Experiment
The corpus callosum is the main commissure connecting left and right cerebral hemispheres, and varies widely in size. Differences in the midsagittal area of the corpus callosum (MSACC) have been associated with a number of cognitive and behavioral phenotypes, including obsessive-compulsive disorders, psychopathy, suicidal tendencies, bipolar disorder, schizophrenia, autism, and attention deficit hyperactivity disorder.
Here, the authors use a large genetic reference panel - the BXD recombinant inbred line - to dissect the natural variation of the MSACC. The authors estimated the MSACC in over 300 individuals from nearly 80 strains. All histologic data for this study were obtained from The Mouse Brain Library (MBL) - a physical and Internet resource that contains high-resolution images of histologically processed slides from over 3,200 adult mouse brains1 with roughly balanced numbers of male and female specimens (Rosen et al., 2003). The ages of the mice used in this experiment ranged from 31 to 432 days with an average of 114.3 +/- 3.6 and a median of 100.
The authors found a 4-fold difference in MSACC between individual mice, and a 2.5-fold difference among strains. MSACC is a highly heritable trait (h2 = 0.60).
QTL analysis was performed using the WebQTL module of GeneNetwork (GN2). This online resource includes all known morphometric data for the BXD strains, adult, and developmental neocortical transcriptome data sets, high density marker maps based on approximately 3,795 informative markers distributed on all chromosomes except Chr Y (Shifman et al., 2006), and a database containing ~8.3 million single nucleotide polymorphisms (SNPs) taken from dbSNP (Frazer et al., 2007), Sanger/UCLA, and from a high quality subset of about 2.8 million SNPs generated by sequencing DBA/2J (~25 x shotgun) performed at UTHSC by R.W.Williams and colleagues. WebQTL incorporates three common mapping methods: (1) simple interval mapping, (2) composite interval mapping, and (3) a scan for two-locus epistatic interactions (Wang et al., 2003). To evaluate candidate genes, the authors used the QTLMiner module of GN (Alberts and Schughart, 2010), which evaluates all genes in an interval against neocortical gene expression databases within gene network.
Initial QTL mapping of MSACC revealed one suggestive QTL (genome coordinates relative to NCBI37/mm9):
Ccmsa1 (corpus callosum midsagittal area 1) maps to Chr 10: 105 - 115 Mb with a peak LRS score of 13.8.
The authors then mapped the corrected MSACC estimates and detected another suggestive locus:
Ccmsa2 (corpus callosum midsagittal area 2) maps to Chr 14: 77.5 - 97.5 Mb with a peak LRS score of 13.2. Haplotype analysis revealed that the strains inheriting the B allele at this interval had smaller MSACC, whereas those inheriting the D allele had larger MSACC values.
Using sequence data and neocortical expression databases, the authors were able to identify eight positional and plausible biological candidate genes within the Ccmsa2 interval: Dgkh (MGI:2444188), Kiaa0564 (Vwa8; MGI:1919008), Naa16 (MGI:1914147), Mtrf1 (MGI:2384815), Diap3 (Diaph3; MGI:1927222), Tdrd3 (MGI:2444023), Pcdh9 (MGI:1306801), and Rgc32 (Rgcc; 1190002H23Rik; MGI:1913464).
Finally, the authors found that MSACC correlated with behavioral traits associated with anxiety and attention.
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