NEW YORK (GenomeWeb) – Researchers from Boston Children's Hospital and elsewhere have discovered a new role for a gene active before birth. They believe that SCN3A, which encodes a voltage-gated sodium channel (VGSC) in the brain, also controls brain folding, swallowing, tongue movement, and oral motor patterns in patients.
When SCN3A is mutated in a patient's brain, the perisylvian cortex — a language area within the brain — develops multiple small folds and causes a rare brain malformation called polymicrogyria (PMG).
"We think than an impaired current in immature neurons disrupts how much [they] are sensing, and that this disrupts their normal migration," Mary Lehtinen, senior author and pathologist in the neurobiology program at Boston Children's, said in a statement. "At the time when SCN3A is active, many progenitor neurons are being produced … [and] if you have disruption in their movement, they migrate over and on top of each other."
In a study published yesterday in Neuron, Lehtinen and her team screened and performed neurological evaluations of six unrelated Finnish families with a history of oral motor or speech deficits. After performing linkage and haplotype mapping, exome sequencing, and Sanger sequencing in a series of families, the team found different forms of PMG.
After screening an additional 258 individuals with PMG and variable oral motor or speech deficits for SCN3A variations, the team found another five families that had mutations in the SCN3A gene, including at alleles R621, L850, I875, I1344, and F1759.
The study's authors noted that "the mutations suggest a range of severity, from inheritance of variably penetrant perisylvian PMG and ID without epilepsy … to the severely damaging recurrent I875T allele, which is de novo in the five families … with widespread PMG, microcephaly, and severe seizures."
In addition, Lehtinen and her team found that sodium channels that expressed mutated SCN3A alleles —F1759 or I875T — showed altered biophysical properties, including increased persistent current. Using whole-cell patching recordings, the team induced sodium currents over a range of voltages and measured the conductance-voltage curve in transfected cells. They saw that both mutant alleles demonstrated abnormal peak conductance and half-maximal voltage of activation.
Th team then evaluated total charge conducted by sodium channels during a voltage step depolarization in F1759Y and I875T alleles, compared to the wild type SCN3A gene. The F1759 variant conducted more sodium current through depolarizing potentials, while the I875T variant conducted less sodium current than controls.
By using the patch clamp method — an experimental approach that allows for a clear biophysical description of the sodium channels — on human primary fetal cortical neurons, the team found small reproducible sodium currents. However, they did not identify regenerative action potentials under tested conditions.
The authors therefore believe that "the absence of action potentials prenatally indicate that during human fetal development, SCN3A sodium currents have functions beyond action potential electrogenesis."
The researchers noted that the disorder growth parallels SCN3A expression, which they observed to be highest in early fetal cortical development but downregulated after birth. Therefore, they believe that different expression patterns between SCN3A may be linked to the ages of onset and severity of VGSC channelopathies such as PMG.
Affected patients demonstrated polymicrogyria in their brains' perisylvian cortex but did not normally exhibit epilepsy. However, they suffered from prominent speech and oral motor dysfunction, implicating SCN3A in prenatal development of human cortical language areas.
In addition, the team saw that SCN3A expression in cultured human fetal neurons caused a modest increase in neurite branching without an increase in neurite length. However, the I875T and F1759 mutants attenuated the branching effect, shortening neurite length. The team believes that the results indicate that SCN3A regulates neuronal development through an action potential-independent system, which are inhibited by mutations that cause PMG.
To validate the results, the group performed in utero electroporationin ferrets, which have brain folds similar to those found in human brains. Deliberately mutating SCN3A and placing the diseased version into ferret brains, the researchers saw that the animals' brains developed a folding pattern mirroring human polymicrogyria. In addition, they saw the mutations disrupted the expected migration of undeveloped neurons toward the outer part of the ferrets' brains.
"The fact that we can pair [speech delay] and [swallowing difficulties] with hereditary structural changes in the brain allows the unpacking of the condition," Lehtinen said in a statement. "We anticipate our study will open doors to better understanding these developmental processes."
The authors further noted that further "understanding the early function of these genes will illuminate development of the Sylvian fissure and its adjacent perisylvian language and oral motor areas."