GeneticsofAffectiveDisorders

ROBERTB.WESNER,MDandGEORGEWINOKUR,MD

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FromDepressiveDisorderseditedbyBenjaminWolberg&GeorgeStricker

Copyright1990byJohnWiley&Sons,Inc.

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TableofContents

TWINSTUDIES

ADOPTIONSTUDIES

FAMILYSTUDIES

GENETICMARKERSTUDIES

SUMMARY

REFERENCES

GeneticsofAffectiveDisorders

ROBERTB.WESNER,MDandGEORGEWINOKUR,MD

A regular finding in the study of affective disorders is that persons with

theseillnessesoftenclusterwithinfamilies.Allmajorpsychiatricdisordersappear

toresultatleastpartiallyfromaninheritedpredispositionandinthatmannerare

not unlike othermedical disorders such as diabetes or coronary artery disease.

The theory that affective disorders and othermedical disorders share common

geneticmodesoftransmissioncanbeadifficultconcepttofollow,ifoneassumes

that mental disorders are purely abnormal psychological reactions to

environmentalevents.Thisisnottosaythattheenvironmentplaysnoroleinthe

development and course of these disorders. In fact, the environment probably

playsa large role in thedevelopmentandcourseofmentaldisordersandother

medicaldisordersaswell.

For example, it is quite clear that coronary arterydisease runs in families

(Wolinsky,1979);havingafirst-degreerelativewiththedisorderisconsideredto

be a risk factor. It is also hypothesized that the environmentmay influence the

course of coronary artery disease. Smoking, obesity, lack of exercise, and high

dietary fatcontentareallenvironmentallycontrolled factors thatmay influence

thecourseoftheillness(Messerli,1986).Apersonwithageneticpredisposition

to coronary artery disease will have different physiological reactions to these

Depressive Disorders 5

environmental factors that one who does not possess that predisposition. The

heartandbloodvesselsare then thesubstrate for theenvironment toactupon.

The physical and metabolic abnormalities that are the product of the genetic

diathesis (congenital predisposition) will interact with the environment to

produce the usual features of coronary artery disease (i.e., atheromas, reduced

coronary blood flow, and angina). Removing or minimizing the environmental

factors in coronary artery diseasemay stall the onset of the disorder, produce

clinically insignificant changes, or entirely prevent the development of any

manifestationofthedisorder.Theunderlyingcongenitalpredisposition,however,

remainsconstant.

Theinteractionbetweentheenvironmentandgeneticfactorsmaybeplayed

outsimilarlyintheaffectivedisorders.Certainenvironmentalorbiologicalevents

in genetically susceptible persons possibly may lead to episodes of affective

illness. The combination of environmental and biological genetic factors is the

liabilitythatagivenindividualwilldevelopanaffectivedisorder.

Thegeneticsofanymentaldisordercanberesearchedinavarietyofways.

In this chapter we will discuss four major research paradigmstwin studies,

adoptionstudies,familystudies,andgeneticmarkerstudies.

Genetics of Affective Disorders 6

TWINSTUDIES

Twinstudiescomparetheconcordanceratesformonozygoticanddizygotic

twin pairs. Monozygotic twins possess identical sets of genes. Dizygotic twins

shareonly50percentofthegenestheyinheritedfromtheirparentsandinthat

manner are exactly the same as other sibling pairs. If a disorder is genetically

transmitted,thenmonozygotictwinsshouldshowahigherconcordanceratethan

dizygotictwinsbecauseoftheiridenticalgeneticmaterial.Amajorassumptionin

these studies is that environmental factors are controlled, since twins raisedby

their biological parents share identical environments. Although environmental

influencesmaybeimportant,studiesofpsychologicaltraitsintwinsindicatethat

monozygotictwinswhohavehighlysimilarenvironmentsdonotonaverageshow

anygreaterconcordanceforpersonalitytraitsorIQthanmonozygotictwinswith

lessenvironmentalsimilarity(Loehlin&Nicholas,1976).

FivetwinstudiesofaffectiveillnessareshowninTable6.1.Thehypothesis

of a genetic component for affective disorders is supported by the repeated

findingofahigherconcordanceinmonozygoticthanindizygoticpairs.Poolingthe

datafromallstudies,anoverallconcordancerateof63.8percentisobservedfor

monozygoticand14percentfordizygoticpairs.InalargestudyusingtheDanish

twin register, Bertelsen and colleagues identified 110 same-sex twin pairs in

whichatleastonehadaffectivedisorder(Bertelsen,1979).Twinswerepersonally

interviewedandzygocitywasestablishedusingserologicmarkersor,ifbothtwins

Depressive Disorders 7

were not alive, anthropometric methods. Of 55 monozygotic twin pairs, 58.3

percentwereconcordantforaffectivedisorderwhereasonly17.3percentofthe

52dizygoticpairswereconcordant.

In a study of 12monozygotic twin pairs reared apart, using data gleaned

frompublishedseries,67percentwereconcordantforaffectivedisorder,afigure

strikinglysimilartothepooledfigureof63.8percent(Price,1968).

TABLE6.1.ConcordanceRatesforAffectiveIllnessinMonozygoticandDizygoticTwins*

Study MonozygoticTwins DizygoticTwins

ConcordantPairs/TotalPairs

Concordance(%)

ConcordantPairs/TotalPairs

Concordance(%)

Slater(1953) 4/7 57.1 4/17 23.5

Kallman(1954) 25/27 92.6 13/55 23.6

Harvald&Hauge(1965)

10/15 66.7 2/40 5.0

Allen,Cohen,Pollin,&Greenspan(1974)

5/15 33.3 0/34 0.0

Genetics of Affective Disorders 8

Bertelsen(1979) 32/55 58.3 9/52 17.3

Totals 76/119 63.8 28/198 14.1

*Datanotcorrectedforage.Diagnosesincludebothbipolarandunipolarillness.

Source:Adaptedfrom:Numberger, J. I.,Goldin,L.R.,&Gershorv,E.S.(1986).Geneticsofpsychiatricdisorders. InG.Winokur&P.Clayton (Eds.),Themedical basis of psychiatry (pp. 486-521).Philadelphia:Saunders.

Depressive Disorders 9

ADOPTIONSTUDIES

Adoption studies attempt to separate genetic susceptibility from

environmental factors. A demonstration that offspring reared away from their

biological parents have higher than expected rates of illness would be a

persuasiveargumentforinheritance.

Adoption studies can be carried out in a number of ways. The simplest

methodistoidentifyagroupofadoptedaffectedindividualsandcomparethemto

a control group of unaffected adoptees. The adoptive and biological parents of

bothgroupswouldbestudiedforthepresenceofillness.Ifthebiologicalparents

of illadopteesshowhigherratesof illnessthanthebiologicalparentsofcontrol

adopteesandbothsetsofadoptiveparents,thenageneticfactorcanbesaidtobe

present.Anadoptionstudycanalsobedonebyidentifyingillparentswhoadopt

away theirchildren.Theseoffspringcanbecompared toadopted-awaychildren

born to unaffected parents. A complicated adoption study, the cross-fostering

study, compares children of affected biological parents reared bywell adoptive

parentstochildrenofwellbiologicalparentsrearedbyilladoptiveparents.

Adoption studies do not provide information about the mode of

transmission. They have been criticized because unknown and unquantifiable

factors such as demographics and selection practices among adoption agencies

mayplacechildreninenvironmentswherenongeneticriskfactorsareprevalent.

Genetics of Affective Disorders 10

Thefamiliesofadultbipolaradopteesandthreecontrolgroupswerestudied

by Mendlewicz and Rainer (1977). In addition to the adoptive and biological

parents of 29 bipolar adoptees, the study included the biological parents of 31

bipolar nonadoptees, the adoptive and biological parents of 22 unaffected

adoptees,and thebiologicalparentsof20 individualswhodevelopedpolio.The

polio group was added to control for the effect of raising a disabled child.

Interviewswereconductedblindtotheclinicalstatusoftheadoptees.

Table6.2 shows thediagnosesof the adoptive andbiologicalparents.The

biologicalparentsofthebipolaradopteesandnonadopteesshowedsimilarrates

of affective disorder (31 percent vs 26 percent). These same parents showed

higher rates of affective illness than either the adoptive or biological parents of

normaladoptees,andhigherratesthantheparentsofpoliopatients.Threemajor

pointscanbemadefromthisstudy.First,thehypothesisofageneticcontribution

tobipolaraffectivedisorderissupportedbythesedata.Second,theexcessamount

ofunipolardisorderseeninthebiologicalparentsofallbipolarpatientssupports

the notion that unipolar illness in some cases stems from the same genetic

abnormalityasbipolarillness.Lastly,thedatashowthatunipolarillnessismore

common in theadoptiveparents thanbipolardisorder. In fact, onlyone caseof

bipolardisorderwasfoundintheadoptiveparents.

TABLE6.2.AffectiveIllnessinParentsofBipolarAdopteesandControls

Group Diagnosisof Adoptive Percent Biological Percent

Depressive Disorders 11

Parent Parents Ill Parents Ill

BipolaradopteesN=29

Bipolar 1 4

Unipolar 6 12% 12 31%

Otheraffectivedisorder

0 2

BipolarnonadopteesN=31

Bipolar 2

Unipolar 11 26%

Otheraffectivedisorder

3

NormaladopteesN=22

Bipolar 0 0

Unipolar 3 10% 1 2%

Otheraffectivedisorder

1 0

PoliomyelitisN=20

Bipolar 0

Unipolar 4 10%

Genetics of Affective Disorders 12

Otheraffectivedisorder

0

Data from Adoption Studies Supporting Genetic Transmission in Manic-Depressive Illness by J.MendlewiczandJ.Rainer,1977.Nature,268,326-329.

This suggests that unipolar disorder is probably a heterogeneous group of

illnesses, with some being related to bipolar illness and others perhaps not

geneticallymediatedatall.

Wender, Kety, and Rosenthal (1986) studied the biological and adoptive

familiesof71adopteeswithaffectivedisordersand71matchedcontroladoptees

withnoillness.Biologicalrelativesofilladopteeshadsignificantlyhigherratesof

affectivedisorder (bipolar andunipolar) compared to thebiological relatives of

controls. Alcohol abuse and dependence was also seen in greater frequency

among thebiological relativesof ill adoptees, supportingearlier findings that in

some cases affective disorders and alcoholism may be genetically related. The

most significant finding was a striking increase in completed suicide in the

biologicalrelativesofilladoptees,comparedtothoseofcontrols.

In a study by Cadoret, 83 adoptees were selected from adoption records

showing evidence that their biological parents had mental disorders (Cadoret,

1978).Forty-threematchedcontrolswerealsoobtained.Alladopteesandatleast

one adoptive parent were interviewed. All data were collected blind to the

Depressive Disorders 13

diagnosis of the biological parents. Eight adoptees were found to have one

biological parent with an affective disorder. Three of these adoptees were

diagnosedashavinganaffectivedisorder,allofwhichwereunipolarillnesses(38

percent). The remaining 75 adoptees in the experimental group and the 43

controlshad four (5percent) and four (9percent) casesofunipolardepression

respectively. The differences in the observed rates of affective illness in the

adopteesofbiologicalparentswithaffectivedisordercomparedtothosewithout

suchadisorderaresignificant.

Inalargestudyof2,966adopteesborninSwedenandadoptedoutpriorto

age three, Von Knorring and colleagues identified 56 adoptees with affective

disorders and 59 with substance abuse (Von Knorring, Cloninger, Bohman, &

Sigvardsson, 1983). Each affected adoptee was matched to one of 115 control

adoptees with no mental illness. Adoptees with affective disorder showed no

higherratesofaffectivedisorderineitherthebiologicaloradoptiveparentswhen

compared to parents of matched controls. There was, however, a trend for

nonpsychoticdepressedadopteestohavemoremotherswithaffectivedisorders

than normal controls. Adoptees with affective disorder did show a significant

increase in biological mothers with substance abuse compared to mothers of

adoptees with no illness. No general concordance of any specific psychiatric

diagnosiswasobservedbetweenadopteesandbiologicalparents.

It should be pointed out that all subjects in this studywere diagnosed by

Genetics of Affective Disorders 14

medical records only. Subjects identified as having no psychiatric illness could

have, in fact, had episodes of illness that were treated but not recorded,

misdiagnosed,ornottreatedatall.Insupportofthis,notoneoftheeightparents

ofadopteeswithpsychoticdepressionwasdiagnosedashavingaffectivedisorder.

Althoughthisispossibleandissimilartotheratefoundinthecontrols,itismuch

lower than the rate observed in studieswhere relatives are directly examined.

Despite its shortcomings, the study does suggest that some cases of unipolar

depressionmaynotbegeneticallytransmitted.

Depressive Disorders 15

FAMILYSTUDIES

Familystudiesareeasilyappliedtotheresearchofthegeneticsofaffective

disordersandhavebeenitscornerstone.Studiessuchastheseusuallybeginwith

anaffectedindividual(proband)andstudyallavailablerelatives.Twobasictypes

of familyresearcharerecognized.The familyhistorymethod involvesobtaining

allpedigreeinformationfromtheprobandonly.Studiesofthisnaturehavebeen

criticized because other affected familymembersmay conceal their illnesses or

mayhavemilderformsofillnessthatwouldgounnoticedbyothers(Andreasen,

Endicott,Spitzer,&Winokur,1977).Ingeneral,familyhistorystudiesarebiased

byunderreporting.Thesecond,morecompletetypeoffamilystudyinvolvesthe

directexaminationofallavailablerelatives.

Familystudiesareuseful indelineatingtherangeofdisordersthatmaybe

associated with a single genetic vulnerability. For example, Winokur

demonstrated through a series of family studies that unipolar depression,

alcoholism, and sociopathy cluster in certain kindreds, suggesting that all three

stem from the same genetic abnormality (Winokur, 1972). When two or more

disordersappeartobegeneticallyrelated,theyarereferredtoasaspectrum.

Afamilystudyattemptstoidentifyallaffectedpersons.Inordertoshowthat

anillnessisfamilial,theratesofillnessintherelativesofprobandsmustbehigher

than the rates of the same illness in controls. The statistic used to compare

familiesofprobandstocontrolsiscalledthemorbidrisktheratioofthenumber

Genetics of Affective Disorders 16

ofillrelativesdividedbythetotalnumberofrelativesatriskforthedisorder.The

calculationofthenumberatriskforthedisordermusttakeintoaccountvariable

ageofonset. Inordertodothis, thenumberofrelativeswhoare inaparticular

decadeismultipliedbythepercentageofaffectedpersonswhofirstbecomeillby

that decade of their lives. This product is called the Bezugziffer (BZ). For each

decade, BZs are summed and the total number is used in the calculation of the

morbidrisk(Nurnberger,Goldin,&Gershon,1986).

The familial concentration of affective disorders has been clearly

demonstratedinnumerousfamilystudies.Table6.3showsthelifetimeprevalence

ofaffectivedisordersinfirst-degreerelativesofpatientsandcontrols.Oftheeight

studieslisted,eightidentifiedbipolarprobands,sixidentifiedunipolarprobands,

and two utilized unaffected controls. Bipolar and unipolar probands had more

relatives ill with either disorder than did normal control probands. The most

common affective disorder seen in the relatives of either unipolar or bipolar

probands was unipolar disorder. This suggests that at least in some families

bipolar and unipolar illnesses are different phenotypic expressions of the same

geneticillness.

TABLE6.3.MorbidRiskforBipolarandUnipolarDisordersinFirst-DegreeRelativesofPatientsandControls

NumberatRisk MorbidRisk(%)

Depressive Disorders 17

Bipolar Unipolar

Bipolarprobands:

Angst(1966) 161 4.3 13.0

Perris(1966) 627 10.2 0.5

Winokur&Clayton(1967b) 167 10.2 20.4

Helzer&Winokur(1974) 151 4.6 10.6

Gershon,Baron,&Leckman(1975) 341 3.8 6.8

Smeraldi,Negri,&Melica(1977) 172 5.8 7.1

Taylor,Abrams,&Hayman(1980) 601 4.8 4.2

Gershon,Goldin,Weissman,&Nurnberger(1981)

598(572)* 8.0 14.9

Unipolarprobands:

Angst(1966) 811 0.3 5.1

Genetics of Affective Disorders 18

Perris(1966) 684 0.3 6.4

Gershon,Baron,&Leckman(1975) 96 2.1 11.5

Smeraldi,Negri,&Melica(1977) 185 0.6 8.0

Taylor,Abrams,&Hayman(1980) 96 4.1 8.3

Gershon,Goldin,Weissman,&Nurnberger(1981)

138(133) 2.9 16.6

Normalprobands:

Gershon,Baron,&Leckman(1975) 518(411) 0.2 0.7

Gershon,Goldin,Weissman,&Nurnberger(1981)

217(208) 0.5 5.8

*Total number at risk for bipolar illness appears first. The number in parentheses represents thenumberatriskforunipolardisorderwhenknown.

Source: Adapted from: Gershon, E. S., Hamovit, J., Guroff, J. J., Dibble, E., Leckman, J. F., Sceery,W.,Targum,S.D.,Nurnberger, J. I.,Goldin,L.R.,&Bunney,W.E. (1982).A familystudyofschizoaffective,bipolarI,bipolarII,unipolarandnormalcontrolprobands.Archives ofGeneralPsychiatry,39,1157-1167.

As seen from Table 6.3, morbid risk figures vary widely among studies.

Depressive Disorders 19

There are many reasons for these observed variations. Diagnostic criteria

employed, method of data collection, and cultural factors all vary and may be

responsibleforthedifferencesobserved.Studiesutilizingafamilyhistorymethod

tend to underestimate prevalences, whereas direct examination of all available

relativestendstoproduceahigherfigurethatmaybeamorecorrectestimateof

thetrueprevalence.

Although each study has its own flaws with respect to methodology and

cannotbedirectlycomparedtotheothers,eachshowsaclearincreaseinaffective

disorder among first-degree relatives of ill probands. Taken together, family

studiessupportthegenetichypothesisinthetransmissionofaffectivedisorders.

Family, twin, and adoption studies, regardless ofmethodological flaws, all

support the hypothesis of a genetic component for affective disorder. Just how

affective disorder is transmitted is not known. Segregation analysis of family

study data has failed to show the exactmode of transmission for any affective

disorder(Gershon,Bunney,Leckman,VanEerdewegh,&DeBauche,1976).There

are likely to be several reasons for this, but in order to proceed with this

discussion two definitions are needed: Genotype is the actual inherited genetic

material,andphenotypeistheobservedeffectofthegenotype.

GenotypeandPhenotype

Sincetheenvironmentmayplayaroleinthedevelopmentofthedisorder,

Genetics of Affective Disorders 20

the absence of critical environmental factors may obscure the mode of

transmission: Certain individuals with a genetic predisposition (affected

genotype) and no environmental input may not show the disease (affected

phenotype) at all. The finding thatmonozygotic twins fail to show 100 percent

concordance for affective disorders suggests that the diseasemay notmanifest

itselfinallpersonswhopossessthegene.Reducedpenetrance,thetermapplied

to this phenomenon,may play a role inmanymental disordersschizophrenia

andanxietydisorders,aswellasaffectivedisorders.Anygeneticillnesswithlow

penetrance would clinically appear to be uncommon even though the actual

frequencyofthediseasegenemaybehigh.Thepossibilitythatmanyindividuals

could be carriers without overt illness would make it nearly impossible to

determine the mode of transmission from family study data alone. Reduced

penetranceimpliesthatthediseasegenedoesnotmanifestitselfatanytimeina

carrierslifetime,assumingthatthecarrierhaslivedthroughtheentireageofrisk

forthedisorder.Theacceptedagesofrisk forbipolarandunipolardisorderare

20-50yearsand20-70yearsrespectively(Paykel,1982).Sinceaffectivedisorders

have a variable age of onset, the penetrance of a disease gene may be age-

dependent. In any family study therewill bea largenumberof individualswho

may be gene carriers but who have not yet reached the age where the illness

manifestsitself.Theidealfamilyforsegregationanalysiswouldbealargekindred

withtheyoungestgenerationwellintotheageofriskandwithaclearunilateral

sourceofillness.Sincefamilystudiesexaminepedigreesonlyatonepointintime,

Depressive Disorders 21

many younger family members who may be carrier would be classified as

unaffected,therebyobscuringthepathoftransmission.

Untilnowithasbeenassumedthatasinglediseasegeneleadstoadefined

singlephenotype.Thisassumptionmaynotbevalidforclinicalpsychiatry;there

is evidence that phenotypesmay vary fromperson to person even though they

maycontainthesamegeneticmaterial.Winokurdescribedaclinicalspectrumof

disorders termed depression spectrum disease, which includes sociopathy,

unipolardepression,andalcoholism(Winokur,1972).Thisclusteringappearedto

breed true, and a closer look at the families studied showed that the cases of

depressionwerecommonlyseeninwomenundertheageof40whohadunstable

personalities; alcoholism and sociopathy were more commonly seen in males.

Winokursworksuggestedthatthethreedisordersmaystemfromasinglegenetic

abnormality that manifests itself differently due to sex effect and/or perhaps

other undetermined environmental or biological factors. Other clinical studies

haveshownaffectivedisordertoclusterwitheatingdisordersandpanicdisorder

in certain families (Cantwell, Sturzenberger, Burrows, Salkin, & Green, 1977;

Leckman,Weissman,Merikangas, Pauls,& Prusoff, 1983). Even among affective

disorders, the wide range of possible phenotypes includes bipolar I, bipolar II,

unipolar,cyclothymia,dysthymia,andschizoaffectivedisorder.Whetherallthese

disordersstemfromthesamegeneticdiathesisisamatterfordebate,butthefact

thatmanykindredsdoshowmorethanonetypeofdisordersuggeststhat,forat

leastsomefamilies,singlegeneticabnormalitiesproduceavarietyofphenotypes.

Genetics of Affective Disorders 22

Since affective disorders are not determined from laboratory or other

biological/physical exams, the diagnosis ismade purely from the description of

thedisordermadebythepatientandsometimesbyaknowledgeableinformant.

The absence of objective tests for diagnosismeans that other disorders with a

different pathophysiology may mimic an affective illness. Examples include

hypothyroidism, reactivedepression,Cushingsdisease, andbereavement. Cases

such as these, called phenocopies, may be clinically indistinguishable from

primary affective disorders. Phenocopies are probably common in studies of

unipolar depression (depending upon the diagnostic criteria used) and would

certainlyobscureeffortstofindthemodeoftransmissioninthefamiliesinvolved.

Heterogeneity

The last and perhaps most difficult obstacle to uncovering the mode of

transmissionforanyaffectivedisorderisheterogeneity.Geneticheterogeneityisa

factor inmanydiseases. For example, diabetes in its varietyof formsmay stem

fromseveraldifferentgeneticabnormalities.Earlyonset(insulin-dependent)and

late onset (non-insulin-dependent) have different pathophysiologies and it has

been demonstrated that non-insulin-dependent diabetes is familial but the

insulin-dependent form is not (Unger & Foster, 1985). In a study of a

heterogeneous illness, detecting the mode of transmission would be nearly

impossible. A variety of genetic mechanisms would be in play and each would

haveitsownmodeoftransmission.Inanattempttolimittheheterogeneityfactor,

Depressive Disorders 23

familiesshowingX-linkedinheritancehavebeensingledoutforanalysis.Inthese

cases there is a father-to-son transmission because fathers can pass only a Y

chromosome on to their sons. It is assumed in these cases that the genetic

abnormality is limited to the X chromosome, thereby excluding all of the

autosomes. InX-linkeddominant transmission, femaleswouldbe affectedmore

oftenbecausetheypossesstwoXchromosomesandthereforehavetwicetherisk

ofgettingadiseasegene.Winokurandcolleaguesobservedbothanexcessof ill

femalesandalackofmale-to-maletransmissioninastudyof62bipolarprobands

(Winokur,Clayton,&Reich,1969).TheypostulatedanX-linkeddominantmodeof

transmissionforbipolarillnessintheseselectedkindreds.Thesedataandothers,

however,havebeenreanalyzed,andneitheranX-linkedoranautosomalsingle-

major-locusmodeloftransmissioncouldexplaintotallythecasesofbipolarillness

observedinthesefamilies(Bucheretal.,1981;Bucher&Elston,1982).X-linked

dominanttransmissionwasalsopostulatedbyMendlewiczandFleiss inaseries

ofbipolarprobandsbutareanalysisofthesedatafoundthattheX-chromosome

single-major-locusmodeldidnot fit theirdata (Mendlewicz&Fleiss, 1974;Van

Eerdewegh, Gershon,&VanEerdewegh, 1980).Data from families suggesting X

linkage remain conflicting and the results obtained vary, based on assumptions

madeandstatisticalmethodsused.ThehypothesisofanX-linkedformofaffective

disorderislikelytobeviablebutprobablyaccountsforonlyaminorityofcases.

Asmentioned earlier, variable phenotypemayhinder segregation analysis

studiesandtherebyobscurethemodeoftransmission.Thisissuehasbeentaken

Genetics of Affective Disorders 24

upbyanumberofresearchersintheanalysisof familystudydata.Gershonand

colleagues have hypothesized that unipolar, bipolar I, bipolar II, and

schizoaffective disorders are all part of a continuum stemming from a common

geneticbackgroundwithmultiplethresholds(Gershonetal.,1982).Theirfamily

study data are in support of that model. Further support for the concept of

variable expressivity comes from work by Winokur. In one study (Winokur,

1984), themorbid risks foraffectivedisorder in first-degree relativesofbipolar

and unipolar probands were nearly identical. In another study (Winokur,

Kadrmas,&Crowe,1986), first-degree relativesof schizoaffectivebipolarswere

foundtohavemorbidrisks foraffectivedisordersimilar to thoseof first-degree

relatives of other bipolar probands. Smeraldi, Fiammetta, Heimbuch, and Kidd

(1981)foundthattheprobandspolaritydidnotpredictillnessinrelatives.

Sex differences in the transmission of affective disorders have been

demonstrated. Winokur and Clayton found that ill mothers more often

transmittedtheirillnesstodaughtersthantosonswhereasillfatherstransmitted

their illness with equal frequency to both sons and daughters (Winokur and

Clayton,1967a).Angst,Frey,Lohmeyer,andZerbin-Rudin (1980)demonstrated

that first-degree femalerelativesof femaleprobandshadhighermorbidityrisks

than other relatives. At this point it is clear that the mode of transmission for

affective disorders is not known and is likely to be complex, even though the

familialclusteringoftheseillnessesiseasilydemonstrated.

Depressive Disorders 25

GENETICMARKERSTUDIES

Geneticmarkersposeanewmethodtouncovertheinheritabilityofaffective

disorders.Itispossiblethatgeneticmarkersmayuncoversubtypesofillnessthat

are linked to a particular marker, thereby directly addressing the problems of

heterogeneityandvariableexpressivity.

The search for genetic markers linked to mental disorders has included

studies of color blindness, blood group polymorphisms, HLA (human leukocyte

antigen)types,andDNA(deoxyribonucleicacid)markers.Themethodologyused

inthesestudiesiscalledlinkageanalysis.Thismethoddeterminestheprobability

that a geneticmarker andadisease trait are inherited together, suggesting that

they are physically close to one another on the same chromosome.Marker loci

mustbepolymorphic;thatis,theymusthaveatleasttwoformssothatoneform

canbedemonstratedtobelinkedtoaparticulardisease.Additionally,themodeof

inheritanceforthemarkerlocusmustbeknown.If linkagecanbedemonstrated

betweenmarkerlocusanddiseaselocus,itisconvincingevidenceforthepresence

ofageneticcomponent, since it isunlikely thatenvironmental influenceswould

causesuchanassociation.

Linkage

Todemonstrate linkage, it isnecessary toemploymathematicalmodels to

estimatethelikelihoodoflinkage.Thelod(logoftheoddsratio)scoremethodof

Genetics of Affective Disorders 26

Morton tests the hypothesis of linkage between two loci (marker and disease)

when the mode of transmission of each locus is known (Morton, 1955). This

method assumes that the gene frequency and penetrance for the disease and

markerlociareknown.Itisalsoassumedthatthereisnopopulationassociation

betweenthemarkeranddiseaselociandthatrandommatingoccurs.Thepattern

of segregation seen between the marker and disease loci is compared to the

probabilityofobservingthesamepattern if thetwo lociarenot linked(random

assortment).Theprobabilityoflinkageisexpressedintermsoftherecombination

fractioncalledtheta.Onagivenchromosome,genescanrecombineatthetimeof

meiosis.Genesthat lie farapartwillrecombinemore frequentlythangenesthat

areclosetogether.Ifamarkerlocusanddiseaselocusaretightlylinked,thenthey

shouldrecombineinfrequently.Thetatakesonavaluebetweenzeroandonehalf.

A recombination fraction of one half means that the loci are expected to

recombine50percentof the time; that is, theysegregate independentlyandare

notlinked.Attheotherextreme,arecombinationfractionofzeromeansthatthe

loci are expected never to recombine and always to segregate together. Using

theserecombinationfractionsalodscoreisdefinedas:

LODScore=log10 probabilityofobservingafamilyfor