These findings support two basic conclusions: 1) N100 amplitude is a robust heritable trait; 2) Reduced N100 amplitude, which has been observed repeatedly in schizophrenia patients, is also present in a subset of unaffected first-degree relatives. This simple measure of auditory sensory processing therefore appears to meet two essential criteria of a viable endophenotype for genetic studies. A similar conclusion can not be drawn for N100 gating. Although the gating measure was heritable, it was not abnormal in either schizophrenia patients or their unaffected family members. However, given our finding of reduced S1 but normal S2 amplitude, we cannot entirely rule out the possibility of two opposing abnormalities - impaired excitation of the N100 cortical generator and impaired gating of the response to repeated stimulation - despite the lack of a significant S2/S1 difference.
It is notable that a familial N100 amplitude deficit was not apparent when the entire first-degree relative sample was contrasted with the entire CCS sample. The familial deficit emerged only when subjects were separated into those with or without co-morbid psychiatric or substance use conditions. Family members with co-morbid conditions had reduced N100 amplitudes relative to CCS with similar co-morbidity. Indeed, in this group, the familial impairment was equal to that seen in patients. However, family members without co-morbidity were indistinguishable from CCS without similar co-morbidity. Importantly, although the presence of co-morbid conditions was associated with reduced N100 amplitude in family members, there was no similar reduction associated with co-morbidity in CCS. One explanation for this pattern of findings is that the presence of psychiatric co-morbidity effectively identified those unaffected family members who carried the highest genetic loading for schizophrenia vulnerability. This would be consistent with our findings that 1) "broad" family members who had co-morbid conditions also had the highest ratings of schizotypy, and 2) among the “broad” family members, it was specifically those with other Axis I diagnoses (i.e., mood and anxiety disorders) who differed from comparable CCS. In the absence of other psychiatric co-morbidity, substance use may be more environmentally influenced and, therefore, less strongly associated with genetic vulnerability to schizophrenia. An alternative explanation is that the genetic risk of schizophrenia was comparable in both "broad" and "narrow" relatives, but that the joint effect of schizophrenia vulnerability and co-morbid conditions resulted in a significant N100 decrement. We consider this explanation less likely, as we would then expect to see evidence of smaller independent effects of psychiatric co-morbidity in the "broad" CCS and genetic vulnerability in the "narrow” family members, neither of which was present.
As noted above, four of five previous studies of N100 amplitude in unaffected family members (28
) reported normal responses in the relatives. Our sample, which was obtained from seven collaborating sites nationwide, was larger and therefore had more power to detect small effects. Nevertheless, we also had normal responses in our unaffected family member sample as a whole. The critical difference between this and previous studies was that we did not assume that family members were all the same and therefore could be grouped together for analyses. Unaffected family members are known to have an increased prevalence of behavioral and psychiatric conditions other than schizophrenia (48
). We therefore adopted a very liberal set of inclusion criteria for these subjects and recruited a similarly broad sample of CCS. Most importantly, we considered these co-morbid conditions to potentially represent associated phenotypes within the at-risk sample and therefore split the sample based on the presence or absence of these co-morbid conditions. Previous studies tended to either exclude family members with history of other Axis I psychiatric disorders or substance abuse (e.g., 29
), or to group them together with relatives without these conditions (e.g., 31
). If, as our data suggest, these co-morbid conditions are not independent of genetic vulnerability then excluding carriers, or co-mingling carriers and non-carriers, would necessarily reduce the power to detect significant differences.
There are, however, reasons to believe that our observed effect sizes are at the low end of what might be expected for N100 amplitude. The COGS database likely reflects an ascertainment bias that skewed the composition of the sample. The COGS design required that both parents and one healthy sibling of each patient proband participate in the research. This necessarily selected for both family members and patients who maintained a cohesive relationship with each other and were motivated to participate in an extensive experimental protocol. This cohort likely represents a somewhat atypical and less severe manifestation of the illness. This ascertainment bias probably also contributed to our failure to observe an N100 gating deficit. Our CCS sample exhibited S2/S1 ratios that closely mirrored previous published values (22
), but our patients had substantially lower ratios (18
In addition, the stimulus protocol that we employed was designed to investigate P50 auditory gating, not N100 amplitude. Typical N100 studies use pure tone stimuli of substantially longer duration (e.g., 50ms), shorter inter-stimulus intervals (ISI) (e.g., 1–2 seconds) and tasks that require subjects to attend to the stimuli (e.g., target detection tasks). Under these conditions, the effect size of the N100 amplitude decrement in patients is typically around 1.0 (e.g., 15
). It is likely that physical stimulus characteristics and task demands are more critical than inter-stimulus interval. Studies of N100 recovery cycle (23
) have consistently demonstrated schizophrenia decrements at all ISIs > 1.5–2.0 seconds and normal responses at ISIs < 0.5–1.0 seconds. Within the range of 2.0–10.0 second ISI, N100 amplitudes tend to increase with longer ISI, but patient decrements remain relatively constant. However, an experimental protocol in which subjects attended to longer duration pure tones would produce more robust N100 decrements than those observed here.
It is important to emphasize that, beyond its utility as a quantitative endophenotype for human genetic studies, the auditory N100 also provides a way to investigate the neural substrates of schizophrenia. This evoked potential abnormality reflects impairment in early auditory sensory processing localized to primary and secondary auditory cortex (52
). Analogous responses can be elicited in simple mouse models, allowing both its neurochemical and genetic underpinnings to be explored via pharmacological probes and/or reverse genetic strategies (e.g.,53
). This will be especially important as we strive to move beyond the initial identification of schizophrenia candidate genes to a fuller understanding of their functional significance (2