No Silver Bullet in the Treatment of Autism

When I was a sophomore in college, I spent my summer as a paraprofessional for twin seven-year-old boys with Autism Spectrum Disorder (ASD). They lacked the ability to use verbal language, and the little communication they did have was in the form of sign language for “bathroom” and “hungry.”

I worked day after day to gain their trust, looking forward to an unrealized moment when they might look me in the eye and smile or say goodbye at the end of the day. Whether or not my twin friends have forgotten me, I will never know, but I will certainly never forget them.

Ever since that summer, I’ve had a deep-seated desire to understand the cause of Autism—to the point that I did my doctoral research in basic neural development. While my work at the bench eventually transitioned into a new career path, my personal drive to understand the foundation of autistic behaviors never faded.

For years, the causes of ASD were shrouded in mystery. During an unfortunate scientific scandal, one man instigated a dangerous camp of people who promulgated the theory that childhood vaccinations are at the core of Autism, an idea that has over and over again been dismissed by the scientific community.

Vaccinations clearly do not cause ASD, so what does?

A large number of twin studies have shown that ASD has a genetic basis. If one child from a set of identical twins has ASD, the other twin is very likely to have it as well. While the fact that genetics has a lot to do with whether or not a child will have ASD, it has been notoriously difficult to pinpoint which gene, or set of genes, accounts for any significant portion of ASD cases.

There is a reason for this; according to three new publications in this month’s issue of Neuron, a large percentage of ASD cases are relatively unique in their origin. The researchers found that specific mutations in any of 130-230 places in our DNA can lead to Autistic behaviors.

The mutations, called copy number variants (CNVs), are seen as large chunks of missing DNA or strands of DNA that have been repeated too many times. For example, if each letter of the ABCs represents a piece of our DNA, it would be like having ABCEFG... or ABCDDDDDEFG...

While the genes affected by these CNVs are numerous, one of the three groups found that many of the genes control activities at the synapse (the place where one neuron passes a signal to the next neuron). This implies that disruption of common biological functions controlled by a number of different genes is responsible for a portion of ASD cases.

An important take-away point from these studies is that ASD does not have one cause, and therefore it does not have one cure. It’s important to understand that there will never be a universal treatment for ASD. Instead, doctors may be able to use new sequencing technologies to determine genetic abnormalities in individual ASD patients, allowing tailored treatment options depending on the unique genetic background.

The Bottom Line: Mutations along many places in our DNA can cause Autism Spectrum Disorder, which suggests that future treatments will need to be tailored to an individual’s unique case.

References:
Sanders et al. (2011). “Multiple Recurrent De Novo CNVs, Including Duplications of the 7q11.23 Williams Syndrome Region, Are Strongly Associated with Autism.” Neuron, 70(5): 863-885.

Levy et al. (2011). “Rare De Novo and Transmitted Copy-Number Variation in Autistic Spectrum Disorders.” Neuron, 70(5): 886-897.

Gilman et al. (2011). “Rare De Novo Variants Associated with Autism Implicate a Large Functional Network of Genes Involved in Formation and Function of Synapses.” Neuron, 70(5): 898-907.

Shy neurons in a model of Alzheimer's disease

What exactly is wrong with Grandma’s brain when she can’t remember the day of the week, or maybe even the year? Sensory information is processed in the brain by sending electrochemical signals along a specific cell type called a neuron. If neurons fail to transmit a signal properly, neurodegeneration can result.

The underlying causes of Alzheimer’s disease are still being discovered in an intense laboratory race against the clock. Like all organs, the brain is composed of an assortment of cells that are shaped and controlled by instructions stored in DNA. One contributing factor to familial Alzheimer’s, a form of the disease that can be passed down through family members, is a gene mutation. Genes are segments of DNA that code for a single piece of the cellular machinery. The set of genes in question are called the presenilin genes, of which humans have two normal versions called presenilin-1 and presenilin-2.

Chen Zhang from the Harvard Medical School recently connected another piece of the puzzle by identifying a potential role of presenilin genes in Alzheimer’s pathogenesis. Mutations in the presenilin genes have been thought to cause neurodegeneration by disrupting the function of the synapse (the site where one neuron communicates with another). Zhang’s team sought to determine if mutated presenilin genes created a problem in the synapse by affecting neurons sending a signal or by affecting those receiving a signal. Using transgenic mice, Zhang disrupted presenilin genes in both types of neurons.

Following stimulation of the mutated neurons with a pulse of electricity, the strength of neuron-to-neuron communication was measured. Zhang’s team found that disruption of presenilin in the neuron sending a signal decreased the strength of signal transmission, while no effect was observed when presenilin activity was eliminated in neurons receiving signals.

In stimulated neurons, calcium plays an important role in the neuron’s ability to release chemicals into the synapse. Upon further investigation, Zhang and colleagues found an upset in calcium balance within the neurons upon presenilin elimination. Subsequently, the signal-sending neurons were no longer able to release the proper amount of chemicals to communicate with their down-stream partners.

Prior to this study, a large portion of research had been focused on cellular events in the signal-receiving neurons. It is now clear that disruption in the neurons that send signals may play an equally important role in the propagation of Alzheimer’s disease pathogenesis. Indeed, other neurodegenerative diseases, such as Parkinson’s disease, are also associated with gene mutations that affect the signal-sending neuron. Perhaps a flaw in the signal-sending events during neural information processing is a more general contribution to the pathogenesis of neurodegeneration than previously thought.

The Bottom Line: A mutation in mouse presenilin genes affects a neuron’s capacity to send signals, stemming from an inability to control chemical release during neuron-to-neuron communication.

Reference:
Zhang et al. “Presenilins are essential for regulating neurotransmitter release.” Nature Vol 460, 30 July 2009, doi: 10.1038/nature08177.