f. prychiar.

Res..

1976.

Vol.

13.pp.99-117.Pergamon Press. Printedin Great Britain.

THE QUESTION OF X-LINKAGE IN BIPOLAR MANIC-DEPRESSIVE ILLNESS ELLIOT

S. GERSHO~ and WILLIAM E. BUNNEY, JR.

Section on Psychogenetics, Adult Psychiatry Branch, Building 10, Room 3N218, National Institute of Mental Health, Bethesda, Maryland, U.S.A. (Received 7 November 1975. Revised 3 May 1976) GENETIC

linkage is a most powerful strategy for demonstrating the presence of a genetic factor and its mode of transmission. The hypothesis of X-chromosome transmission of manic-depressive illness was first advanced by ROSANOFF et al.,l but it was WINOKUR and his co-workers who first presented evidence using genetic markers to support this hypothesisgm4 A large series of informative families was recently reported by MENDLEWICZ and his co-workers.5-7 Combining their cases with the previously reported cases, they concluded that bipolar manic-depressive illness is linked to marker loci on the X-chromosome. This finding represents a potentially crucial breakthrough in our understanding of affective disorders, and therefore deserves a most careful review. In the present paper, we will review the investigative methods and the reported evidence for linkage in all these studies and the analysis of this data as recently reported.’ We will consider whether the investigative strategy and analytical methods are appropriate, whether the findings reported are subject to ambiguity of interpretation, and if the conclusions offered are consistent within themselves. On each of these points, there are unresolved questions. These are: (1) The analytic method used is not designed for a disorder with incomplete penetrance and variable age of onset; (2) in the data on color blindness, there appears to be an association between color blindness and manic-depressive illness, which makes it very difficult to conclude that there is linkage in the same data; (3) some of the families included are subject to other pedigree analysis than the one presented; and (4) the reported close linkage of bipolar illness to both Xg and the protan-deutan area of the X-chromosome is not compatible with the known large chromosomal map distance between Xg and protan-deutan. SEX CHROMOSOME

LINKAGE AND RECOMBINATION TO BIPOLAR ILLNESS

AS APPLIED

Chromosomes that pair during meiosis are transmitted from parent to child in one of two forms-either as replications of one of the original chromosomes or as crossover products of both members of the pair (Fig. 1). Two markers on the same chromosome can be transmitted as recombinants (crossover products) or non-recombinants. The recombination frequency can be observed by analyzin g gametes produced by an individual who is heterozygous at both marker loci as in Fi g. 1, where the Ab and aB gametes result from recombination. Onlyadoubly heterozygous individualproduces informativegametes. If the recombination 99

100

ELLIOT S. GERSHON and WILLIAV E. BUSXEY.JR.

Pairing

1st meiotic division

t meiotic

from

2nd division

Cavalli-Sforza

and Bodmer (1971)

FIG. 1. Paired chromosome replication.

frequency (e) is 0.50, the two markers are unlinked, since this is the expected “recombination” between genes on separate chromosomes. Demonstration of less than 50% recombination between a known X-chromosome locus and bipolar illness would constitute proof that bipolar illness is transmitted in large part by a single locus on the X-chromosome. Previous studies of linkage on the X-chromosome have allotted a map of the chromosome to be produced (Fig. 2), which provides reference points for placement of newly discovered loci, such as the putative bipolar locus. The human X-chromosome is paired in the female but not in the male, and is not transmitted from fathers to sons. Examination of the sons of women heterozygous at two loci, therefore, allows analysis of the gametes produced by such women. In the current case, bipolar manic-depressive illness and color blindness (or Xg blood group) are the two marker loci studied. The hypothesis bein, 0 tested in the reported linkage studies is that a single gene will produce bipolar illness in all males who carry it, and that absence of bipolar illness in these males is satisfactory evidence that the gene is absent. Several investigations also require that a woman be demonstrably ill to be considered informative,1*5-7 hypothesizing that the gene for bipolar illness is dominant, and that a woman without affective illness lacks the gene. Given these hypotheses, it is necessary to demonstrate that a woman is heterozygous for bipolar illness before her offspring are considered informative. Thus she must have either a well father or a well son; otherwise she may be homozygous for the illness. As will be noted below, these requirements are valid only if there is complete penetrance of the gene for bipolar illness, and if the variable age of onset of this illness is ignored. STRATEGY AND METHODS ANALYSIS A typical investigative

strategy is to review known patients with bipolar illness for families in which there was unambiguous female to male transmission of the disorder, as evidenced by a woman and her son(s) having a major affective illness. From among these families, those in which segregation of the marker loci could be observed can be analyzed for linkage transmission.

THE QUESTION OF X-LISUGE xx BIPOL.+RMASK-DEPRESSIVE1ttx~s.s

101

LEGE!ID HPRT =

cbD

=

rhP

=

x3

=

ich

=

FA

=

xi;

region.

:Apted

f:om Pe.?.rsone: al. (1975).

FIG. 2. Map of the X-chromosome

based on family studies.

Such data have been analyzed5-’ using the method of EDWARDS.~ Although this method accepted, it is worth noting its limitations and its assumptions. One limitation is that it is not applicable to analyzing multigeneration pedigrees in a single analysis. Each segment of the pedigree must be considered as if it were ascertained independently, with resulting distortions in anaIysis that are not well understood.B Second, the Edwards method assumes that the phenotype of each person is unambiguously observable. In a psychiatric disorder with multiple manifestations and a variable age of onset, the use of an analytic method which requires that each informative person be definitely ill or well does not allow for errors in classification of well persons who have not passed through the age of risk. This problem might be dealt with through introducing an age of onset probability density function into the Edwards method.‘O An alternative has been adopted by MENDLEWICZ et al.j-’ of not including persons as negative for illness unless is generally

102

ELLIOTS. GERSHONand WILLIA.%E. BLJKXY, JR.

they have passed through an appreciable portion of the ages at risk for the disorder. Clearly, this is only a rough approximation of the probability of a well relative ever becoming ill. The problems of penetrance and of the multiple manifestations of affective illness are less easily handled. Two devices are used by Mendlewicz et al. to minimize the number of persons whose genotype may be ambiguous: (1) The choice of families in which two generations of illness are manifest, and (2) calling of any affective disturbance in the pedigree a manifestation of the genotype for bipolar illness. As reviewed elsewhere” this method may select atypical families from among all persons with affective illness. But the use of these methods of selection would not be expected to produce a falsely positive linkage analysis in the families studied and must be considered valid if ascertainment is otherwise unbiased. The presence of multiple forms of the illness, for example, unipolar and bipolar forms of affective disorder, presents problems in analyzin, 0 for mode of transmission. Family studies are required to demonstrate that each of the multiple manifestations is indeed related to bipolar illness. Finding that significantly more relatives of bipolar patients have unipolar illness or cycloid personality than is expected from the population incidence would be a satisfactory demonstration that some cases of these disorders are related to bipolar illness. The possibility of phenocopies or of independent unipolar illness appearing in these pedigrees, and producin, 0 falsely positive findings of illness, is not taken into account by straightforward application of the Edwards method.* The possibility of nonpenetrance causing a mistaken negative finding of psychiatric normality is also not taken into account. An assumption of the Edwards method is that coupling* and repulsion are equally likely to occur in the population from which the subjects are drawn and that the method of proband selecton does not favor one or the other types of family.12 This assumption does not appear valid in the reported data on color blindness and manic-depressive illness. In that data, all of the male manic-depressive probands in the informative families found by MENDLEWICZ et aL5-’ are color-blind (as is the male proband in the Alger family3 included in their analysis). Since there are 10 male probands, this apparently would not occur in complete examination of a population of probands and their sibs, since one would expect half of the informative sibships to be found in repulsion. Several possibilities could explain the reported results: there may have been preferential referral of clinic patients for family investigation if the patient was both color-blind and manic-depressive, or there may be an association between color blindness and manic-depressive illness, such that, in families of manic-depressive patients, color-blind males are more likely to have manic-depressive illness than normal vision males. If there were preferential referral, linkage might still be present, but the method of Edwards would not be valid because the ascertainment corrections would be incorrect.13 However, preferential referral would not account for coupling being more likely in the *Coupling and repulsion refer to the alleles present on a single chromosome. In the case of color blindness, a chromosome in repulsion would have a not-manic-depressive and a color blindness allele, or the alleles for manic-depressive illness and not-color blindness. In coupling, chromosomes would have colorblind and manic-depressive, alleles, or alleles for normal color vision and non-affective illness. Chromosomes in coupling may produce chromosomes in repulsion through recombinaton during the process of replication (Fig. 1).

THE QLESTION OF X-L~NUGE

IN B~FOLAR MASK-DEPRESSIVE 1~~xs.s

103

families of female probands who were not demonstrably deficient in color vision. In the data of MENDLEWICZ et a1.j -’ and in the reported cases of REICH et a1.3 the preponderance of coupling is present in the informative relatives of both male and female probands (Table 1). This would imply that the second possibility is more likely; that there is an association TABLE

1. TESTS

FOR A?SOCUTtONBETWEENTRAIT

AT ,MARKER

LOCUS

AND MANIC-DEPRESSIVE

ILLh?%S

Color blindness

Trait I. Color blindness (Protan or deutan) Relatives of male probands’

Manic-depressive illness Absent Present Manic-depressive illness Absent Present Manic-depressive illness Absent Present

Relatives of female probands

II. xg

*All informative sons for linkage analysis, determination, included in this tabulation.

and informative

Absent 1s 3

Present 1 25

8 3 Xg1s 19

2 12 Xg+ 7 25

maternal

x2 = 29.op

< 0031

x2 = 6.5

p < 0.02

x2 = 367, NS

uncles or grandfathers

for phase

between

manic-depressive illness and color blindness. A third possibility is that there was incorrect ascertainment of families, such that whole families would be more likely found if they contained affectively ill color-blind males, thus producing an artefactual association. An association would not exist on the basis of genetic linkage alone, which would produce an association of the two traits within individual pedigrees but not across pedigrees. The presence of an association across families if not due to procedural errors, would suggest phenomena such as differential viability, selective pressures, linkage disequilibrium or a common etiology for the two traits considered. Whatever the explanation, the assumptions of the lod score method are clearly not met in this caseI and it does not appear possible to proceed with statistical analyses for linkage that are based on these assumptions, such as the method of EDWARDS.* Analysis of linkage of color blindness to bipolar affective disorder

thus

does

In the data or repulsion

not appear

of Xg

blood

possible group

in the informative

on the basis and bipolar

sons (Table REVIEW

The

available

pedigrees

in the

of currently

illness,

there

available

data.

is no preponderance

of coupling

1). OF PEDIGREES

literature2*3*5-7

were reviewed for informativeness. The criterion for rejecting a pedigree was a positive answer to any of these questions: (1) Is the mother of informative sibships ambiguously heterozygous for the traits being studied; (2) does the variable age of onset of affective illness make any pedigrees of doubtful informativeness; (3) are there tenable alternative interpretations to the analysis of the pedigree recently presented by MENDLEWICZ et al.v-’ Those pedigrees that met any of these criteria are discussed here; on the remaining pedigrees, we are in agreement with the analyses of MENDLEWICZ

et aI.5-7

In the current

review, we required

that persons

be over 25 to be classified as not having

ELLIOTS. GERSHONand WILLUM E. B~TEY, JR.

104

major affective illness (bipolar or unipolar), since by this age half of the cumulative risk for bipolar disorder has passed.2 In an early paper the investigators being reviewed used 30 as a dividing age5 but in later studies they used 20aS7 so the current choice of over 25 seemed a reasonable age to apply to both sets of data. As noted above, this computational device is only an approximation to the required application of an age of onset cumulative distribution function. Since all investigators agree that some male-to-male transmission of bipolar illness exists, it appears to us desirable to require that the father of informative males be known not to have affective illness. This is a valid means of selecting families in which X-linked transmission might be present, and would not produce falsely positive linkage analyses. COLOR BLINDNESS

AND MANIC-DEPRESSIVE

ILLNESS

Analysis As noted above, these pedigrees cannot be analyzed for linkage until the observed association of color blindness and bipolar illness is understood. We have reviewed these pedigrees here to provide a basis for the statistical analysis that supports an association (Table l), since the same criteria of informativeness would apply. Protan color blindness (Fig. 3) Family 1.5 The proband’s two sons are both over age 25. One (III-I) Fomily

I

from Bipolar

depression

Unipolar

depresslon

Possible

depression

No disorder Unknown

status

has affective illness

67 / L:

Mendlewicz, Fleiss and Fieve (1972)

Propositus ‘,:

Color-blind

(P+) Coirler (Dil Neither color-blind nor carrier Unknown stotus (?I

FIG. 3. Protan color blindness

and affective illness.

and is color-blind, the other (111-2) is well and is not color-blind. The proband’s father (l-2) was well and not color-blind; she, the proband, is therefore in coupling, and her two sons are non-recombinants. AZger family.4 The male bipolar proband has an alcoholic father (111-Q and a unipolar paternal uncle (111-4). This suggests that the alcoholism may be the form found in relatives of unipolar patients, which may share a genetic etiology with unipolar illness.”

THE QLESTION OF X-LISKAGE IS BIPOLARMANIC-DEPRESSIVE ILLNESS

i

105

106

ELLIOTS.GERSHON

and

WILLIAM E. BuN?*+Y,JR.

Both the proband’s sisters (IV-14 and IV-15) are unipolar and heterozygous for protan color blindness. They must have received the color blindness gene from mother, since father has normal vision. However, they may not be counted as non-recombinants because they may have inherited unipolar affective illness from their father or mother or both. The sisters’ offspring (V-3 and V-6) may not be included as informative sibships in the analysis using Edwards’ method because each sister has only one son over age 25, and the phase in each sister is unknown except through her sons. The proband’s uncles (III-12 and 111-14) may not be included in the analysis because there is no evidence that their mother was heterozygous for color blindness. The proband’s great aunt (11-16) is not clearly heterozygous for color vision, and is not included in the analysis. II-16 also had an ill daughter (111-29) who was either color-blind or a carrier, but who cannot be included in the analysis because there is no data on her father. The pedigree may thus be considered not to contain informative sibships. MENDLEWICZ

and FLEES’ (Fig. 3) Family RS. The proband has one color-blind brother with hyperkinesis (11-3) who is 16 yr old, and a well normal vision brother (11-2) who is 20 yr old. Both are too young to be included in the analysis and the mother therefore may not be heterozygous for affective illness. The entire family is not considered informative. Family

I

40

A + 45

51

I

RS

P+

2

3

(P+l

P

/,‘gXg--Ygp+

II

Fomlly

62+

BR

63’

I

From

Mendlewicz

and

Flelss

(1974)

Family BR. The proband’s sister’s grandchildren (IV-l, IV-2 and IV-3) are ascertained in an unbiased way through their maternal grandparents, with the maternal grandfather (11-3) in coupling. However, it is not valid that the granddaughter (IV-l) is considered negative for color blindness, since some women who are heterozygous are not detectable by

THE QLTSTIONOF X-LISKAGE xx BIPOLAR MAMC-DEPRESIVE Irr~~ss

107

even the most sophisticated tests. Only a large number of non-color blind sons would give a reasonable likelihood that a woman is not a color blindness carrier. The two brothers (IV-2 and IV-3) are informative in that generation. Deutan color blindness (Fig. 4) Family .5.5 The proband’s maternal grandfather I-l) has normal vision and is well. His maternal grandmother (I-2) is a carrier for color blindness and is ill. Thus his mother (II-2), who is ill, is in coupling, and the proband is a non-recombinant and his brother (111-3) is a recombinant. Cafvert family.3 The proban’s father (U-8) is alcoholic. The proban’s sister (111-5) who

from

Reich, Cleycon.Uinoku: (1969)

Legend for pediqrcer

Unipoior

depression

Possible

depressiona No disorder

Unknown

q eCo’or @C$;;J;~ 0

’’

Pt Pt

b”ndD+

O+ I,‘:\

Oco~or blind nor comer @‘j:,k,n,q”” I?)

stotusa , Prooositusr

FIG. 4. Deutan color blindness and affective disorder.

108

ELLIOTS. GERSHOKand WILLIAME. Buxr.~~, JR.

is heterozygous for color blindness, may not be informative, since she may have inherited unipolar affective illness from her father, if both suffer from variants of depressivedisease.li The proband’s three ill color-blind brothers (III-S, III-1 1, 111-12) and two well normal vision brothers (111-9, 111-10) are informative.

Xg LINKAGE Fonlly 5 74+ I

I

ILLNESS

Q to+1

69+

Q

n

TO MANIC-DEPRESSIVE

~~+~

2

6$-q-from Mendlewicz,

FIG. 5. Xg blood group and manic depressive

Fleiss and Fieve (1972)

illness.

(a) MENDLEWICZ, FLEISS and FIEW? (Fig. 5) FumiZy P. A paternal aunt (H-5) of the proband has bipolar illness and is Xg-t. Her husband (H-6) is well and Xg-. Therefore her sons and daughters are informative on X-

from Mendlewicz, Fleiss and Fieve (1975)

linkage. One daughter is 23 (111-6) and well, but is too young to be included in the analysis. Three children are informative: a son (11-3) and a daughter (111-5) who both have affective disorder and are Xgf, and another daughter (1114) who is well and Xg - .

109

THE QUESTIONOF X-LISKAGE IN BIPOLAR URANIC-DEPRESSWE ILLSESS

Futnily R. There are two brothers in this family, one (11-l) is Xg-+ and ill, and the other (11-2) is Xg - It is not possible to consider him well psychiatrically because he is only 23 yr

,’ Family From

old. The mother uninformative.

R

Mendlewlcr,Fleiss

is thus not clearly

and

heterozygous

Fieve

(1975)

for affective

illness,

and the family

is

(b) WINOKUR and TANNA~ Family W-6. The proband (II-I) has one brother (11-2) who has never been ill, but he is only 24 yr old. The other sibs are psychiatrically ill (II-1 and 11-3) and the heterozygosity 6 52

50

Xg!a+)

2 XQ(a+)

I

Ii

24 24 26 2II 3 Xq(o+)Xq(a-1 xgto+1

0

Female w!th depresston

M

Female

mole

W-6

:fi

cl l

Well

Fomlly

wel I female only

has shown monlc

From

Wlnokur

of the mother for affective disorder tive for the current analysis.

n

H behOvlOr /

and Torna

Mole

with depression

only

Mole

has shown momc behowor

Probano

(1969)

is thus in doubt.

The family is not considered

informa-

(c) MENDLEWICZ and FLEISS’ (Fig. 5) Family MO. The proband’s father (I-l) has unipolar illness and his mother (I-2) has bipolar illness. Since male-to-male transmission is known to occur in this disorder, it would be conservative not to include this family in the present analysis. Family CR. The proband (U-2) is a woman who has one son (III-l) who is ill and Xg+. Since the other son (111-2) is 20 yr old, he is not through enough of the age of risk to consider him as psychiatrically well. He does demonstrate that his mother carries an Xg allele. The proband’s maternal grandfather (I-l) is well and Xg-l-, thus- demonstrating that

110

ELLIOTS. GERSHON

and WILLIAM E. BUNNEY,JR.

FAMILY MO

FAMILY GR

the mother is in repulsion. The ascertainment condition is then grandfather in repulsion (CGR), and the likelihood of one son in coupling is simply 8. Edwards’ ascertainment correction is not needed here since the probability of ascertainment in this case is not a function of 8. FamiIy RS. This family is discussed above, since it was informative for both Xg and protanopia (Fig. 3). As noted above, it does not appear possible at the current ages of the proband’s children to consider the family informative for linkage to mood disorders. Family E. The proband (11-4) has two brothers who are less than 25 yr of age (II-2 and 11-3) and one (II-l) who is 27 and ill. No evidence is present to indicate that the mother (I-2) is heterozygous for manic-depressive illness. The family is therefore considered uninformative. ANALYSIS

OF LINKAGE

OF Xg AND BIPOLAR ILLNESS

In the data reviewed (Table 2) here the maximum likelihood estimate is 0 = O-2 and

THE QUESTIONOF X-LISKAGE IN BIPOLAR MANIC-DEPRESSIVEILLSES

FIGURE

111

j

(conhu+d)

Family RS from Mrndlerviz

,nci Fleiss (1974)

FAMILY

RS

\\ lod score = 1.88 (odds favoring linkage 77 : 1) (Table 3). These data are thus compatible with the hypothesis of X-linkage, although because of the statistical nature of lod scores, the odds should be greater than 1000 : 1 (lod score > 3) for linkage to be accepted.l? In view of the methodologic and statistical problems described, it would be premature to conclude that linkage is present. The importance of age of onset as a methodologic problem in this data may be illustrated by the fact that in 8 out of the 21 informative pedigrees, the evidence that the mother is heterozygous for affective disorder is a single well individual between the ages of 26 through 30. In each case, if that person became ill the family would not be informative. Without these families, the linkage would be even further from statistical significance. Family E from Mendlewicz and Fleiss (1974) FAMILY

CHROMOSOMAL MATTHYSSE’~

E

MAP DISTANCES

has pointed out the apparent inconsistency of the reported close linkage with known map distances. As determined by MENDLEWICZ et al.,’ the linkage of bipolar illness to both the protan/deutan region of the X-chromosome, and to the Xg locus, is so close that these latter regions (protan/deutan and Xg) should be closely linked to each other. Even if we maximize the implied distance between Xg and protan/deutan by assuming that the putative BP locus lies between them, the total map distance Xg-BP-

ELLIOT S. GERSHON and WILLIAM

112

E. BUNNEY,JR.

TABLE2. SUMURY OF REVIEWEDPEDIGREES Reported

Findings?

analysis Xi ,MDMarker

Protan color blindness

Source

MENDLEWICZ~

blindness

~%,lNOKUR2 ME~~LEWI~Z~ ME~DLEwI~zJ REICH’

xg

MENDLE~VICP

Deutan color

ME~DLEWICZ~ WNOKUR’ MENDLEWICZ~ MEP*~LEWICZ’ MENDLEWICZ’ MENDLEWICZ~

Familq

I

Alger RS 5 Calvert P R W-6 GR RS E MO

astertainment’

MBC MBC MBC MBR S2 MBC s2 S2 MBC MBC MBR MBR

or X- MD-

1 0 2 1 6 4 2 3 2 2 0 0

Xt

Review MD-

or X- MD +

I 3 0 1 0 0 0 0 0 0 2 2

analysis

Findings? )lD-X-

or

Y+fD +

GC -

2

0

6-C S2 MBC

; 3’ -

T 0 0

GR

1 _ 0

0

s2 _

2

*Ascertainment terminology of EDWARDS.~ MBC and MBR refer to mother’s brother in coupling or repulsion. GC and GR denote grandfather in coupling or repulsion. S 2 refers to sons and mother examined simultaneously. EDWARDS~provides tables to calculate the likelihood of each sibship according to the method of ascertainment. TFindings in informative offspring. X refers to X-chromosome linkage marker studied, MD refers to manic-depressive (or other affective) disorder. For color blindness, offspring who are Xf, MD+ are color-blind and manic-depressive, whereas X-, MD- offspring have no abnormalities of color vision or mood. If there is very tight linkage between two loci, all the informative offspring in a given family will fall in the same “Findings” column. If there is no linkage detectable, offspring in any one family would fall with equal probability into both columns.

protanjdeutan should be 26 map units (centimorgans) (Table 4). Linkage between Xg and protan/deutan should thus be directly observable, yet studies of 26 pedigrees informative for Xg and protan or deutan did not support linkage. 16 In that study, the maximum lod score was + 0.148, with a maximum likelihood estimate of the recombination fraction (0) equal to O-42 (corresponding to 48 map units). The lod score method of evaluating sequential tests for linkage” requires a score > 3.0 to conclude that linkage is present. Using 95% confidence limits suggested by RENWICKE and SCHULZE, l6 the implied distance of 26 map units can be rejected. Conversely, the 95 % confidence limits of the Xg-BP and BP-protan/ deutan distance are 28 and 18 units which sum to Iess than the maximum likelihood estimate of 48 units between Xg and the color blindness loci. Thus, there is a possible incompatibility between the two close linkages reported between each locus and bipolar illness. More recent studies suggest more strongly that there is no observable linkage between Xg and the color-blind loci. As recently reviewed by PEARSON et al.” studies of translocated chromosomes using cell culture methods find that Xg is on the short arm of the X-chromosome and that the color-blind loci are at the distal end of the long arm. [Color blindness is not detectable in tissue culture, of course, but G6PD (glucose 6-phosphate dehydrogenase), which is closely linked to both color-blind loci, is detectable.] The enzyme HPRT (hypoxanthine guanine phosphoribosyl transferase) has been located on the long arm of the Xchromosome, closer to the centromere than G6PD and therefore closer than G6PD to the short arm of the X-chromosome. Yet family studies suggest that HPRT is not within

THE

TmLE 3.

Qtxs-rrox

CO.WUTATION

O-III G-III G-II KA B Sum of lods

X-LNUGE

ILLSESS rx B~EOLARMANIC-DEPRESSIVE

OF XAXfWJ~l LIKELIHOOD RECOWXNATON

Source and family designation MENDLEWICZ, F~E~Eand FLEX& P MO L M K SA C R S F co WWOKVRand TANNA~ w7 MENDLEWICZand FLEISs7 LO SI G%

OF

FREQUESCY

FORX~XW

Recombination frequency (9) 0.2 0.25 0.3

o-45

0.24 0.24 - 0.10 0.14 - 0.05 0.10 0.19 - 0.08 0.03 0.03 0.03

0.13 0.13 - 0.05 0.07 - 0.02 0.05 0.09 - 0.04 0.02 0.02 0.02

0.04 0.04

0.17

0.10

0.05

0.01

0.22 0.22 0.22 0.06 0.08 0.08 0.08 0.08 0.17 1.48

0.14 0.14 0.16 0.03 0.05 0.05 0.05 0.05 0.10 0.92

0.07 0.08 0.10 0.02 0.02 0.02 0.02 0.02 0.05 0.50

0.02 0.02 - 0.05 0.00 0.00 0.00 OGI 0.00 0.01 0.13

0.1

0.81 @81 - 1.47 0.52 - 0.76 0.51 0.80 - 1.45 0.22 0.22 0.22

0.74 0.74 - 0.92 0.47 - 0.47 0.45 0.71 - 0.90 0.19 019 0.19

066 066 - 0.62 0.42 - 0.32 0.38 0.62 - 0.59 0.15 0.15 0.15

0.57 0.57 - 0.42 0.36 - 0.21 0.31 0.51 - 0.39 0.12 0.12 0.12

0.47 0.47 - 0.28 0.29 - 0.14 0.24 0.41 - 0.25 0.09 0.09 0.09

0.36 0.36 -0.18 0.22 - 0.08 0.17 0.30 - 0.15 0.06 0.06 0.06

0.51

0.45

0.38

0.31

0.24

0.52 0.52 - 0.22 1xlO

-

-

-

0.47 0.47 0.70 0.19 0.47 0.17 0.47 0.47 0.45 0.54

0.42 0.42 0.52 0.15 0.32 0.32 0.32 0.32 0.38 1.61

0.36 0.36 0.40 0.12 0.21 0.21 0.21 0.21 0.31 1.88

0.29 0.29 0.30 0.09 0.14 0.14 0.14 0.14 0.24 1.77

0.76 0.76 0.76 0.76 0.51 - I.33

-

-

-

-

BIPOLARlLL.NESS

0.4

0.05

0.15

113

0.35

-

-

- 0.01

0.02 0.00 0.01 0.03 - 0.01 0.00 0.00 OXKI

Values in table are the log of the likelihood of the pedigree given for value of 0 of that column. rMa?timum likelihood estimate of 0 is approximately 0.2. The lod score (logarithm of ratio of the likelihood of 8 = 0.2 to likelihood of 8 = 0.5 (no linkage)) is 1.88. For accepting linkage, lod score should be 3.0.12 TABLE~.&RO.MOSOMAL

Xg and protan blindness MENDLEWICZ~~~FLEISS~ Bipolar illness blindness MENDLEW~CZ and FLEISS~ Bipolar illness blindness MENDLEWICZand FLEISS~ Bipolar illness ‘Methodof

mapping

DISTANCES DERIVED FROW

Markers analyzed

Source RENWICKand

hf.e

SCHULZE16

REPORTED

LINKAGE ANALYSIS

Recombination frequency Maximum 95% likelihood confidence estimate limit

Map distance (morgans)* Maximum 95% likelihood confidence estimate limit

or deutan color

0.42

0.30

0.48

0.31

and protan color

0.10

0.18

0.10

0.18

and deutan color

0.07

0.15

0.07

0.15

and Xg

0.19

0.27

0.19

0.28

CARTER~~~FALCONER~~

distance

of either the G6PD color blindness complex (as reviewed by PEARSON it unlikely that Xg and G6PD color blindness are within confirmable mapping distance of each other, as would be required by the observations closely linking bipolar illness to both regions. et ~1.“) or of the Xg locus,‘* making

ELLIOT S. GERSHON

111

and

Wrrruac E. BUNSEY, JR

If the recombination

fractions and map distances between bipolar illness and both Xg as suggested by ~IESDLEWICZet ~7l.,j-~ much of the available data on observable recombination and translocation between other X-chromosome loci17-1g would need to be called into question. and protan-deutan

were as small

DISCUSSION

The map distance incompatibilities prevent us from accepting the data presented as supporting the current hypothesis that bipolar affective illness is linked to both Xg and to protan and deutan color blindness. It is possible, however, to consider the evidence for each linkage separately. The question of linkage to protan and deutan color blindness must remain open until the association of bipolar illness with color blindness in the reported data is better understood. An associaton could be generally present, as a result of linkage disequilibrium, selective pressure, or a common biochemical factor in affective illness and in the alterations in visual pigments that produce protan or deutan color blindness. Alternatively, the apparent association may have resulted from sampling or procedural artefacts. Linkage of bipolar illness to the Xg locus is compatible with the reported data, although there are still methodological problems and the statistical significance required by MORTON’~is not reached. The evidence on transmission of bipolar illness via the X-chromosome may be divided into evidence from the studies of illness in relatives of patients with affective disorder, and studies of linkage to known markers of the X-chromosome in pedigrees which were reviewed above. In a recent review of family study data” there was a ratio of male to female probands of l-2.9 for unipolar illness and l-l.8 for bipolar illness. This female predominance is compatible with a dominant X-linked disorder although other causes of association of femaleness with propensity to develop affective disorder may exist. If transmission by a single locus on the X-chromosome was generally present, then an ill male proband would always receive the allele from his mother, never from his father, His brothers should have at least a 50% probability of receiving the same illness from their mother. These predictions were tested by GERSHONef nl.” in their review. When mothers and daughters of bipolar males were compared with sons and fathers, there was a statistically significant preponderance of ill mothers and daughters, as predicted by the hypothesis. However, 10 out of 116 sons and fathers at risk had affective illness,which is higher than reported population prevalences and is not expected if there is X-chromosome transmission. The preponderance of illness in mothers and daughters over sons and fathers might result from a preponderance of females to males with the i!lness which is unrelated to the mode of transmission. A comparison of males with males would not be affected by a greater tendency of women to manifest the disorder. The X-chromosome transmission hypothesis predicts a higher probability of illness in male siblings of probands than in male parents and offspring, but this prediction was not supported in the reviewed data (Table 5). These data suggest that the paucity of male-to-male transmissions which is expected in X-linked disorder is not generally found in family studies of affective illness, and the observed difference of male and female parents-offspring may be attributable to autosomal or environmental factors in the sex ratio of affective disorders. In one of the reviewed family studies4 the predicted preponderances of male-to-female

THE QUESTIOM OF X-LINKAGE IN BIPOLAR MASK-DEPRESSWE

TABLE

5.

ILLNESS

115

BROTHERS vs FATHERS-SONS OF MALE BIPOL;ZR PROBANDS rx FIVE STTJDKES l

Relative

Ill?

Diagnosis Well

Male siblings

18

129

10

106

x’ = 0.90, 1 DF, NS: Male parents

and offspring

*From GERSHO~ et n/.1*. Data of STENDSTEDT,~~ WINOKUR,?~ HELZER, 24 GOETZL,~~ GESHON er nl.,26 and CADORET et al.27. tMajor aRestive illness (unipolar and bipolar). : In a disordx transmirted via the X-chromosome, the brothers would have a greater morbid risk than fathers and sons.

transmission and absence of male-to-male transmission were observed. The family study data is thus compatible with heterogeneity of the disorder in different populations studied, with some populations or subpopulations showing X-linked transmission. These statistical tests on prevalence in one or two types of relatives do not constitute a comprehensive test of a genetic model. The methodological problems of penetrance, age of onset, phenocopies and pleiotropy, which were discussed in relation to linkage studies, are also barriers to analysis of the mode of transmission through segregation analysis. Recent developments in multiple-threshold models allow these problems to be taken into account’O and will undoubtedly be applied in the near future. Detection of the ‘affective genotype’ through a phenotypic biologic characteristic demonstrable early in life and in all cases would allow most of the issues raised here to be definitely settled. Alternatively, application of genetic models of linkage for all-or-none disorders with incomplete penetrance, variable age of onset, and multiple forms (unipolar and bipolar) is theoretically feasible. B*1oProgress in both areas may be required for the question of X-linkage to be resolved. SUMMARY

Recent reports supporting linkage of bipolar affective disorder to X-chromosome markers raise certain questions, which must be settled before linkage can be accepted. These are: (1) Association vs lixkage The available data, on color blindness and affective illness shows an association of the two traits across pedigrees, such that persons who are color-blind tend to be affectively ill. This association would not exist on the basis of chromosomal linkage alone, which would produce an association of the two traits within individual pedigrees but not across pedigrees. (2) Analytic methods In a psychiatric disorder with multiple manifestations and a variable age of onset, the use of analytic methods that require that each informative person be definitely ill or well does not allow for errors in classification of persons who have not passed through the age of risk. Unipolar illness in families of bipolar patients has been assumed to be a manifestation of bipolar disorder; the possibility of phenocopies or of independent unipolar illness

116

ELLIOT S. GERSHONand WILLIAM E. BUXNEY, JR.

appearing in these pedigrees taken into account.

and producing

falsely positive

(3) Ambiguous pedigrees Some of the reported families are subject to alternate for linkage.

findings

analyses

of illness has not been

and may not be informative

(4) Chromosomal map distance incompatibilit> The reported close linkage of bipolar illness to both the Xg blood group and the protan and deutan forms of color blindness is not compatible with the known large chromosomal map distance between the Xg locus and the protan-deutan region of the X-chromosome. Acknowlerigemenf-We gratefully acknowledge the editorial work and manuscript preparation of AMrs JANICEN. OHLERand Miss M. BELINDAWHALEN. REFERENCES 1. ROS~OFF, A. J., HANDY, L. and PLESSET,I. R. The etiology of manic-depressive syndromes with special references to their occurrence in twins. Am. J. Psychiat. 91, 725, 1935. 2. WINOKUR,G. and TANNA, V. L. Possible role of X-linked dominant factor in manic-depressive disease. Dis. nerv. Syst. 30, 89, 1969. The genetics of mania. &I. 1. 3. REICH, T., CLAYTON,P. J. and WIKOKUR,G. Family history studies-V. Psychiat. 125, 1358, 1969. 4. WINOKUR,G., CLAYTON,R. J. and REICH, T. Manic-Depressive Illness. C. V. Mosby, St. Louis, MO, 1969. J., FLEISS,J. L. and FIEVE, R. R. Evidence for X-linkage in the transmission of manic5. ME~ZDLEWICZ, depressive illness. J. Am. med. Ass. 222, 1624, 1972. 6. MENDLEWKZ,J., FLEISS,J. L., FIEVE, R. R. Linkage studies in affective disorders. The Xg blood group and manic-depressive illness. In: Genetics and Psychuparhology. FIEVE, R., ROSENTHAL,D. and BWL, H. (Editors). Johns Hopkins, Baltimore, MD., in press, 1975. 7. ME~~LEWICZ, J. and FLEISS, J. L. Linkage studies with X-chromosome markers in bipolar (manicdepressive) and unipolar (depressive) illnesses. Biol. Psychiat. 9, 261, 1974. 8. EDWARDS,J. H. The analysis of X-linkage. Ann. hum. Genet. 34, 229, 1971. oedigree data. Hum. Hered. 9. ELSTON.R. C. and STEWART.J. A general model for the genetic analysis of _ 21,5i3, 1971. 10. EL&N, i. C. Ascertainment and age of onset in pedigree analysis. Hum. Hered. 23, 101, 1973. II. GERSHON.E. S.. BUNNEY.W. E.. JR.. LECKMAN.J. F.. VAN EERDEWEGH.M. and DE BAUCHE,B. A. The inheritance df affective disorders:‘a review if data and of hypothesei. In press, Behav. Genet. 1976. 12. MORTON,N. E. Sequential tests for the detection of linkage. Am. J. hum. Genet. 7, 277, 1955. 13. GERSHON,E. S. and MATTHYSSE,S. X-linkage: ascertainment through doubly ill probands. Submitted for publication. 14. WISOKUR, G., CADORET,R., DORZAB,J. er al. Depressive disease-a genetic study. Archs gen. Psychiar. 24,135,1971. S., CHILDS,B. and GERSHON,E. S. Frontiers of psychiatric genetics. Nelrrosci. 15. WORDEN,F., MATTHYSSE, Res. Prog. Bull. 14, 1, 1916. 16. RENWICKE, J. H. and SCHULZ, J. An analysis of some data on the linkage between Xg and color btindness in man. Am. J. hum. Genet. 16,410,1964. 17. PEARSON,P. L., SANGER,R., BROWN,J. A. Report of the committee on the genetic constitution of the X-chromosome, in Human Gene Mapping 2, Rotterdam Conference 1974. Second International Workshop on Human Gene Mapping, Birth Defects Original Article Series 11: 190, 1975. L., CARPENTER,D. G. and CARTER,C. H. Genetics of an X-linked 18. NYHAN,W. L., PESEK,J., SWEETMAN, disorder of uric acid and metabolism and cerebral function. Pediat. Res. 1, 5, 1967. 19. GERALD,P. S. and BROWN,J. A. Cytogen. Cell Genet. 13,29, 1974. J. W. and Moms, C. A. The use of multiple thresholds in determining the mode 20. REICH, T., Jms, of transmission of semi-continuous traits. Ann. ham. Gener. 36, 163, 1972. I

_

THE QCTSTIONOF X-LINMGE IF BIPOLARMASK-DEPRESSIVE 1rrx~s.s

117

21. CARTER,T. C. and FALCO~ZR,D. S. Stocks for detecting linkage in the mouse and the theory of their design. J. Genet. 50,307, 1951. 22. STE~STEDT, A. A study in manic-depressive psychosis. Acra. psychiat. neural. stand. 79, 111, 1952. 23. WINOKIJFGG., CLAYTON,P. Family history studies-I. Two types of affective disorders separated according to genetic and clinical factors. In: Recent Advances in Biological Psychiatry, WORTIS,I. J. (Editor). Plenum Press, New York 9, 35, 1967. 24. HELZER,J., WINOKIJR,G. A family interview study of male manic-depressives. Archs gen. psych&. 31, 73, 1974. 25. GOE+ZL, U., GREEN, R., WHYBROW,P. and JACKSON,R. X-linkage revisited-a further family history study of manic-depressive illness. Archs gen. Psychiat. 31, 665, 1974. 26. GERSHON, E. S., MARK, A., COHEN, N., BELIZON, N., BARON, M. and KNOBE, K. E. Transmitted factors in the morbid risk of affective disorders: a controlled study. J. psychiat. Res. 12, 283, 1975. 27. CADORET,R., WINOKUR, G., CLAYTON,P. Family history studies: VII. Manic-depressive disease vs depressive disease. Br. J. Psychiat. 116, 625, 1970. 28. CAVALLI-SFORZA,L. L. and BODMER,W. F. The Genetics of Human Populations, pp. 33, 640. W. H. Freeman, San Francisco, 1971.