Cytogenetic nomenclature

OBSTETRICS Cytogenetic nomenclature JOE LEIGH SIMPSON, M.D. ALICE 0. MARTIN, PH.D. Chicago, Illinois The advantage of a standardized nomenclature is apparent in any discipline. A standardized method for designating carbon molecules in steroid chemistry facilitates communication in endocrinology; consistent staging methods permit comparison of data in oncology. Ukewise, a standardized method for designating cytogenetic data facilitates communications in genetics. Therefore, investigators should adhere to the official recommendations summarized in this report. (AM. J. OssTET. GYNECOL. 128: 167, 19n.)

Several years later, techniques to determine the chromosomal complement of bone marrow cells and lymphocytes became available. With these techniques, cytogenetic studies of many individuals were performed and subsequently reported. In these early reports, chromosomes were usually arranged on the basis of size and the relative position of the centromere. However, prior to 1960, investigators did not necessarily designate the same complement in the same manner. Recognizing the difficulties caused lack of standardization, those investigators who had published karyotypes conferred in 1960, at a conference subsequently known as the Denver Conference. 1 Recommendations concerning chromosomal identification were offered. These recommendations were subsequently updated at two other conferences, the London Conference (1963) 2 and the Chicago Conference (1966). 3 The nomenclature recommended at the Chicago Conference ( 1966) was adopted by most genetic journals and by many clinical journals, and manuscripts published in those journals adhered to this nomenclature. Based upon the recommendations of the Chicago Conference, chromosomes are arranged according to their relative size and the position of their centromere. The cytogenetic techniques generally available prior to 1969 permitted one to distinguish only certain chromosomes (1, 2, 3, 16, andY), although by the relatively laborious technique of autoradiography Nos. 4, 5, l3, 14, and 15 and the late-replicating X chromo-

CYTOGENETICS IS becoming increasingly important in the clinical practice of obstetrics and gynecology. Both clinicians and investigators are aware of the antenatal detection of chromosomal disorders, the role of chromosomal abnormalities in the etiology of spontaneous abortion, and the chromosomal causes of infertility and primary amenorrhea. However, relatively few gynecologists appear to be aware that an official chromosomal nomenclature exists. Adherence to this nomenclature not only saves space in journals by permitting the chromosomal complement to be designated in a standard manner that requires no additional explanation but also increases accuracy of communication. This article summarizes those aspects of the nomenclature that are of most immediate relevance to the obstetrician-gynecologist.

bl

Historical aspects That human beings have 46 chromosomes was first elucidated in 1956, by the study of embryonic tissue. From the Section of Human Genetics, Prentice Women's Hospital and Maternity Center, Northwestern University Medical School.

Received for publicati(m]uly 22, 1976. Revised November 3, 1976.

Accepted December 14, 1976. Reprint requests: Dr. Joe Leigh Simpson, Section of Human Genetics, Prentice Women's Hospital and Maternity Center, 333 E. Superior St., Chicago, Illinois 60611.

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2

3

6

7

8

13

14

15

19

20

9

21

22

4

5

10

II

12

16

17

18

X

y

Fig. l. Karyotype of a lymphocyte derived from a normal male subject, illustrating the G-banding technique. The chromosomes are stained by a modification of the o·ypsin-Giemsa method. (Prepared by one of the authors in the laboratory of M. Neil Macintyre. )

some could be identified. Other chromosomes could be placed into one of several groups (A= Nos. 1-3; B = 4-5; C = 6-X-12 ; D = 13-15; E =16-18; F = 1920; G = 21 , 22; andY). Members of one group could be distinguished from members of another group but not from other chromosomes within the same group. Between 1969 and 1971, the field of cytogenetics was advanced by the development of staining techniques that produced patterns of horizontal bands on chromosomes (Fig. 1) .4 A chromosomal band is defined as a region distinguishable from adjacent regions on the basis of its lighter or darker appearance. A darkly stained or brightly fluorescent band is a positive band ; a lightly stained band is a negative band. (A band was not actually so defined until the Paris Conference! to be deseribed below.) The bands produced if a cell is stained with quinacrine and analyzed with fluorescent microscopy are Q-bands (Table I). A similar banding pattern results if chromosomes are pretreated in certain ways and stained with Giemsa stain; the bands thus produced are called G-bands. Other kinds of pretreatment also followed by Giemsa staining produce

R-bands. R-bands are complementary to Q- and G-bands; a region that stains lightly by the R-banding technique stains darkly by the G-banding technique and vice versa. Another method is C-banding, a method that utilizes vigorous pretreatment to produce darkly staining bands near the centromeres. Each of these banding techniques (Table I) is useful in particular situations. Moreover, additional techniques and modifications of previously reported techniques continue to be reported. Because of advances in banding techniques, additional terminology was needed. Thus, in 1971, a fourth nomenclature conference (Paris Conference) was held.; At the Paris Conference, a chromosome region and a chromosome band were defined, a system to designate chromosomal bands was recommended, and other aspects of chromosomal nomenclature were updated. Minor addendums were added in 1975,6 for example, suggestions for denoting various banding techniques. There are many parts of the _official nomenclature, some complicated and of less than immediate clinical relevance to the obstetrician-gyne-

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Cytogenetic nomenclature

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Table I. Chromosome banding techniques* Q-banding Banding technique

Banding patterns produced

in human subjects

G-banding

Pretreatment by: (a) Staining with concentrated sait solufluorescent dyes tions at high tempera(e.g., quinacrine dihydrochloride); tures (e.g., 60° C.), analysis by fluo(b) proteolytic enzymes, (c) other methods. rescent microscopy Staining with Giemsa Positive bands Positive bands, darkstaining; negative fluorescent; negative bands bands, light-staining. The pattern of bands nonftuorescent corresponds closely to the pattern of Q-bands

R-banding

C-banding

Pretreatment by high temperature and controlled pH; staining with Giemsa stain. Fluorescent methods may also produce R-bands Positive bands, dark-staining; negative bands light-staining, as in G-

banding. However, Rbanding pattern is usually the reverse of that observed for Qand G-bands

Pretreatment by: (a) bases (e.g., sodium hydroxide), (b) acids (e.g., hydrochloric acid), (c) other methods. Staining with Giemsa Positive bands, darkstaining; negative bands, light-staining.

Positive bands usually present only at centromeric regions

*The techniques used by a given laboratory depend upon the availability of equipment (e.g., fluorescent microscope) and the types of investigations undertaken. A given pattern of bands (e.g., G-bands) can be produced by a variety of methods. In addition, the same method of pretreatment (e.g., sodium hydroxide) can produce different banding patterns, depending upon the length of exposure and various other parameters. Although the mechanism of banding is not entirely understood, bands appear to be related to variations in the deoxyribonucleic acid composition and their interaction with acidic proteins.

Table II. Some symbols recommended by the Paris Conference (1971) 6 and its supplement (1975j1 Chromosome parts Centromere Short arm Long arm Isochromosome Deletion Translocation

Reciprocal translocation Mosaicism Chimerism Ring Dicentric Duplication Inversion Break without reunion (e.g., terminal deletion) Break and join From ... to ...

Symbol cen p q i del rep mos chi r

die dup inv

cologist. This article will summarize only the most pertinent features of the nomenclature. Symbols

In Table II, some symbols used to designate parts of chromosomes and certain rearrangements are listed. In particular, chromosome bands are numbered according to a particular system, an example of which is shown in Fig. 2. On every chromosome, certain consistent and distinctive morphologic landmarks exist (e.g., centromere, certain bands) that aid in the identification of that chromosome. A region is defined

Table III. Representative chromosomal complements, written according to the recommendations of the Paris Conference (1971) 6 and its supplement (1975) 7 Paris designation* 45,XY

46,XX 45,X 47,XXX 47,XY,+21 46,XX,1q+ 46,X,del(X)(p21) or 46,X,del(X)(qter~p21:)

46,X,i(Xq) or 46,X,i(X)(qter~

cen.....,.qter) 46,X,r(Y) 46,X,t(X;3)(q21;q31) 45,X/46,XX or mas 45,X/46,XX Xp21

Chromosomal complement !'-.J"ormal male karyotype Normal female karyotype MonosomyX Polysomy X Trisomy 21 Increase in length of the long arm of No. 1 Terminal deletion of the short arm of X distal to band p21 Isochromsome of the long arm of X Ring Y chromosome Balanced translocation involving band 21 ofthe X long arm and band 31 of the long arm of No.3 45,X/46,XX mosaicism Region 2 band 1 on short arm of X

*The shortened system is illustrated for each complement; the detailed system is also illustrated for deletions· and isochromosomes.

as the area between any two adjacent landmarks. A region is further divided into bands. A band is designated by listing the chromosome, the arm (p or q), the region, and the specific band. Within a given region, bands are numbered consecutively, from the centromere distally.

170 Simpson and Martin

.\>fay 15, 1977 Am . .J. Obstet. Gym•ml.

(13-15)

Chromosome No. 3

•

~

Iii Iii

Chromosome No.4

H

n

(13- 15)

Breakage at Positions 3q27 and 4q23

~

/

H

Reunion Leading to a Reciprocal Translocation Fig. 2. A reciprocal translocation between chromosome Nos. :l and 4. A normal chromosome No. 3 and a normal No. 4 are shown in the top row, along with diagrammatic drawings of the expected G- or Q-banding patterns, according to the Paris Conference. 5 In the middle row the positions of the breaks are designated, and the sections interchanged after breakage are shown. The two translocation chromosomes formed as a result of reunion and reciprocal exchange are shown in the bottom row. An individual who has both these translocation chromosomes, as well as a normal No. 3 and a normal No. 4, is said to be a carrie'· of the translocation in its balanced state (translocation heterozygote) . Such an individual is expected to be phenotypically normal, because no duplication or deficiency of genetic material has occurred. In the shortened system, this translocation would be designated !:l;p(3;4) (q27;q23); in the detailed system !:l;p(3;4) (3pter_,. 3q27:: 4q23 _,. 4qter) (4pter ---+ 4q23:: 3q27 ---+ 3qter). The diagrams in this figure are based not upon actual measurements but rather upon the relative sizes and positions of the bands (Paris Conference, 1971 ). 6 (Cytogenetic cultures were pl-epared in the laboratory of M. Neil Macintyre.)

Designation of chromosomal complements The chromosomal complement-abnormal or normal-is designated by writing sequentially: (I) the total number of chromosomes, (2) a comma, and (3) the sex chromosomal complement (XY in normal male subjects; XX in normal female subjects). Thus, the normal male chromosomal complement is designated 46,XY (Table III). A complement containing an ab-

(13-15) Fig. 3. Partial karyotypes of Group D chromosomes (Nos. 13-15) from two women. eac h of whom had repeated spontaneous abortions. The upper row shows the appearanre ot the chromosomes with nonbanding staining techniques. Both women would be said to have the complement 45,XX,t(DqDq). Autoradiography might have provided additional information on the identit y of the chromosomes involved. The middle row shows that the chromosomes involved in the translocation (arrow) of the first woman are Nos. 13 and 14 [45,XX,t(l3q14q)] (trypsin-Giemsa staining). Such a u·anslocatiun can lead to both genetically balanced and genetically unbalanced gametes; thus, both normal and abnormal offspring can be produced. The bottom row shows the appearance of the chromosomes of a second woman (trypsin-Giemsa staining). The two chromosomes involved in the translocation (arrow) are homologues, the two ch romosome Nos. 15, [45,XX,t(J 5q I 5q)]. An individual with such a translocation can produce only genetically unbalanced gametes. which lead to either trisomic or monosomic zygotes. Because both trisomy 15 and monosomy 15 are lethal, all pregnancies of the second woman 'i
normal number of sex chromosomes is designated by listing the total number of chromosomes and the appropriate sex chromosomal complement. For example, 45,X is the complement most commoniy associated with the Turner syndrome. "45,XO" is not a correct designation. One designates a complement contain-

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ing additional or m1ssmg autosomes by writing: ( 1) the total number of chromosomes, (2) the sex chromosomal complement, and (3) a + or -sign followed by the number of the missing or additional chromosome. Thus, a male subject with trisomy 21 (Down's syndrome) is designated 4 7,XY, + 21; a female subject believed to have monosomy 21 would be designated 45,XX,- 21.

Structural aberrations Involving one chromosome One designates a complement containing a structurally abnormal chromosome by writing ( 1) the total number of chromosomes, (2) the sex chromosomal complement, (3) the symbol for the particular structural aberration present (Table II), and (4) the number of the aberrant chromosome. Two systems exist (Table III). In one system, the detailed system, altered chromosomes are defined by their band composition. In the other, shortened system, altered chromosomes are defined only by their break points. For example, in the shortened system 46,XX,del(X)(q21) indicates that a terminal deletion occurred at band q21 of the X [i.e., long arm (q), region 2, band l]. The genetic material distal to that band was lost, with the remaining portion of the X consisting of the entire short arm and the portion of the long arm located between the centromere and band q2l. In the detailed system, the same complement would be designated 46,X, del(X)(pter-q2l :), i.e., the portion of the X remaining extends from the terminal (ter) portion of Xp to band Xq21, where a break without reunion (:) occurred. A ring chromosome is designated by its two break points. If a ring X chromosome forms following a break in the short arm at band p21 and in the long arm at band q31, the chromosome is designated r (X)(p21 q31 ). Incidentally, if different breaks occur in the same chromosome, such as in a ring, no punctuation is required between the designated bands [e.g., r(X)(q2lq31]. If breaks occur in different chromosomes, as in a translocation, a semicolon is usually required between the designated bands, e.g., t(X;3)(q2l;q31); however, a semicolon is not always required in designating Robertsonian translocations (see below). Structural aberrations that may be encountered by the obstetrician-gynecologist include translocations, isochromosomes, duplications, inversions, and dicentric chromosomes.

Structural rearrangements Involving more than one chromosome There are two basic types of translocations-Robertsonian and reciprocal (Figs. 2 and 3). Their for-

Cytogenetic nomenclature

171

mations and hence their designations are more complex than the types of chromosomal abnormalities cited above. In Robertsonian translocations, two acrocentric chromosomes (chromosomes whose centromeres are nearly terminal, hence essentially onearmed) join at their centromeric regions to form a single chromosome that appears metacentric (twoarmed). In reciprocal translocations, interchange of chromosomal material occurs between any two nonhomologous chromosomes, acrocentric or not, at any locations. The symbol for translocation is!· If one wishes to emphasize that a translocation is either Robertsonian or reciprocal, the symbols rob or !fi!· respectively, may be used in lieu of 1· One may also need the punctuation ;...,; to indicate break and join,_;_ to indicate break without joining, or _;_ to separate the designations for two or more chromosomes. Translocations may be designated in several ways, depending upon whether the shortened system or the detailed system is used. The detailed system may appear unwieldl}', but it is conceptually simple. However, the shortened system will usually suffice. In either system one designates a translocation by listing those chromosomes that are not present in their usual form followed by information concerning the translocation. Robertsonian translocations (Fig. 3) are the translocations most likely to interest obstetricians because they can lead to Down's syndrome. An example of a Robertsonian translocation is 46,XY,-l4,+ t(l4q2lq). This indicates an unbalanced chromosomal complement, consisting of two separate No. 21 chromosomes, one separate No. 14, and one translocation chromosome, consisting of the long arm of No. 14 and the long arm of No. 21. In such an individual, the long arm of No. 21 is present in triplicate; thus, that individual has Down's syndrome. The complement 45,XX,-13,l4,+t(l3ql4q) indicates that a female subject with 45 chromosomes carries a balanced Robertsonian translocation between chromosomes Nos. 13 and 14 (Fig. 3); only one separate No. 13 and only one separate No. 14 are present. Individuals with balanced Robertsonian translocations are phenotypically normal. although the gametes may be chromosomally unbalanced. Balanced translocations may also be designated in very brief form, namely, 45,XX,t(l3ql4q); however, unbalanced translocations should always be written in full, according to either the shortened system or the detailed system. Reciprocal translocations are designated by first listing the chromosomes involved and then listing the bands involved. Fig. 2 shows a reciprocal translocation that occurred between chromosome Nos. 3 and 4;

172 Simpson and Martin

this translocation is designated 46,XY,rcp(3;4). The chromosome with the lowest number is designated first, although if a sex chromosome is involved it is listed first. Methods to designate more complex arrangements, such as translocations involving three or more chromosomes, have also been recommended. However, individuals with such rearrangements are more rare than individuals with most of the other rearrangements discussed above. In addition, the Paris nomenclature recommends designations for meiotic preparations5 • 6 and chromosome variants. 6

X-chromatin and Y-chromatin X chromosomes in excess of one condense tightly during interphase. These X chromosomes, believed to be genetically inactive because of their tightly condensed state (Lyon hypothesis). form a planoconvex

May 15, 1977

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body that can be detected along the nuclear membrane during interphase. This mass is termed X-chromatin. Older terms for the X-chromatin (mass) include Barr body or sex chromatin mass. After quinacrine dihydrocholoride staining and fluorescent microscopy. the distal two thirds of the long arm of the Y chromosome fluoresces brilliantly in interphase nuclei, as it does throughout the cell cycle. The fluorescent spot produced by the Y during interphase is called Y-chromatin; an older term is Y-body. The number of Y chromosomes may be deduced from the numbers of Y-chromatin. their numbers being equal. However, the testicular determinant(s) are located not in the fluorescent portion but in the nonfluorescent portion, specifically in the short arm close to the centromere. 7 • H Thus, male subjects lacking Y-chromatin may be normal.

REFERENCES l. Denver Conference: A proposed standard system of nomenclature of human mitotic chromosome, Lancet 1:

1063, 1960.

2. London Conference: The normal human karyotype, Ann. Hum. Genet. 27: 295, 1963. 3. Chicago Conference: Standardization in Human Cytogenetics, Birth Defects 2(2): I, 1966. 4. Caspersson, T .. and Zech, L.: Chromosome Identification, Nobel Symposium 23, New York, 1973, Academic Press, Inc.

5. Paris Conference (1971): Standardization in Human Cytogenetics. Birth Defects 8(7): l, 1972. 6. Paris Conference (1971), Supplement (1975): Standardization in Human Cytogenetics, Birth Defects 10(9): l, 1975. 7. German, J., Simpson, J. L., and McLemore, C.: Abnormalities of human sex chromosomes. I. A ring Y without mosaicism, Ann. Genet. 16: 225, 1973. 8. Simpson, J. L.: Disorders of Sexual Differentiation: Etiology and Clinical Delineation, New York, 1976, Academic Press, Inc.

Nutritional care of the HospitaliZed Patient The University of California School of Medicine, San Francisco, Extended Programs in Medical Education will present "Nutritional Care of the Hospitalized Patient," June 9-11, 1977, at the Hyatt Regency Hotel, Five Embarcadero Center, San Francisco,. California. For information, write: Extended Programs in Medical Education, 1308 3rd Ave., University of California, San Francisco, California 94143. Telephone: (415) 666-2483.