In this study we report decreased SPEM velocity gain and PPI of startle in children with 22q11DS compared to typically developing children of the same age. Decreased SPEM in 22q11DS is a novel finding because to date no studies have reported on SPEM performance in 22q11DS children. This study replicates the previously reported finding of decreased PPI in 22q11DS subjects (Sobin et al, 2005
We further demonstrate a significant interactive effect of COMT158 allele status×proline level on SPEM; proline negatively affects SPEM in 22q11DS subjects who are hemizygous for the COMTmet allele. With regard to PPI such interactive effect of proline and COMT158 was not found; however a trend towards a main effect of the COMT158 genotype was demonstrated; individuals with the COMTmet allele showed decreased PPI performance.
Individuals with the 22q11DS carry one, instead of two copies of the genes that reside in the deleted region. This abnormal situation may have two consequences: first, for some genes one copy may be insufficient to generate adequate amounts of gene product. Second, any functional variant in the remaining allele of these affected genes can be expected to have a critical effect, as there is no compensating normal allele. Thus, in 22q11DS, the enzyme COMT may be generated in lesser amounts and in addition, the common functional variant COMT158 has a dominant impact on the enzyme’s effectiveness.
Consistent with both the central hypothesis of this study and previous findings in mice (Paterlini et al, 2005
), we found that the effect of proline on SPEM was contingent upon the COMT158
genotype. These findings are also partly consistent with recently reported results (Raux et al, 2006
), although the main outcome measure in that study was psychiatric diagnosis rather than SPEM. Findings of both studies indicate a negative effect of high proline on brain function in 22q11DS subjects with the COMTmet
genotype. The consistency of these results is not surprising given the reported association between SPEM abnormalities and psychosis in numerous studies (Thaker, 2008
). We did not analyze psychosis as a phenotypic outcome because the young age of the study sample excluded a reliable partition of subjects in this respect. Raux et al
, also reported that increased proline levels were significantly associated with lower FSIQ, independent of the COMT158
genotype. Our post hoc
analyses did not replicate this correlation.
Although P50 and PPI are both thought to reflect the brain’s capacity to filter information, they tend to be uncorrelated within individuals and, therefore, are likely to be mediated by different neurobiological mechanisms (Schwarzkopf et al, 1993
; Braff et al, 2007
). For PPI, a significant regulatory influence of the striatum is suggested by the findings of impaired PPI in Huntington’s disease patients (Swerdlow et al, 1995
) and in animals with striatal lesions (Kodsi and Swerdlow, 1997
). In contrast, several studies indicate a more critical (though not exclusive) role for the PFC (Grunwald et al, 2003
; Knight et al, 1999
; Nagel et al, 2008
; Kurthen et al, 2007
) and the hippocampus (Grunwald et al, 2003
; Tanabe et al, 2006
; Tregellas et al, 2004
) in the regulation of P50 gating and SPEM. Although the anatomical loci of control for P50 gating and SPEM may overlap, the regulatory neurotransmitter systems are thought to be different. For P50, there are strong indications that cholinergic transmission is an essential part of its regulation (Adler et al, 1992
). In SPEM, a regulatory role for DA is strongly suggested by the fact that the COMT158
allele affects SPEM performance in both healthy subjects and schizophrenia patients (Thaker et al, 2004
). Similarly, DA signaling is likely involved in the regulation of PPI, as the administration of DA agonists attenuates PPI (Hutchison and Swift, 1999
; Abduljawad et al, 1998
) and a common functional variant of the DA D3 receptor significantly affects PPI in humans (Roussos et al, 2008
Given the putative roles of the hippocampus and PFC in regulating SPEM, it is notable that evidence now supports an influence of proline on excitatory neurotransmission in these regions. A high affinity proline transporter has been identified on a subset of glutamatergic neurons (Crump et al, 1999
; Renick et al, 1999
; Fremeau Jr et al, 1992
) and proline-mediated modulation of glutamatergic neuron terminals has been demonstrated (Cohen and Nadler, 1997a
; Martin et al, 1992
). Regions with the highest levels of proline transporter expression include hippocampal (Schaffer collateral commissural and lateral perforant pathway) and corticostriatal pathways (Renick et al, 1999
). Importantly, in a PRODH knockdown study, it has been shown that PRODH deficiency not only alters hippocampal glutamatergic transmission, but also significantly potentiates DA release (Paterlini et al, 2005
In summary, the hypothesized model holds that the action of proline on specific glutamatergic neurons in the hippocampus induces two events: interference with glutamate transmission and secondary potentiation of the DA response in the PFC (). Under this model, one can anticipate that the effect of proline will be accentuated in individuals with the low activity COMT enzyme, given their decreased capacity to effectively catabolize the augmented DA response in the forebrain. Our finding that proline significantly affects SPEM in the low activity COMTmet allele, but not in the high activity COMTval allele subgroup is both consistent with and supportive of this model.
Figure 2 Schematic representation of the hypothesized model. High proline levels induce glutamatergic signaling in the hippocampus. Increased glutamatergic tone causes a release of DA in the PFC. In 22q11DS subjects hemizygous for COMTmet (in blue), the inefficiency (more ...)
The effect reported in this study accords with the hypothetical model of an inverted U
-shape relationship between DA signaling and prefrontal function (Mattay et al, 2003
; ). In this model, optimum prefrontal function occurs within a restricted range of DA signaling, with decreased function in conditions of too high or too low DA availability.
Paterlini et al (2005)
, reported that PRODH deficient mice with elevated proline levels demonstrated significantly increased expression levels of COMT in the frontal cortex. Similarly, high proline levels may have led to an upregulation of COMT expression in the 22q11DS subjects of this study. However, given the haploidy of COMT in individuals with 22q11DS, the capacity for upregulation may be diminished, thereby restricting the effect of this compensatory mechanism. In addition, in those with the low activity COMTmet
allele, a further reduction of the net effect of this compensatory mechanism can be expected.
Finally, the attenuating effect of the COMTmet
allele, but not of proline, on PPI in this study is likely the result of haploinsufficiency, with the low activity COMT allele increasing DA availability in the striatum. Apparently, this effect occurs despite COMT expression being lower in the striatum than the PFC (Matsumoto et al, 2003
). The absence of any influence of proline on PPI is consistent with the fact that proline appears to potentiate DA transmission in the murine cortex, but not in the striatum (Paterlini et al, 2005
A limitation of the current study is the absence of plasma proline values from age-matched controls. This makes it difficult to delineate, which 22q11DS subjects have elevated plasma proline levels and to draw firm conclusions regarding the effects of increased proline. If all 22q11DS subjects have abnormally elevated proline levels, then an effect of proline on either of the physiological measures cannot be entirely ruled out. However, it is unlikely that this is the case for two reasons: (1) Abnormally high levels are not reported in all, but in approximately 50% of 22q11DS subjects (Goodman et al, 2000
). (2) The variance of proline values in 22q11DS cases and controls shows considerable overlap (see Supplementary Figure 3
), suggesting that many 22q11DS children in this study have plasma proline levels within the normal range.
Another limitation is that it is unknown to what extent changes in peripheral plasma concentrations of proline correlate with similar changes in proline concentration in the brain. This is an issue that needs to be addressed in future studies.
Findings of this study may contribute to our understanding of the pathophysiological mechanisms that lead to the increased vulnerability for psychosis in 22q11DS subjects. This finding not only is relevant to our understanding of 22q11DS-related psychopathology, but also contributes to our understanding of how factors such as proline influence DA metabolism and transmission in the brain.