their paper the authors suggested, “Analysis of many ROBs will be required to understand how individual features affect their propagation and carrier fertility. By integrating insights from genomic, cytogenetic and evolutionary studies, we can gain a more comprehensive understanding of the role of rDNA and SST1-based recombination in genome evolution and reproductive biology.”
Genome scientist Glennis Logsdon, PhD, from the University of Pennsylvania, who was not involved in the work, called the development “exciting,” and stated, “This is a landmark study…As the first group to identify the precise breakpoint at which Robertsonian chromosomes combine, Gerton and her colleagues have lit a flame that could ignite a broader understanding of how these chromosomes function.”
