The biological causes of autism have been a source of inquiry and debate for half a century. The wide range of cognitive and social deficits that are diagnosed as autism spectrum disorders are often quite disabling, so there is an urgent desire to find treatments. Now, scientists are beginning to discover that not all autism-related disorders are alike. They appear to have different implications for brain function, and consequently, treatment.

Working with genetically engineered mice, researchers have discovered that tuberous sclerosis – a rare disease on the autism spectrum – is caused by too little protein synthesis within synapses of the brain. This is in stark contrast to Fragile X syndrome, another rare disease on the autism spectrum, which had been previously shown by many laboratories to be caused by too much protein synthesis.

Any drug given to a patient with autism must be carefully matched with their genetic background to ensure it does more good than harm.

Both tuberous sclerosis (TSC) and Fragile X syndrome can be traced to specific mutations on a single gene, but they are different types of mutations on different genes. In the case of TSC, the mutation is either on the gene TSC 1 or TSC 2. In the case of Fragile X syndrome, the mutation is found on a gene called Fragile X Mental Retardation Protein (FMRP). Both syndromes are characterized by severe mental retardation and are the subjects of clinical trials to develop drugs to treat them.

The idea that two autism-related disorders could have different causes suggests that the gene mutations do not produce the same disease, even though both are considered forms of autism.

The new study suggests that drugs developed to treat Fragile X may not work for TSC and that any drug given to a patient with autism must be carefully matched with their genetic background to ensure it does more good than harm.

The investigators, based at Massachusetts Institute of Technology, had previously found that genetically engineered mice with Fragile X syndrome produce too much protein synthesis when a specific type of glutamate receptor, known as mGluR5, is activated. The protein FMRP normally acts as a brake for protein synthesis, so when it is mutated, the result is an overabundance of synaptic proteins. Drugs that block activation of mGluR5 restore the normal balance of protein synthesis and ameliorate the symptoms of Fragile X syndrome in the mice.

But the same investigators found the exact opposite occurs in mice with mutations in TSC. When mGluR5 is activated in the TSC mice, synapses have too little protein synthesis. Drugs that block activation of mGluR5 only make things worse, but drugs that stimulate activation of mGluR5 restore the balance and improve symptoms of TSC in the mice.

FMRP and TSC are just two of several autism-related genes that have been genetically engineered into lab mice. Another one, called Shank3, has also been found to reproduce in mice the compulsive, repetitive behavior seen in humans.

But the investigators at MIT believe that all genetic causes of autism could have one underlying theme: a functional pathway of neurotransmitters and receptors that in the end results in the synthesis of new proteins at synapses.

“The study identified one functional axis, and it will be important to know where a patient lies on this axis to devise the therapy that will be effective," said lead researcher Mark F. Bear, Picower Professor of Neuroscience, at MIT.

So far, there are no good biomarkers or indicators of which types of mutations a human with autism will have. However, drugs that inhibit or stimulate mGluR5 for Fragile X and autism are currently under development. How patients respond to these drugs could indicate which type of genetic mutations are the root cause of the patients’ autism.

The study was published online on Nov. 23 in the journal, Nature.