Anticipation and CAG*CTG repeat expansion in schizophrenia and bipolar affective disorder

Unstable repeat expansion in major psychiatric disorders: two decades on, is dynamic DNA back on the menu?

For a period in the mid-1990s, soon after the discovery of the involvement of trinucleotide repeat expansions in fragile-X syndrome (both A and E), Huntington's disease, myotonic dystrophy, and a number of hereditary ataxias, there was a clear sense that this new disease mechanism might provide answers for psychiatric disorders. Given the then failures to replicate initial genetic linkage findings for schizophrenia (SCZ) and bipolar disorder (BD), a greater emphasis was placed on the role of complex and non-Mendelian mechanisms, and repeat instability appeared to have the potential to provide adequate explanations for numerous apparently non-Mendelian features such as anticipation, incomplete penetrance, sporadic occurrence, and nonconcordance of monozygotic twins. Initial molecular studies using a ligation-based amplification method (repeat expansion detection) appeared to support the involvement of CAG•CTG repeat expansion in SCZ and BD. However, subsequent studies that dissected the large repeats responsible for much of the positive signal showed that there were three main loci where CAG•CTG repeat expansion was occurring (on 13q21.33, 17q21.33-q22, and 18q21.2). None of the expansions at these loci appeared to segregate with SCZ or BD, and research into repeat expansions in psychiatric illness petered out in the early 2000s. The 13q expansion occurs within a noncoding RNA and appears to be associated with spinocerebellar ataxia 8 (SCA8), but with a still unexplained dichotomy in penetrance - either very high or very low. The 17q expansion occurs within an intron of the carbonic anhydrase-like gene, CA10. The 18q expansion is located within an intron of the TCF4 gene. Mutations in TCF4 are a known cause of Pitt-Hopkins syndrome. Also, pertinently, genome-wide association studies have shown a well-replicated association between TCF4 and SCZ. Two decades on, in 2016, it appears to be an appropriate juncture to reflect on what we have learned, and, with the arrival of newer technologies, whether there is any mileage to be made in revisiting the unstable DNA hypothesis for psychiatric illness.

Paternal age effect on age of onset in bipolar I disorder is mediated by sex and family history

This study investigated for the first time in the psychiatric literature the effect of parental age on age-of-onset (AO) in bipolar I disorder (BPI) in relation to proband sex and family history (FH) for major psychoses in a sample of 564 BPI probands. All probands, 72.68% of their first-degree and 12.13% of their second-degree relatives were directly interviewed. The FH-method was used for all unavailable relatives. The diagnoses were made according to DSM-IV(TR) . The impact of parental age on proband early/late AO was evaluated through logistic regression with the cut-off for early AO determined through commingling analysis. We found evidence for a significant influence of increasing paternal age, and especially age ≥ 35 years, on AO of BPI disorder in the total sample (OR = 0.54, CI: 0.35-0.80), in the female subsample (OR = 0.44, CI: 0.25-0.78), in the sporadic subsample (OR = 0.64, CI: 0.38-0.95), and in the subsample with FH of recurrent unipolar major depression (Mdd-RUP) (OR = 0.55, CI: 0.34-0.87). No significant effect of paternal age on disease AO was found in patients with FH of bipolar (BP), schizoaffective disorders (SA), or schizophrenia (SCZ), nor in males. Mean age was significantly higher in fathers of sporadic cases and of cases with FH of Mdd-RUP than in fathers of cases with FH of BP/SA/SCZ (P = 0.011). Maternal age had no significant effect either in the total sample or in subsamples defined by proband sex or FH. In conclusion, in our sample increasing paternal age lowered the onset of BPI selectively, the effect being related to the female sex and FH-type.

Unstable genes unstable mind: beyond the central dogma of molecular biology

Schizophrenia has a polygenic mode of inheritance and an estimated heritability of over 80%, but success in understanding its genetic underpinnings to date has been modest. Unlike in trinucleotide neurodegenerative disorders, the phenomenon of genetic anticipation observed in schizophrenia or bipolar disorder has not been explained. For the first time, we provide a plausible molecular explanation of genetic anticipation and pathophysiology of schizophrenia, at least in part, with supporting evidence. We postulate that abnormally increased numbers of CAG repeats in many genes being expressed in the brain, coding for glutamine, cumulatively press for higher demand of glutamine in the respective brain cells, resulting in a metabolic crisis and dysregulation of the glutamate-glutamine cycle. This can adversely affect the functioning of both glutamate and GABA receptors, which are known to be involved in psychosis, and may also affect glutathione levels, increasing oxidative stress. The resulting psychosis (gain in function), originating from unstable genes, is described as an effect "beyond the central dogma of molecular biology". The hypothesis explains genetic anticipation, as further expansions in subsequent generations may result in increased severity and earlier occurrence. Many other well described findings provide proof of concept. This is a testable hypothesis, does not deny any known facts and opens up new avenues of research.

Can epigenetics help in the discovery of therapeutics for psychiatric disorders, especially schizophrenia?

Psychiatric disorders have a heterogeneous biological basis, where environmental factors interplay with non-mendelian genetics to produce complex cognitive/behavioural syndromes such as schizophrenia. Recent findings indicate a proportion of schizophrenia is associated with genomic copy number variation, suggesting that alteration of gene expression levels rather than direct mutation may play a role. Epigenetic mechanisms could be the crucial link between external stimuli and gene expression, influencing schizophrenia risk. These are dynamic reversible systems that offer much promise as targets for future therapies.

The influence of parental age and gender on anticipation in familial B-cell malignancies

Anticipation occurs when a disease manifests at an earlier age and/or with an increased clinical severity in the next generation. Its relationship with parental age and gender is relevant to the patterns of genetic transmission of familial B-cell neoplasms. One hundred and sixty pairs [44 non-Hodgkin's lymphoma (NHL), 38 Hodgkin's disease (HD), 48 HD/NHL, and 30 CLL] were analyzed retrospectively for presence of anticipation in paternal (PT), maternal (MT), and overall transmission. Overall mean anticipation measured -23.0 yr, and varied between -18.93 and -26.46 yr with no significant difference among different diseases, except between CLL and mixed HD/NHL (mean difference -7.68 yr, p = 0.03). A significant Pearson correlation (PC) between the parental age at conception and anticipation was found for all malignancies (PC = -0.339, p < 0.0001), with the exception of mixed HD/NHL pairs of MT. Higher PCs were observed with PT than MT for all diseases. Anticipation manifests in all familial B-cell malignancies analyzed and it correlates with the parental age at conception. Although less prominent than with neurological disease, this phenomenon indicates a possible germline inheritance of B-cell malignancies and a common genetic basis for HD and NHL.

Anticipation in families with Hodgkin's and non-Hodgkin's lymphoma in their pedigree

Anticipation is an earlier onset and/or increasing severity in successive generations. This study was conducted to determine whether anticipation occurs in families that exhibit both Hodgkin's (HD) and non-Hodgkin's (NHL) lymphoma in their pedigrees. Nine published reports of multi-generational lymphoma and 33 previously unreported families with both lymphomas were analysed for evidence of anticipation. The difference between age at onset for each affected related pair was tested against the null hypothesis that there is no difference in age at onset. Differences between disease-free survival in affected generations were determined. These analyses were also conducted separately using only parent - child pairs with an age of onset above 25 years in an effort to avoid ascertainment bias. Age at onset in studied cases was also compared with the HD and NHL series from the Surveillance Epidemiology and End Results (SEER) Program of the US National Cancer Institute. The mean age at onset in the child and parent generations of all case families (60.2 and 35.7 years, respectively) and in the selected pairs (61.5 and 45.6, years) were significantly different (mean difference -24.5 years; P < 0.00001, and -15.9 years, P < 0.00001, respectively). Mean anticipation for parents with HD and children with NHL was -6.8 years (P = 0.01) for the unpublished and -14.4 years (P = 0.002) for the published families (overall anticipation -10.1 years). Mean anticipation for parents with NHL and children with HD was -34.4 years (P < 0.0001) for the unpublished and -32.7 years (P < 0.0001) for the published families (overall anticipation -34.2 years, P < 0.0001). The signed rank test rejected the null hypothesis which stated that there was no difference in age at onset between parents and children for overall, as well as selected pairs (P < 0.00001). The null hypothesis was also rejected for both the parents with HD/children with NHL group and the parents with NHL/children with HD group pairs (P < 0.0001). Age at onset distributions were significantly different for all generations with HD or NHL when compared to the SEER population (P < 0.00001), except for the parents with NHL, which showed no difference. In addition, this study reports four previously unpublished families with three generations of lymphoma in their pedigrees. These data suggest that anticipation occurs in families that exhibit both HL and NHL and that both neoplasms may have a common genetic basis.

In search of anticipation in unipolar affective disorder

Controversial evidence exists regarding the presence of the phenomenon of anticipation in affective disorder. To further evaluate this hypothesis on the unipolar pattern of the disease, we examined 21 two-generation pairs of first and second degree relatives with unipolar recurrent major depression. Biases from index-patient and from unaffected sibs were taken into consideration. A significant difference in the age at onset and episode frequency (as measure of disease severity) between parental and offspring generation was observed. The median age at onset of the parental generation was 37+/-8.2 years compared to 22+/-8.3 years in the offspring generation (p=0.001). The offspring generation also experienced an episode frequency two times greater than the parent generation (p=0.001). Anticipation was demonstrated in 95% of pairs regarding age at onset and in 84% of pairs in episode frequency. However, the observation of a birth cohort effect may possibly explain the differences in age at onset between generations in our sample.