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Lab-grown neurons showcase effects of autism mutations

by  /  15 January 2018
Neurons missing a part of chromosome 16 (left) are larger and have more branches than control neurons (middle); neurons with duplications of the region are small and sport short dendrites (right).
Formative effects: Neurons missing a part of chromosome 16 (left) are larger and have more branches than control neurons (middle); neurons with duplications of the region are small and sport short dendrites (right).

Neurons derived from people with mutations linked to autism display diverse abnormalities that may help explain the origins of these individuals’ features, according to three new studies.

In all three studies, researchers reprogrammed skin cells from individuals with one of these mutations into stem cells that can mature into any cell type. They then turned these induced pluripotent stem (iPS) cells into neurons.

In one study, Lauren Weiss and her colleagues crafted neurons from people who have a mutation in a region of chromosome 16 associated with autism. These neurons display abnormalities in size, shape and function that correspond with features seen in people with the mutations, the team reported1.

The researchers also reprogrammed skin cells from people with cardiofaciocutaneous syndrome (CFC), a condition often accompanied by intellectual disability and autism. They found problems in the cultures that suggest an imbalance of brain cell types in people with the condition2.

Another team generated neurons from people with a deletion or duplication of a region of chromosome 15 linked to autism, epilepsy and related conditions. These neurons show abnormalities that could disrupt their signaling, the team found3.

Together, the studies provide new insight into the cellular consequences of genetic variants associated with autism and related conditions.

“The [researchers] found different mechanisms for how the genes had their effects,” says Anthony Wynshaw-Boris, professor of genetics at Case Western Reserve University in Cleveland, Ohio, who was not involved in any of the studies. “It underscores the heterogeneity of autism.”

The studies also illustrate the promise of the iPS cell approach for understanding how genetic mutations lead to functional impairments, says Alysson Muotri, professor of pediatrics and of cellular and molecular medicine at the University of California, San Diego, who was not involved in the study. “This is a nice representation of the type of work that’s possible,” he says.

Size matters:

Weiss and her colleagues derived neurons from the skin cells of three people with deletions and three with duplications of the 16p11.2 chromosomal region; both types of mutations are associated with autism. The researchers also generated neurons from four typical individuals.

Neurons derived from people with a 16p11.2 deletion have unusually large cell bodies and unusually long dendrites, or signal-receiving branches. By contrast, those from people with a duplication in this region are atypically small and have truncated dendrites.

The results jibe with the observation that people with a 16p11.2 deletion tend to have macrocephaly, or abnormally large heads, whereas those with an extra copy of the region are likely to have microcephaly, or small heads.

Neurons with either type of mutation have fewer synapses, the junctions between neurons, than do those from controls, the researchers found.

The findings help explain certain features in people with a 16p11.2 variant. They also suggest that different genes within the region underlie the disparate features seen in people with a mutation in the region. (The region contains 29 genes.)

“The mechanism for cell size, which we think is likely to be related to macrocephaly and microcephaly, did appear to be distinct from that of synapse density, which seems more likely to be related to behavioral features,” says Weiss, associate professor of psychiatry at the University of California, San Francisco. The study appeared 5 December in Cell Reports.

Signal problem:

The next stage is to find specific genes that cause each cellular abnormality, Weiss says.

“I think this is definitely a step forward,” Muotri says. “Each one of these cells comes from a specific individual with a different genetic background, so I think this makes the data very robust.”

In a separate study, Weiss and her colleagues generated neural progenitor cells — precursors to neurons and other brain cells — from four people with CFC and three sex- and age-matched controls. The people with CFC all have mutations in BRAF, a gene involved in cell maturation.

Progenitor cells derived from people with CFC mature faster than those from controls, rapidly depleting the pool of progenitors, the researchers found. This early depletion of the progenitor pool leads to an unusually high proportion of brain cell types that form early in development. This throws off the balance of cell types in the brain, which could, in turn, lead to an imbalance in neuronal signals.

“A lot of brain function is really dependent on having all the right cell types in the right ratios in the right places,” Weiss says. The work appeared 21 November in Molecular Psychiatry.

Cell surprises:

In the third study, Christian Schaaf and his colleagues derived neurons from three people with a deletion of the chromosomal region 15q13.3, three with a duplication of the segment, and three controls.

One gene in this segment, CHRNA7, codes for a component of calcium channels on the surface of neurons. The flow of calcium ions through these channels is essential to neuronal signaling. When the researchers exposed the cells to compounds that cause the channels to open, less calcium flowed into neurons missing a copy of CHRNA7 than flowed into control neurons.

The results suggest that children with CHRNA7 deletions have too few functional calcium channels on neurons, says Schaaf, assistant professor of human and molecular genetics at Baylor College of Medicine in Houston, Texas.

Surprisingly, neurons from children with an extra copy of CHRNA7 also show unusually low calcium influx. Too much CHRNA7 in these children appears to impair neurons’ ability to assemble the proteins and transport them to the cell surface, Schaaf says.

The results illustrate that the effects of mutations are not always predictable from the sequence alone. “It’s kind of humbling,” Schaaf says. “We actually need to do that basic science research to understand the consequences of these genetic changes.”

A next step is to turn these iPS cells into 3-D clusters of neurons, Muotri says. These ‘organoids’ resemble the human brain more than ordinary cell cultures do, and may yield additional clues to how certain mutations lead to biological changes that underlie autism.


References:
  1. Deshpande A. et al. Cell Rep. 21, 2678-2687 (2017) PubMed
  2. Yeh E. et al. Mol. Psychiatry Epub ahead of print (2017) PubMed
  3. Gillentine M.A. et al. Am. J. Hum. Genet. 101, 874-887 (2017) PubMed
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