Psychiatr & Psychobio/ (1990) 5, 283-287 © Elsevier, Paris Editorial Genetic Research in Psychiatry: Update from the Society of Biological Psychiatry and the American Psychiatrie Association, New York, May 1990 MA Crocq, F Duval, JP Mâcher Centre Hospitalier Spécialisé, 68250 R ouffach, France Summary — A stimulating variety of papers on genetic and elinical research in psychiatry was discussed at the latest meeting of the American Psychiatrie Association and the Society of Biological Psychiatry in May 1990 in New York. Conflicting results point out that extreme caution must be taken in interpreting linkage studies of psychiatrie disorders. Difficulties stem from complex rnodels on inheritance as well as the genetic heterogeneity of psychiatrie disorders. Progress lias been made in the approach to the régulation of receptor genes that hâve been implicated in the pathogenesis of psychiatrie disorders. In some cases, gene régulation may be tissue- dependent, as is suggested by the alternative splicing of D, receptor mRNA. genetics / linkage / manic-depressive illness / Amish / Alzheimer’s diseuse / schizophrenia / D2 receptors / American Psychiatrie As­ sociation / Society of Biological Psychiatry Résume - Recherche génétique en psychiatrie : mise à jour de la société américaine de psychiatry biologique et de l’association améri­ caine de psychiatrie. Les congrès annuels de l'association américaine de psychiatrie et de ta société américaine de psychiatrie biologi­ que se sont tenus en mai 1990 à New York. Les recherches génétiques (études de liaison, études chez des jum eaux, biologie moléculaire des gènes des récepteurs impliqués dans les maladies mentales) ont occupé une place de choix. Les controverses se poursuivent autour des liaisons qui ont été rapportées antérieurement entre la psychose maniaco-dépressive, la schizophrénie, la maladie d ’Alzheimer, et des marqueurs génétiques. Ces difficultés sont expliquées par des caractères en partie propres à la psychiatrie, tels que la complexité des modes de transmission génétique, l ’hétérogénéité génétique et clinique des troubles, la difficulté de définir les phénotypes et la révélation tardive de certaines affections. La connaissance de la régulation des gènes impliqués dans les maladies mentales, notam ­ m ent celle des gènes des récepteurs D ,, a aussi progressé notablement. génétique / liaison / psychose maniaco-dépressive / Amish / maladie d’Alzheimer / schizophrénie / récepteurs 1), / association amé­ ricaine de psychiatrie / société américaine de psychiatrie biologique Introduction Genetics has emerged as a highly fruitful area of research in psychiatry. It has contributed to the évo­ lution of psychiatrie nosology by indicating the étio­ logie heterogeneity of seemingly identical elinical conditions, and conversely by suggesting the com- mon nature of apparently unrelated disorders. Research strategies hâve varied over the years. They hâve relied on twin and adoption designs, familial ségrégation analyses and chromosomal studies. More recently, linkage analyses were made possible by the identification of polymorphie DNA fragments (restriction fragment length polymor- phisms). Finally, today’s molecular biology aims at identifying receptor genes and unravelling the régu­ lation of their expression in the brain. The sheer number of présentations at the latest annual meetings of the Society o f Biological Psy­ chiatry and the American Psychiatrie Association in May 1990 in New York confirms that genetic research is an active field. No particular research strategy is outdated ; each one is particularly suited to approaching certain questions. Regardless of the method used, thc field of psychiatry lias its own ad- ditional complexities, such as thc difficulty of iden- tifying phenotypes and cases. The l'ata morgana of linkage in manic-depressive illness Manic-depressive illness has been linked with the X chromosome in independent samples in thc USA, Belgium, Israël and Sardinia (Winokur et al, 1969; Mendlewicz et al, 1974; Baron et al, 1977 ; DelZom- po, 1984). Linkage has been established notably with markers o f thc terminal fragment (28) of the long arm (q) of the X chromosome, including pro­ tan and deutan color blindness, and G6PD deficien- cy. A report by Egeland et al (1987) supported the existence of a gene conferring a strong prédisposi­ tion to bipolar affective disorder linked to two loci on the short arm of chromosome 11 ( 11 p 15), the Harvey-ras-1 oncogene locus (HRAS1) and the insulin (INS) locus. These studies had a noticeable impact on psy­ chiatrie thinking. They came as thc logical molecu- lar biology sequel to the adoption and twin studies of the previous décades and suggested that the iden­ tification of m ajor susceptibility genes was just around the corner. However, linkage studies ol psy­ chiatrie disorders hâve had notorious difficulties withstanding the trial o f time. Controversies were revivcd by the re-analysis of the linkage between chromosome 1 lp loci and affective disorders in thc Old Order Amish, presented at the APA meeting by Kelsoe (1990), in the wakc of thc article published in the 16 november 1989 issue o f Nature. As far as is known, the chromosome 11 linkage has not been confirmed. The initial results of Egeland (1987) were based on restriction fragment length polymorphism (RFLP) studies at the HRAS-1 and INS loci in an 81-member pedigree (pedigree 110) with 6 ill pro- bands front thc Old Order Amish community in southeastern Pennsylvania. The Amish are an ex- ceptional study environment because their commu­ nity is extremcly endogamous; they arc a genetic isolate with minimal heterogeneity. Kelsoe presented results on an extended version of pedigree 110. In addition to the 81 members o f the original core pedigree, the new pedigree includes a 31-member (8 affected) “ right” extension and a smaller six- member (2 affected) “ left extension” . Furthermorc, two members o f the core pedigree had their clini- cal status changed front unaffected to affected, and thc genotyping of 10 unaffected individuals of the core pedigree was completed. With this new data, linkage to an affective disorder locus can be ex- cluded to 5 centimorgans foreither HRAS1 or INS, in the total 120-member pedigree and in the right extension alone. The new data, notably the présence of obligate recombinants in the core pedigree, has causcd thc lod score to drop front an original value ol 4.08 to non-significance (at 0 = 0 in the two-point analysis between HRAS1 and bipolar affective disorder). Besides the importance o f diagnostic follow-up and ol at least yearly visits of ail the pedigree mem­ bers, what particular or general conclusions can be drawn Iront the Amish study? First, soute factors are related to the genetic paradignts that are pur- sued. It is important to bear in mind that linkage norntally occurs in relation to a putative m ajor sus- ceptibility gene. Linkage is a strategy directed toward the search for a major Mendelian suscepti- bility locus, as opposed to additive “ G altonian” polygénie effects. Lack of réplication in large popu­ lations casts doubt on the universal validity of a single-gene etiology. Secondly, other factors such as the problents of genetic and clinical heterogeneity, and définition of penetrance and caseness, are closely dépendent upon the particular psychiatrie disorder that is studied. There are scveral possible interprétations of the non- rcplication in the Amish group: either the initial report is simply not reproducible, or else there is genetic heterogeneity, which means that at least two different genes may be rcsponsible for affective ill­ ness in the pedigree. It has been suggested by Egeland et al (1987) that several genes may be rcsponsible for affective disorder within the Am­ ish population itself, as several of the founding cou­ ples had mental illness among their first and second génération descendants. Thirdly, questions arise concerning thc exception- al genetic isolate that is studied and the gcneraliza- tion of linkage results beyond this highly inbred community. The Amish are descended frorn the Swiss anabaptists who appeared in the wake o f the 16th century religious reformation and later emigrated to Southern Gcrmany and Alsace to flee persécution. It would be interesting to investigate whether (lie chromosome 11 linkage could be repli- cated in thc European parent population, which is considerably less inbred. It may be noted that the American Old Order Amish population is exception- ally inbred; genes l'Iow out when subjects marry out- side the community, but almost no genes flow in. The very closcd American Amish community which is reported herc might be expected to differ from its European forefathers by the combined resuit of two effects: the “ founder effects” (oniy thosc who emigrated contributed to the new genetic pool) and the “ genetic drift” (the genetic pool of the new com- munity evolved due to different fertility rates). Three paths to linkage ln general, the search for linkage can be guided by three possible strategies (Gershon, 1990). The first strategy is to investigate linkage with “ candidate genes” , such as those involved in neurotransmis­ sion or neuroreceptors. However, this approach has so far failed to show a m ajor susceptibility locus. For instance, linkage with the D2 receptor région ( 11 q22-23) has been excluded in bipolar and schizophrénie pedigrees. Similarly, Lentes et aI (1989) failed to show linkage with beta-1 (chromo­ some 10q24) and beta-2 (chromosome 5q31) adrenergic receptor genes in manie dépressive pedigrees. The second strategy is the exploration of markers in the région of a rare cytogenetic event found in rare pedigrees. This has been the case in schizophre- nia where a report of a trisomy implicating the prox­ imal long arm of chromosome 5 (5q 11-13) inherited with schizophrenia (Basset et al, 1988) was followed by the finding that schizophrenia might be linked to a gene locus with a dom inant susceptibility al- lele on chromosome 5q in British and Icelandic fa- milies (Sherrington et al, 1988). However, négative findings were published at the same time by other researchers in a Swedish population (Kennedy et al, 1988). Kennedy et al (1990) presented additional results at the American Psychiatrie Association meeting, and confirmed the absence of linkage to schizophrenia of the genes for tyrosine hydroxylase and the D2 receptor (chromosome 11); other areas of interest were also excluded for linkage in the same Swedish population : the genes for nerve growth fac­ tor, pro-opiomelanocortin, and the pseudoau- tosomal boundary (MIC2) of the sex chromosomes. The third strategy is systematic mapping, and several papers based on this approach were present­ ed in New York (Berrettini et al, 1990; Byerley et al, 1990; Polymeropoulos et al, 1990). Systematic mapping, leaving no région unexplored, is now pos­ sible, mainly because of the availability of poly­ m o rp h ie in fo rm a tiv e m a rk e rs d is trib u te d throughout the genome and the ability to screen the whole genome in a less costly and more automa- tized manner. It had been estimated as carly as 1980 (Botstein) that 150 evenly spaced markers would en- sure that no disease locus could be more than a 10% recombination distance from a marker locus. Preliminary results on systematic mapping of genes for manic-depressive disease were reported in May in New York. Berrettini et al (1990) studied a sériés of 20 bipolar pedigrees consisting of 375 inform a­ tive subjects (160 bipolars, unipolars or schizoaffec- tives) under the assumption of a dominant gene, 1 % frequency and 85% penetrance. Results with 20 DNA markers exclude linkage to 5p (short arm of chromosome 5), 5 q ll- q l3 , 5q32-34, 11 p 15 (région reported to contain the susceptibility gene for bipolar disease in the 1987 Old Order Amish study) and llq22-23 (D2 receptor région). In the same line, Byerley et al (1990) conducted a systemat- ic genomic mapping study using over 250 DNA mar­ kers and also reported absence of linkage on chromosome 5,11 and 19 in eight manic-depressive kindreds and eight schizophrénie families. Late onset of Alzheiiner’s Disease. hinders linkage Several Iinkages hâve also been reported for Al- zheimer’s disease (AD). A number of authors exa- mined chromosome 21 because Alzheimer-like neuropathology develops among patients with Down’s syndrome. St George-Hyslop et al (1987) reported linkage o f the rare disorder Familial Al­ zheimer Disease (FAD) to anonymous polymorph­ ie DNA sequences mapping to the proximal long arm of chromosome 21. Positive LOD scores on 21q were also reported in FAD families in England by Goate et al (1989). However, some authors could not confirm these results ; others even excluded link­ age to the same markers by mapping other pedigrees (Pericak-Vance et al, 1988; Schellenberg et al, 1988). More recently, Roses et al (1990) reported positive LOD scores between AD and several m ar­ kers on the long arm of chromosome 19 in an en- larged set of families. In this case too, genetic heterogeneity, ie multiple predisposing genes result- ing in indistinguishable phenotypes, has been sug- gested to account for these discrepant or négative linkage results. In addition, clinieal heterogeneity has been suggested. One form o f the illness may be caused by a genetic del'ect on chromosome 21 and show early onset, typically in the fifth decade, while another more common form o f the disorder may be a different genetic defect and show later onset, typically around the âge of 60. However, proving genetic etiology and showing linkage mcet with considerably more difficulties in AD than in manic-depressive illness or schizophre­ nia. It has been said that a large proportion of sporadic cases argue against the presence of genet­ ic causes in most cases of AD. However, the dis­ cussion of results must take into account the particular characteristics of the disorder that is studied. Breitner et al (1988, 1989, 1990, and un- published data) hâve emphasized that the very late onset of AD may preclude the occurrence of disease in many relatives whose earlier death has censored the expression of any inherited disease tendency. A characteristic of AD is age-dependent expression and variable âge of onset. In the m ajority o f cases, onset occurs in the eighth and ninth decade and therefore only one or two family members are af- fected. Such a limited number o f affected subjects precludes the démonstration of linkage. It has been suggested that most AD cases may in fact be shown to hâve genetic causes. When appropriate allowance is made for censorship of disease tendency in rela­ tives by other causes of death, the risk of Alzheimer- like dementia in first degree relatives of unselected clinically diagnosed AD probands increases with âge and is about 50% over a lifespan o f 90 years. One cannot help noticing that this figure is the risk ex- pected in an autosomal dominant Mendelian illness. Because of the late onset of AD, other methods such as twin studies may prove particularly useful. A pi­ lot study in the National Academy of Sciences Registry of aging twin vétérans was reported by Breitner et al (1990). The study o f a sample of 442 pairs (884 subjects) suggests a 60% concordance rate for Alzheimer’s disease in monozygotic twin pairs. This concordance rate will likely increase with on- going longitudinal observation and exceeds prior estimâtes. New Approachcs to D2 receptor genes Our understanding of neurotransm itter action has been refined by molecular biology studies of recep­ tor genes expressed in the brain. This is particular­ ly the case with D2 receptors, which hâve been implicated in the pathogenesis of schizophrenia. Some data suggested an increased number of D2 receptors in the brains of schizophrénies. Two main types o f dopamine receptors, D, and D2, hâve been identified by pharm acological studies. Bunzow et al (1988) hâve reported the clon- ing of a rat cDNA (complementary DNA copied from mRNA by the enzyme reverse transcriptase) with the expression characteristics o f a D2 recep­ tor. Todd et al (1990) reported that the overall or- ganization of this D2 gene is similar in rats, mice and man. It contains 8 exons (expressed DNA sequences), a very long first intron (intervening DNA sequence that is not translated in the poly­ peptide gene product), and spans at least 50 kilo- bases in the rat. In contrast to the genomic organization of most G-protein-coupled receptors, this D2 gene contains multiple introns. The in- teresting finding was that two forms of D2 mRNA were detected in various brain régions, and it has been suggested that the D2 receptor gene identified by Bunzow gives rise to at least two m ajor D2 receptor mRNA subtypes (Todd et al, 1989, 1990). The two forms are generated by alternative splic- ing of an additional exon (exon 6) within intron 5 ; consequently the two resulting D2 proteins differ by the insertion of a 29 amino acid sequence. Splic- ing refers to the stage o f mRNA processing where- by introns are removed. The relative proportion of the two alternative transcripts varies 25-fold in different brain tissues. The exon 6 containing mRNA predom inated in areas where D2 receptor number is highest-basal ganglia, anterior pituitary, and olfactory bulb. These findings imply the presence of tissue-specific factors that control not only D2 gene transcription, but also alternative splicing of the pre-mRNA. Knowledge of the genetic organization o f D2 receptors is of obvious im por­ tance in schizophrenia and neuroleptic drug treatments. New methods for the approach to neurons ex­ pressing D2 receptors were also discussed. In situ hybridization of D2 receptor mRNA was reported in the prim ate brain by M eador-W oodruff et al (1990) after being reported in rats a short time ago (M eador-W oodruff et al, 1989). This technique compléments studies using receptor autoradiogra- phy and PET ; it reveals cell bodies and allows the study of earlier events in receptor synthesis. Régu­ lation at the gene transcription or translation level might thus be explored. Conclusion No one doubts that genetics is a promising field in psychiatry. From pedigree studies to the molecular biology of receptor genes expressed in the brain, no particular approach is outdated. Each one is ideal- ly suited tow ard investigating spécifie questions. Some results that were first presented a few years ago become the object of renewed controversy as recent findings emerge. Particular caution must be taken in interpreting the results of linkage studies until models o f inheritance can be delineated with more clarity. Acknowledgments The authors are indebted to Ms Hein who kindly read the original manuscript. Référencés Berrettini W H, Detera-Wadleigh S, Goldin L, Martinez M, Hsieh W T, Gershon E (1990) Systematic genomic mapping o f genes for manic-depressive disease : stu- dies o f chromosomes 5 and 11. 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