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Largest-Ever Alzheimer’s Gene Study Brings New Answers

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Alzheimer's Risk Genes

Predicting whether someone will get Alzheimer’s disease (AD) late in life, and how to use that information for prevention, has been an intense focus of biomedical research. The goal of this work is to learn not only about the genes involved in AD, but how they work together and with other complex biological, environmental, and lifestyle factors to drive this devastating neurological disease.

It’s good news to be able to report that an international team of researchers, partly funded by NIH, has made more progress in explaining the genetic component of AD. Their analysis, involving data from more than 35,000 individuals with late-onset AD, has identified variants in five new genes that put people at greater risk of AD [1]. It also points to molecular pathways involved in AD as possible avenues for prevention, and offers further confirmation of 20 other genes that had been implicated previously in AD.

The results of this largest-ever genomic study of AD suggests key roles for genes involved in the processing of beta-amyloid peptides, which form plaques in the brain recognized as an important early indicator of AD. They also offer the first evidence for a genetic link to proteins that bind tau, the protein responsible for telltale tangles in the AD brain that track closely with a person’s cognitive decline.

The new findings are the latest from the International Genomics of Alzheimer’s Project (IGAP) consortium, led by a large, collaborative team including Brian Kunkle and Margaret Pericak-Vance, University of Miami Miller School of Medicine, Miami, FL. The effort, spanning four consortia focused on AD in the United States and Europe, was launched in 2011 with the aim of discovering and mapping all the genes that contribute to AD.

An earlier IGAP study including about 25,500 people with late-onset AD identified 20 common gene variants that influence a person’s risk for developing AD late in life [2]. While that was terrific progress to be sure, the analysis also showed that those gene variants could explain only a third of the genetic component of AD. It was clear more genes with ties to AD were yet to be found.

So, in the study reported in Nature Genetics, the researchers expanded the search. While so-called genome-wide association studies (GWAS) are generally useful in identifying gene variants that turn up often in association with particular diseases or other traits, the ones that arise more rarely require much larger sample sizes to find.

To increase their odds of finding additional variants, the researchers analyzed genomic data for more than 94,000 individuals, including more than 35,000 with a diagnosis of late-onset AD and another 60,000 older people without AD. Their search led them to variants in five additional genes, named IQCK, ACE, ADAM10, ADAMTS1, and WWOX, associated with late-onset AD that hadn’t turned up in the previous study.

Further analysis of those genes supports a view of AD in which groups of genes work together to influence risk and disease progression. In addition to some genes influencing the processing of beta-amyloid peptides and accumulation of tau proteins, others appear to contribute to AD via certain aspects of the immune system and lipid metabolism.

Each of these newly discovered variants contributes only a small amount of increased risk, and therefore probably have limited value in predicting an average person’s risk of developing AD later in life. But they are invaluable when it comes to advancing our understanding of AD’s biological underpinnings and pointing the way to potentially new treatment approaches. For instance, these new data highlight intriguing similarities between early-onset and late-onset AD, suggesting that treatments developed for people with the early-onset form also might prove beneficial for people with the more common late-onset disease.

It’s worth noting that the new findings continue to suggest that the search is not yet over—many more as-yet undiscovered rare variants likely play a role in AD. The search for answers to AD and so many other complex health conditions—assisted through collaborative data sharing efforts such as this one—continues at an accelerating pace.

References:

[1] Genetic meta-analysis of diagnosed Alzheimer’s disease identifies new risk loci and implicates Aβ, tau, immunity and lipid processing. Kunkle BW, Grenier-Boley B, Sims R, Bis JC, et. al. Nat Genet. 2019 Mar;51(3):414-430.

[2] Meta-analysis of 74,046 individuals identifies 11 new susceptibility loci for Alzheimer’s disease. Lambert JC, Ibrahim-Verbaas CA, Harold D, Naj AC, Sims R, Bellenguez C, DeStafano AL, Bis JC, et al. Nat Genet. 2013 Dec;45(12):1452-8.

Links:

Alzheimer’s Disease Genetics Fact Sheet (National Institute on Aging/NIH)

Genome-Wide Association Studies (NIH)

Margaret Pericak-Vance (University of Miami Health System, FL)

NIH Support: National Institute on Aging; National Heart, Lung, and Blood Institute; National Human Genome Research Institute; National Institute of Allergy and Infectious Diseases; Eunice Kennedy Shriver National Institute of Child Health and Human Development; National Institute of Diabetes and Digestive and Kidney Disease; National Institute of Neurological Disorders and Stroke


Study Shows Genes Unique to Humans Tied to Bigger Brains

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cortical organoid

Caption: Cortical organoid, showing radial glial stem cells (green) and cortical neurons (red).
Credit: Sofie Salama, University of California, Santa Cruz

In seeking the biological answer to the question of what it means to be human, the brain’s cerebral cortex is a good place to start. This densely folded, outer layer of grey matter, which is vastly larger in Homo sapiens than in other primates, plays an essential role in human consciousness, language, and reasoning.

Now, an NIH-funded team has pinpointed a key set of genes—found only in humans—that may help explain why our species possesses such a large cerebral cortex. Experimental evidence shows these genes prolong the development of stem cells that generate neurons in the cerebral cortex, which in turn enables the human brain to produce more mature cortical neurons and, thus, build a bigger cerebral cortex than our fellow primates.

That sounds like a great advantage for humans! But there’s a downside. Researchers found the same genomic changes that facilitated the expansion of the human cortex may also render our species more susceptible to certain rare neurodevelopmental disorders.


Of Mice and Men: Study Pinpoints Genes Essential for Life

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Many people probably think of mice as unwanted household pests. But over more than a century, mice have proven to be incredibly valuable in medical research. One of many examples is how studies in mice are now helping researchers understand how mammalian genomes work, including the human genome. Scientists have spent decades inactivating, or “knocking out,” individual genes in laboratory mice to learn which tissues or organs are affected when a specific gene is out of order, providing valuable clues about its function.

More than a decade ago, NIH initiated a project called KOMP—the Knockout Mouse Project [1]. The goal was to use homologous recombination (exchange of similar or identical DNA) in embryonic stem cells from a standard mouse strain to knock out all of the mouse protein-coding genes. That work has led to wide availability of such cell lines to investigators with interest in specific genes, saving time and money. But it’s one thing to have a cell line with the gene knocked out, it’s even more interesting (and challenging) to determine the phenotype, or observable characteristics, of each knockout. To speed up that process in a scientifically rigorous and systematic manner, NIH and other research funding agencies teamed to launch an international research consortium to turn those embryonic stem cells into mice, and ultimately to catalogue the functions of the roughly 20,000 genes that mice and humans share. The consortium has just released an analysis of the phenotypes of the first 1,751 new lines of unique “knockout mice” with much more to come in the months ahead. This initial work confirms that about a third of all protein-coding genes are essential for live birth, helping to fill in a major gap in our understanding of the genome.


Gene Duplication: New Analysis Shows How Extra Copies Split the Work

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Word cloudThe human genome contains more than 20,000 protein-coding genes, which carry the instructions for proteins essential to the structure and function of our cells, tissues and organs. Some of these genes are very similar to each other because, as the genomes of humans and other mammals evolve, glitches in DNA replication sometimes result in extra copies of a gene being made. Those duplicates can be passed along to subsequent generations and, on very rare occasions, usually at a much later point in time, acquire additional modifications that may enable them to serve new biological functions. By starting with a protein shape that has already been fine-tuned for one function, evolution can produce a new function more rapidly than starting from scratch.

Pretty cool! But it leads to a question that’s long perplexed evolutionary biologists: Why don’t duplicate genes vanish from the gene pool almost as soon as they appear? After all, instantly doubling the amount of protein produced in an organism is usually a recipe for disaster—just think what might happen to a human baby born with twice as much insulin or clotting factor as normal. At the very least, duplicate genes should be unnecessary and therefore vulnerable to being degraded into functionless pseudogenes as new mutations arise over time

An NIH-supported team offers a possible answer to this question in a study published in the journal Science. Based on their analysis of duplicate gene pairs in the human and mouse genomes, the researchers suggest that extra genes persist in the genome because of rapid changes in gene activity. Instead of the original gene producing 100 percent of a protein in the body, the gene duo quickly divvies up the job [1]. For instance, the original gene might produce roughly 50 percent and its duplicate the other 50 percent. Most importantly, organisms find the right balance and the duplicate genes can easily survive to be passed along to their offspring, providing fodder for continued evolution.


Largest Study Yet Shows Mother’s Smoking Changes Baby’s Epigenome

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Pregnant woman smoking

Credit: Daniel Berehulak/Getty Images

Despite years of public health campaigns warning of the dangers of smoking when pregnant, many women are unaware of the risk or find themselves unable to quit. As a result, far too many babies are still being exposed in the womb to toxins that enter their mothers’ bloodstreams when they inhale cigarette smoke. Among the many infant and child health problems that have been linked to maternal smoking are premature birth, low birth weight, asthma, reduced lung function, sudden infant death syndrome (SIDS), and cleft lip and/or palate.

Now, a large international study involving NIH-supported researchers provides a biological mechanism that may explain how exposure to cigarette toxins during fetal development can produce these health problems [1]. That evidence centers on the impact of the toxins on the epigenome of the infant’s body tissues. The epigenome refers to chemical modifications of DNA (particularly methylation of cytosines), as well as proteins that bind to DNA and affect its function. The genome of an individual is the same in all cells of their body, but the epigenome determines whether genes are turned on or off in particular cells. The study found significant differences between the epigenetic patterns of babies born to women who smoked during pregnancy and those born to non-smokers, with many of the differences affecting genes known to play key roles in the development of the lungs, face, and nervous system.


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