The transmission of manic depressive illness—II. Segregation analysis of three sets of family data

Data were collected on six large multigenerational pedigrees, four ascertained through a proband with major depression and two ascertained through a proband with a bipolar form of illness. Diagnoses were made using the SADS-L structured interview and Research Diagnostic Criteria (RDC). Complex segregation analyses were conducted on the bipolar and the major depression pedigree sets using a model allowing for both major locus and polygenic inheritance; in these analyses a variety of diagnostic schemes and assumptions concerning the lifetime population prevalence were examined. Linkage analyses on standard markers were conducted using parameters for transmission of susceptibility to illness derived from the segregation analyses. Results of the segregation analyses were quite sensitive to the diagnostic and prevalence assumptions. In the pedigrees ascertained through probands with a bipolar form of illness, we were unable to discriminate between major gene and polygenic inheritance. The data were compatible with Mendelian major gene transmission of susceptibility to illness when bipolar and schizoaffective manic diagnoses were considered as affected and the lifetime population prevalence was between 0.04 and 0.06. Outside this narrow prevalence range, or when additional diagnoses, such as major depression or hypomania, were included as expressions of liability to disease, major gene transmission of susceptibility to disease could be rejected. Similarly, in the pedigrees ascertained through probands with major depression, it was not generally possible to discriminate between major gene and polygenic transmission of susceptibility to illness. For a diagnostic scheme including only major depression as a manifestation of susceptibility to illness, there was a narrow range of lifetime population prevalence values (female prevalence ranging from 0.20 to 0.25, male prevalence set to 1/2 female prevalence) which yielded results compatible with major gene transmission. Linkage analyses for all markers yielded negative or inconclusive results. In one bipolar pedigree a lod score of 1.65 was found with a marker in chromosome 1 recommending further studies of this chromosome.

Studies of the underlying components of affective disorders are particularly difficult because of the confounding effects of both genetic and environmental factors. Linkage analysis is a useful tool in delineating the etiology of affective disorders, as it is unlikely that linkage between behavioral traits and blood group polymorphisms could result from environmental effects. The present study used the robust Haseman and Elston sibpair method to analyze linkage between 24 genetic markers and affective disorder in 34 nuclear families from 25 pedigrees (195 people). The probands were ascertained as part of the ongoing NIMH Collaborative Depression Study. Indications of linkage between familial pure depressive disease and MNS and depression spectrum disease and ORM were found, as had been previously suggested. There was also suggestive evidence for linkage between the latter and GC. Results are discussed in terms of methodological differences with previous studies.

From the Yale-NIMH collaborative family study, the 1482 first degree relatives of 90 bipolar 1, and 163 major depression probands were examined to test the hypothesis that bipolar 1 and major depression are due to a single underlying genetic liability. We attempted to fit multifactorial-polygenic and single-major-locus multiple threshold models for sex and severity to the relatives in the major depression and bipolar 1 families. With relatives classified as affected only if they met criteria for major depression or bipolar 1, there was at best only marginal support for these models. Differences between these and previously reported results were examined in relation to differences in underlying assumptions. Additional analyses of these and other data from families of NIMH bipolar 2 and schizoaffective probands suggest that different methods of age adjustment, the relative placement of bipolar 2 and schizoaffective disorders in a hypothesized liability continuum and the inclusion or exclusion of sex thresholds were not primarily responsible for differences in the fit of genetic threshold models. Factors which do appear to be important are variability in rates between samples, the possibility of genetic heterogeneity, and the presence of a large secular increase in affective illness over the past three generations; the secular trend cannot be accommodated in the models of genetic transmission examined.

The single major locus (SML) model of Bucher and Elston (1981) was applied to data collected in a long-term follow-up and family study of major effective disorders. Pedigree segregation analysis was used to test the hypothesis that bipolar and unipolar disorders are phenotypic variants of the same genotype at a SML. The particular SML model examined did not adequately describe the familial pattern of these disorders in families of bipolar and unipolar probands. Although other SML models may be superior, it is likely that the existence of a SML will be obscured by genetic heterogeneity.

In the present study we tested the fit of the Single Major Locus (SML) hypothesis, using segregation analysis for single families of affective probands treated with lithium salts, with second degree relatives included. We tested the segregation pattern for every family with two different sets of parameters: one dominant for a group that did not relapse on lithium treatment and one recessive for a group that did.

We calculated the likelihood ratios for each family. The results of this study partially confirm the importance of outcome on lithium treatment in susceptibility to affective disorders. However, application of segregation analysis suggested that there is genetic heterogeneity that cannot be completely detected when using only the simple pharmacological criterion of outcome on lithium.

A long-term epidemiological genetic study was conducted in which all new patients were evaluated prospectively at the Foundation for Depression and Manic Depression and two Lithium/Affective Disorders clinics at the Columbia-Presbyterian Medical Center between the years of 1972 and 1978. All patients met Feighner, RDC and DSM III criteria for Major Depressive Disorder after initial clinical screening interviews and were further subtyped using the Fieve-Dunner 7-point criteria. All 604 probands and 90% of 2711 first-degree relatives were interviewed blindly by diagnosticians trained in the use of the SADS structured interview. Comulative morbid risk in parents, siblings and children of 490 bipolar probands was 15.6± 3% and 14.0± 1.7% in the first-degree relatives of 114 unipolar probands. A number of biological and genetic marker studies were simultaneously performed on samples of the overall population.

The enzymes catechol O-methyltransferase and dopamine β-hydroxylase, and dexamethasone suppression test (SDT) did not show any biological marker value for outpatients even though both enzymes were determined to have hereditability. The HLA system, monoamine oxidase and acetylcholinesterase segregated differently from normal controls in samples of the patient population. The positive association findings with monoamine oxidase and the HLA system conflicted with the positive findings of other investigators, leaving doubtful their biological marker value. Red cell acetylcholinesterase was found to be significantly lower in affective disorder patients than in controls. This positive association finding was recently replicated by MATHEWS et al. (1982) but needs further confirmation.

Using 28 blood group markers, a prior association study between the trait defining susceptibility to affective disorder and the genetic marker was positive for haptoglobin GC, and properdinfactor B, confirming earlier findings. Using the sib-pair method of the remaining 25 blood groups revealed that none other than peptidase A showed significant linkage with affective disorder since one significant finding is expected by chance.

We conclude from the overall morbid risk data and segregation analyses that bipolar manic- depressive illness is a spectrum disease inherited through a multifactorial mode of genetic transmission (which is not synonymous with polygenetic inheritance) with possible genetic heterogeneity and find no evidence for X-linkage. Additional studies with acetylcholinesterase, haptoblobin, GC, and properdin-factor B are needed to confirm their positive biological/genetic marker value suggested by our long-term epidemiological study.