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First mouse model of Timothy syndrome debuts

by  /  5 September 2011
P. Bader  Cold shoulder: Mice carrying the Timothy syndrome mutation tend to spend more time near an empty cage (red areas), whereas controls gather near a cage holding another mouse (blue areas).

Cold shoulder: Mice carrying the Timothy syndrome mutation tend to spend more time near an empty cage (red areas), whereas controls gather near a cage holding another mouse (blue areas) P. Bader.

Researchers have created the first mouse model of Timothy syndrome, a rare genetic disorder that causes heart defects and autism. Researchers first presented the findings, published 30 August in the Proceedings of the National Academy of Sciences1, at the Society for Neuroscience annual meeting last November.

Individuals with the syndrome carry mutations in the CACNA1C gene, which controls the flow of calcium ions in and out of cells. The mutant mice, which express the defective gene at one-third the normal level, show mild social deficits, repetitive behavior and shorter vocalizations than controls.

“We have here all three elements of the classic autistic triad: the social behaviors, the repetitive behaviors and the communications difficulty,” says lead investigator Richard Tsien, professor of molecular and genetic medicine at Stanford University in Palo Alto, California.

There are only a few dozen reported cases of the Timothy syndrome mutation, which causes an unchecked influx of calcium ions in cells. This disturbs heart muscle contractions, sometimes leading to fatally slow heartbeats. It also upsets the firing patterns of neurons. About three-quarters of individuals with Timothy syndrome also show symptoms of autism.

Although the mutation is rare, the autism community is paying attention to Timothy syndrome because CACNA1C has much in common with other autism candidate genes. It encodes a protein called the L-type channel that sits at the synapse and regulates calcium signaling, which in turn controls gene expression.

Most of the autism genes identified to date fall into one of two categories: They either affect the synapse, the junction between neurons, or they’re involved in the translation of DNA into protein. “The L-type channel just sits beautifully at the watershed between these two camps,” Tsien says.

Cassette players:

Researchers’ first attempts at making a mouse model of Timothy syndrome failed because animals expressing the mutation at normal levels did not survive.

Tsien’s group cleared this hurdle using a trick of molecular engineering. One way to create a mutant mouse is to insert the desired genetic mutation along with a so-called neomycin cassette, a string of DNA that codes for resistance to an antibiotic called neomycin. By later exposing the cells to the antibiotic, the researchers can select for those that successfully received the target mutation.

Once this is done, researchers typically remove the cassette from the transformed cells. In this study, however, they left it in. The cassette interferes with the transcription of nearby genes, ultimately dampening their expression. In this case, that meant that the mutant mice only show one-third the normal level of CACNA1C expression — not enough to be lethal, but apparently enough to cause behaviors reminiscent of autism.

The most striking feature, according to the researchers, is the mutants’ strong resistance to change. In a test called the Y maze, mice typically swim along a straight path and are taught to make a turn in order to find a dry platform. They are first trained to go to one side, but then the platform is switched to the other side.

Normal mice learn to switch to the correct side within a few trials. Many of the Timothy mutants, however, steadfastly insist on taking the old route, even after 25 trials (see video). The researchers tried putting up a plastic partition to prevent the mice from making the wrong turn. “Even then, they would bump their head against the block,” says Patrick Bader, a trained psychiatrist and postdoctoral fellow in Tsien’s lab. “It was very, very exciting to see that.”

The mice show similar problems during a memory test in which they are trained to associate a certain sound with a mild electrical shock. Once they learn the association, mice respond to the tone, even when not accompanied by a shock, by freezing in fear. When they hear the sound more than two weeks after learning the initial association, however, normal mice are less afraid, freezing for much shorter lengths of time, whereas the mutants tend to stay frozen.

“Our hypothesis is that the Timothy syndrome mice might have more problems un-learning something,” Bader says.

Subtle effects:

The mutants also have mild social deficits, as determined by a new behavioral test developed in collaboration with Mehrdad Shamloo, also of Stanford.

In the four-hour test, mice roam in a dark cage that has two unfamiliar objects: an empty pencil cup, and a cup holding another mouse. After the mice acclimate to the cage for two hours, researchers use an infrared camera to measure where the mice spend their time. In the second two hours of the test, normal mice tend to spend more time with the other mouse, whereas mutants veer toward the empty cup.

“I really like this test, and I’m thinking to myself very selfishly, I wouldn’t mind using it,” notes Valerie Bolivar, director of the Mouse Behavioral Phenotype Analysis Core at the Wadsworth Center, New York State Department of Health. The test is useful because it’s akin to the long-term social interactions of people with autism, she says.

“If we want to apply this to autistic individuals, we’d want to consider the social behavior in the home, with people they know and spend time with, not strangers,” Bolivar says.

However, it’s difficult to say whether this mimics the social behaviors of people with Timothy syndrome, researchers say, because the disease is so rare and each individual has a unique mix of symptoms and traits.

“The psychiatric manifestations of the disease are often very subtle,” notes Silvia Priori, who was part of the team that first defined the syndrome in 2004 and now treats eight individuals with the disease2.

What’s more, the mutation’s other effects — such as webbed fingers, fainting and a gag reflex — could in themselves lead to strained social interactions.

“The fascinating part of this disease is that, maybe because calcium is such an important mediator of embryological development in different tissues, these patients seem to have a uniquely complex phenotype,” says Priori, professor of medicine at New York University.

The new mutant mice do not have heart defects, perhaps because not enough of the faulty gene is expressed, or because mouse hearts beat much faster than human hearts do. “It’s a shame that the model cannot be used to study the cardiac phenotype,” Priori says.

Still, the mutants could test whether drugs that act on calcium channels — such as calcium blockers, which are commonly used to treat high blood pressure — have a beneficial effect on the animals’ social and communicative defects.

“It could be a good tool to try to shed some light on the psychological and developmental abnormalities in these patients,” Priori says.

 


References:
  1. Bader P.L. et al. Proc. Natl. Acad. Sci. USA Epub ahead of print (2011) Abstract
  2. Splawski I. et al. Cell 119, 19-31 (2004) PubMed

7 responses to “First mouse model of Timothy syndrome debuts”

  1. Sue Gerrard says:

    Re autism: Perhaps if researchers were to pay more attention to the physiological abnormalities that accompany autism, rather than the autism per se, the causal picture would become clearer?

  2. Sue Gerrard says:

    Thanks for the links, Virginia. Problem is, in terms of theoretical modelling, many researchers would consider autistic characteristics to be core features, and physiological abnormalities to be secondary issues – but without the quotation marks. So many physiological clues are ignored as biological markers for developmental disorders that result in autism, because those physiological factors are not always associated with autistic characteristics.

  3. virginiahughes says:

    Which physiologic features do you think should be included in theoretical models of autism?

  4. RAJensen says:

    One of the largest sub-groups in autism is the group whose underlying etiology involves auto-immune disease and immune deficiencies. Michael Rutter has proposed a novel ‘two-hit’ mechanism that states that the genes underlying the broad autism phenotype may not be the same as the the genetic and environmental factors involved in the disruption of early brain development and the transition to the handicapping disorder. The most powerful evidence for the complex interrelationships between the environment, auto-immunity, autism, early and long-lasting disruption of brain development, aberrant synaptic connections and the Rutter hypothesis is found in DiGeorge Syndrome/ Velo-cardio-facial syndrome (22q11.2 deletion syndrome). The 22q11.2 region harbors the broad autism phenotype related COMT gene which is not an ‘autism’ susceptibility gene at all but is associated with a wide array of broad autism phenotype features and in general population studies is associated with self reported schizotypy personality in healthy males, anxiety related traits, depression, obsessive compulsive disorder and panic disorder without the 22q11.2 deletion syndrome ( Avramopoulas et al 2002) ( Hettema et al 2008), (Liu et al 2010 ) ( Woo JM et al 2002). The 22q11.2 region also harbors genes that influence cell cycle or mitochondrial function that are expressed in the developing cerebral cortex. When dosage of these genes is diminished, numbers, placement and connectivity of neurons essential for normal behavior may be disrupted.( Meechan et al 2011 ). In the 22q11.2 deletion syndrome, 93% of cases are the consequence of a germ line reproductive error (egg or sperm) in contrast to being an inherited event. Where transgenerational does occurs the parent is not affected as far as autism is concerned. ( McDonald-McGinn et al 2005). Seventy-seven per cent of individuals have an immune deficiency including Graves disease regardless of clinical presentation ( Gosselin et al 2004) ( Greasdal et al 2001). The population prevalence of 22q11.2 syndrome is 1 in 4,000 which is exactly the same population prevalence of Fragile X (males=1-4,000, females=1-8,000) as sourced in the NIMH genetic reference table ( 22q11.2 deletion syndrome ) ( Fragile X ). In a Swedish clinic the prevalence of narrowly defined autistic disorder in individuals with the 22q11.2 syndrome is 5% which is 25 times greater than narrowly defined autistic disorder in the general population ( Nkilasson et al 2009 ) (Fombonne 2009 ).

    • RAJensen says:

      Sue;
      You are quite right physiological abnormalities are primary. The long term studies of Romanian orphans abandoned at birth and placed into horrendous Romanian orphanages clearly demonstrate that severe sensory and emotional deprivation is primary to the secondary autistic characteristics present in the orphans. When these orphans were adopted into well functioning UK families 9% of the orphans met diagnostic criteria for autism using gold standard diagnostic tools (ADOS-R, ARI-R).

      Completly unrecognized is that at intake many of the orphans were in poor physical health with immune deficiencies that included chronic respiratory and intestinal diseases.

      The orphans suffered multiple environmental insults, sensory deprivation, emotional deprivations and chronic respiratory and intestional infections.

      It is hard to argue that the Romanian orphans had a heritable genetic disorder nor is it hard to ignore the immune deficiencies a common theme associated with a large sub-group.

      Immune deficiencies has also been recognized in the newly recognized 17q12 deletion syndrome. The 17q12 region harbors the serotonin transporter gene (SCL6A4). The SCL6A4 gene is not an ‘autism’ susceptability gene, general population studies have shown it is a gene associated with a wide array of broad autism phenotype features. The 17q12 region also harbors the HNF1B locus responsible for renal cysts and a diabetes syndrome.

      In a clinical study children diagnosed with a 17q12 deletion and cystic or hyperechogenic kidneys were followed up. Three of the fifty three children examined were diagnosed with co-occuring autism a prevelance rate (6%) far greater than population prevelance rates for autism.

      In the cases reported in the literature so far, all cases were caused by a de novo mutation in contrast to being an inherited event.

      http://ndt.oxfordjournals.org/content/25/10/3430.abstract

  5. Sue Gerrard says:

    RAJensen has explained, in far more detail than I could, some of the mechanisms by which physiological factors could be related to autism.

    A key point is that ‘autism’ is a set of behavioural characteristics that frequently co-occur – it’s a syndrome. But ‘autism’ is only one factor in other syndromes; different groups of children exhibit different clusters of frequently co-occurring symptoms, one of which is autism.

    There is a range of abnormalities that frequently co-occur with autism, affecting, for example, the immune system, sensory processing, digestion, sleep and motor function. These could be coincidental or could be related in some way to the child’s autism, but if there is no obvious reason for the child’s autism, then their disorder is diagnosed as ‘autism’ – when in fact the autism is only one of several other impairments they exhibit.

    The diagnostic process means that children from different groups (ie with different causes for their autism) are inevitably going to be lumped together in research designs, with the inevitable consequence that features that don’t co-occur in all cases of autism get sidelined in research programmes.

    Because the sidelined features are often low-level characteristics such as variations in urinary peptides or differences in auditory filter bandwidth they are much better candidates for identifying causes of autism than high-level features such as impairments in social interaction or communication.

    What we need to be looking at is the causes of individual children’s symptoms (all of their symptoms) or groups of children showing the same co-occurring symptoms, not at the causes of autism per se.

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