On 2015 Apr 18, Dorothy V M Bishop commented:

As a psychologist interested in the genetics of lateralization, I frequently come across this paper, which is cited as evidence for early genetic influences on brain asymmetry. As of today, 160 citations are shown in Web of Science.

When I read the paper a couple of years ago, I found some details that did not seem to support the conclusions of the authors. These are described in a blogpost: http://deevybee.blogspot.co.uk/2012/12/genes-brains-and-lateralisation-how.html

I will summarise the main issue below, but I was interested to note that, since that time, other papers have appeared, using larger datasets, which have stressed the remarkable symmetry of early gene expression, notably:

Johnson, M. B., et al (2009). Functional and evolutionary insights into human brain development through global transcriptome analysis. Neuron, 62(4), 494-509. doi: 10.1016/j.neuron.2009.03.027

and

Pletikos, M., Sousa, A. M. M., Sedmak, G., Meyer, K. A., Zhu, Y., Cheng, F., . . . Sestan, N. (2014). Temporal specification and bilaterality of human neocortical topographic gene expression. Neuron, 81(2), 321-332. doi: 10.1016/j.neuron.2013.11.018

Here is a brief account of the main issue I raised about the study. Please see the blogpost for more details.

Sun et al used a method called Serial Analysis of Gene Expression (SAGE) which compares gene expression in different tissues or – as in this case – in corresponding left and right regions of the embryonic brain. The analysis looks for specific sequences of 10 DNA base-pairs, or tags, which index particular genes. SAGE output consists of simple tables, giving the identity of each tag, its count (a measure of cellular gene expression) and an identifier and more detailed description of the corresponding gene. These tables are available for left and right sides for three brain regions (frontal, perisylvian and occipital) for 12- and 14-week old brains, and for perisylvian only for a 19-week-old brain. The perisylvian region is of particular interest because it is the brain region that will develop into the planum temporale, which has been linked with language development. One brain at each age was used to create the set of SAGE tags.

To verify asymmetrically expressed genes the authors performed chi square tests. The chi square involves testing whether the distribution of expression on left and right is significantly different from the distribution of left vs. right expression across all tags in this brain region – which is close to 50%. In the left-right perisylvian region of a 12-week-old embryonic human brain, there were 49 genes with chi square greater than 6.63 (p < .01): 21 were more highly expressed on the left and 28 more highly expressed on the right. But for each region the authors considered several thousand tags. My analysis indicated that the number of asymmetrically expressed genes appeared to be lower than you would expect by chance – entirely consistent with the conclusions of Pletikos et al.

Unless my analysis is mistaken, it would seem this paper should not be cited as evidence for asymmetric fetal gene expression, as it actually shows the opposite.

On 2015 Apr 18, Dorothy V M Bishop commented:

As a psychologist interested in the genetics of lateralization, I frequently come across this paper, which is cited as evidence for early genetic influences on brain asymmetry. As of today, 160 citations are shown in Web of Science.

When I read the paper a couple of years ago, I found some details that did not seem to support the conclusions of the authors. These are described in a blogpost: http://deevybee.blogspot.co.uk/2012/12/genes-brains-and-lateralisation-how.html

I will summarise the main issue below, but I was interested to note that, since that time, other papers have appeared, using larger datasets, which have stressed the remarkable symmetry of early gene expression, notably:

Johnson, M. B., et al (2009). Functional and evolutionary insights into human brain development through global transcriptome analysis. Neuron, 62(4), 494-509. doi: 10.1016/j.neuron.2009.03.027

and

Pletikos, M., Sousa, A. M. M., Sedmak, G., Meyer, K. A., Zhu, Y., Cheng, F., . . . Sestan, N. (2014). Temporal specification and bilaterality of human neocortical topographic gene expression. Neuron, 81(2), 321-332. doi: 10.1016/j.neuron.2013.11.018

Here is a brief account of the main issue I raised about the study. Please see the blogpost for more details.

Sun et al used a method called Serial Analysis of Gene Expression (SAGE) which compares gene expression in different tissues or – as in this case – in corresponding left and right regions of the embryonic brain. The analysis looks for specific sequences of 10 DNA base-pairs, or tags, which index particular genes. SAGE output consists of simple tables, giving the identity of each tag, its count (a measure of cellular gene expression) and an identifier and more detailed description of the corresponding gene. These tables are available for left and right sides for three brain regions (frontal, perisylvian and occipital) for 12- and 14-week old brains, and for perisylvian only for a 19-week-old brain. The perisylvian region is of particular interest because it is the brain region that will develop into the planum temporale, which has been linked with language development. One brain at each age was used to create the set of SAGE tags.

To verify asymmetrically expressed genes the authors performed chi square tests. The chi square involves testing whether the distribution of expression on left and right is significantly different from the distribution of left vs. right expression across all tags in this brain region – which is close to 50%. In the left-right perisylvian region of a 12-week-old embryonic human brain, there were 49 genes with chi square greater than 6.63 (p < .01): 21 were more highly expressed on the left and 28 more highly expressed on the right. But for each region the authors considered several thousand tags. My analysis indicated that the number of asymmetrically expressed genes appeared to be lower than you would expect by chance – entirely consistent with the conclusions of Pletikos et al.

Unless my analysis is mistaken, it would seem this paper should not be cited as evidence for asymmetric fetal gene expression, as it actually shows the opposite.