Human ROBO1 regulates interaural interaction in auditory pathways

Abstract

In rodents, the Robo1 gene regulates midline crossing of major nerve tracts, a fundamental property of the mammalian CNS. However, the neurodevelopmental function of the human ROBO1 gene remains unknown, apart from a suggested role in dyslexia. We therefore studied axonal crossing with a functional approach, based on magnetoencephalography, in 10 dyslexic individuals who all share the same rare, weakly expressing haplotype of the ROBO1 gene. Auditory-cortex responses were recorded separately to left- and right-ear sounds that were amplitude modulated at different frequencies. We found impaired interaural interaction that depended on the ROBO1 in a dose-dependent manner. Our results indicate that normal crossing of the auditory pathways requires an adequate ROBO1 expression level.

Figure 1.

Abridged pedigree of the family carrying the weakly expressing haplotype of ROBO1. The haplotype cosegregates with developmental dyslexia in a dominant manner. Squares denote males and circles females.

Figure 2.

Electrophysiological assessment of the crossing of auditory pathways. A, A highly schematic presentation of the afferent tracts of the auditory system from the right ear. The majority of the axons cross the midline in the brainstem and enter the contralateral temporal cortex, whereas approximately one-third of the axons remain in the ipsilateral side. B, The applied stimuli. Left monaural, right monaural, and binaural 1-kHz continuous, 2-min-long tones were presented, amplitude modulated (AM) at 41.1 Hz for the left ear and at 39.1 Hz for the right ear. C, Averaged steady-state responses (2 cycles illustrated) in a control subject (top row) and a ROBO1 subject (bottom row) to ipsilateral and contralateral inputs. Approximately 2000 single responses were averaged, time locked to the phase of the modulation; the traces were filtered from 32 to 48 Hz. The same amplitude-modulated tone was presented monaurally (solid lines) and during binaural presentation (dashed lines; comparable with the arrows in B). For binaural stimuli, responses to left- and right-ear inputs were extracted on the basis of different modulation frequencies of the stimuli. D, Mean ± SEM suppression of responses during binaural listening. Values indicate (in percentage) how much smaller the responses were in the binaural than in the monaural situation.

Figure 3.

Dependence of the ipsilateral suppression on lymphocyte ROBO1 mRNA level in ROBO1 subjects. ROBO1 expression levels are shown as ΔΔCt values; the higher number indicates stronger expression (Ct in qRT-PCR were normalized first against the geometric mean of GAPDH and 18S rRNA and then against the highest individual value). The regression lines and the correlation coefficients show that decreasing gene expression is associated with weaker ipsilateral suppression in both hemispheres. The additional source analysis of eight subjects in the LH and nine subjects in the RH confirmed the relationship; the subjects with unreliable source modeling are colored gray.