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hippocampus, normally shrinks with age, but if the process speeds up, it could increase vulnerability to Alzheimer's disease

two genes associated with intracranial volume -- the space within the skull occupied by the brain when the brain is fully developed in a person's lifespan

gene variants identified in the first study do not cause Alzheimer's, but they may rob the hippocampus of a kind of "reserve" against the disease

cause cell destruction and dramatic shrinkage of this key brain site

almost twice the Alzheimer's risk if he or she had these versions of the gene

if a person with one of these variants did get Alzheimer's, the disease would attack an already compromised hippocampus and so would lead to a more severe condition at a younger age than otherwise

Alzheimer's disease causes much of its damage by shrinking hippocampus volume

loses a greater-than-average amount of volume due to the gene variants we've identified, the hippocampus is more vulnerable to Alzheimer's

associations impacting intracranial volume, which is an indirect measure of the size of the brain at full development.

brain volume and intracranial volume are both highly heritable

no associations for brain volume

one of these genes has played a unique evolutionary role in human development, and perhaps we as a species are selecting this gene as a way of providing further advances in brain development

In the last few years, scientists have learned that lincRNAs, as well as other RNAs that are long and noncoding but not intergenic, perform a variety of jobs. Some serve as guides showing proteins where to go, while others tether proteins to different types of RNA, or to DNA. Some work as decoys, distracting regulatory molecules from their usual assignments. Some may even have multiple roles, all the while chattering away to other RNA within cells. (It is not idle gossip; RNA communication within cells may ward off diseases such as cancer.) And as the ultimate multitaskers, lincRNAs keep proper cellular development ticking along and help define what makes mice mice and people people.

That archive contains about 3 billion genetic letters, far more than the genomes of less complex organisms such as roundworms and fruit flies.

In 2005, the research revealed that even though genes that code for proteins make up only 1.5 percent of the mouse genome, more than 63 percent of the genome’s DNA is copied into RNA. In humans the number is even higher, with up to 93 percent of the genome made into RNA, even though protein-coding genes make up less than 2 percent of the genome.

At first, many scientists didn’t know what to make of the excess RNA. Some thought it was overexuberance on the part of the DNA-copying machinery. But gradually researchers began to realize that many of those extra RNAs had important jobs to do.

Some, though, appear to act like general contractors — not hammering in the nails and pouring the foundations of cells themselves, but dictating how the job should be done.

One of the most famous long noncoding RNAs, known as XIST, is also one of the most hands-on. XIST is in charge of shutting down one of the X chromosomes in every single cell of women and girls

XIST doesn’t have a long commute to work; it coats whichever X chromosome makes it, preventing other genes on the chromosome from being activated

One of the most well-studied linc­RNAs, named HOTAIR, wasn’t lucky enough to get a job close to home. It is copied from DNA on chromosome 12 but has to travel to chromosome 2 to shut down several genes in a group known as the HOXD cluster, genes important for proper development of an organism

Not only does HOTAIR help direct development, but it is also important throughout life to help cells pinpoint their location in the body.

Whether promoting health or mis­directing cells, lincRNAs don’t necessarily act alone.

A lincRNA known as HOTTIP also works with a crew of histone modifiers, but instead of shuttering genes, HOTTIP’s crews hang grand-opening signs to attract gene-activating machinery

In the recipe for humans, lincRNAs are in the thick of things from the very beginning. At least 26 different lincRNAs need to be on to keep an embryonic stem cell a stem cell

Just how lincRNAs choose which genes to turn on and off isn’t yet known. But Pier Paolo Pandolfi, a geneticist at Beth Israel Deaconess and Harvard Medical School, suspects that the lincRNAs are whispering to each other and to other RNAs, keeping tabs on all a cell’s goings-on. Pandolfi laid out his hypothesis for how this chatter might help control protein production and other processes in the Aug. 5 Cell.

The Columbia team and Pandolfi’s team independently found that tweaking levels of a few messenger RNAs that distract microRNAs from PTEN messenger RNA can lead to prostate cancer or a type of brain tumor called glioblastoma. Just messing with levels of a messenger RNA from another gene known as ZEB2 throws off PTEN protein levels and can lead to melanoma in mice, Pandolfi’s group reported in another paper in the Oct. 14 Cell.

Losing one noncoding RNA may be disastrous for a cell, but for want of noncoding RNAs whole species may never have evolved, argues Queensland’s Mattick. He and others say the real function of lincRNAs is to give evolution a sort of molecular clay from which to mold new designs.

Humans have several lincRNAs that are found in no other species. Many of those RNAs are made in the brain, leading scientists to speculate that the molecules may be at least partially responsible for that important organ’s evolution.