Transcript 1. dia
Joseph Schlammadinger
retired associate professor Lectures in March, 2013 (revised versions)
Note and don’t forget, please
• •
Facts, their description, organization and interpretation within one chapter of a given science–in our case: Genetics–may be different as taught by different lecturers, or seen in different sources (e.g. textbook vs. lecture, and so on). The essence, however, is the same even if different terms, words, expressions are actually used, provided the students understand and use them correctly.
This formula is true to exam questions, too.
CYTO GENETICS
2013
Morphologically (microscopically) identifiable chromosomes are seen only in eukaryotic organisms
Cytogenetics is the science of chromosomes, elements of genetics which can be analyzed cytologically. (Better in LM than in EM.) ONE CHROMOSOME = ONE DNA MOLECULE The linear DNA molecule runs form one end of the chromosome (telomere) to the other end. It is organized into chromatin (with histones and other proteins), in a form of multiple coils. The physical length of a metaphase chromosome is approximately 1/10,000 of the total length of the DNA making that very chromosome.
The sequence of the genes within one DNA molecule, i.e. within a given chromosome is determined, and is characteristic and specific to species.
(See details in genetics: linkage, linkage groups, etc., and the results of the Human Genome Program.)
(G 0 ) G 1 M S G 2
Phases of the cell cycle:
G 1 , S and G 2 = interphas Chromosomes are not visible individually*.
M = mitosis .
Individual chromosomes are visible, the best in the metaphase.
* Novel methods, however, may show (and even identify) the individual chromosomes or part(s) of them in interphase, too. (See FISH and next slide.)
Arrangement of chromosomes in an interphase nucleus.
(Computer reconstruction in false colours.)
MITOSIS (first half)
Metaphase
MITOSIS (second half
cytokinesis)
Metaphase chromosome as seen in the electron microscope
Variable stained THE NUMBER*, SHAPE, relative SIZE, BANDING PATTERN and GENE SEQUENCE (= order of loci**) of CHROMOSOMES is a characteristic standard for each species.
Morphological (light microscopic) features of chromosomes of each species are summarized in the IDEOGRAM , most conveniently in diagrammatic representation .
The chromosomes of a given individual are evaluated by comparing them to the ideogram of the very species concerned.
*Higher order eukaryotes are typically diploid organisms. The normal (diploid) human chromosme number is 2n = 46.
** Locus (latin) = place, location (plural: loci). In genetics: the location of a gene in question within a specified chromosome.
LIGHT MICROSCOPIC INVESTIGATION OF HUMAN METAPAHSE CHROMOSOMES
I. TRADITIONAL PROCEDURE 1/ Dividing cells (predominantly: in vitro cell culture, e.g. PHA [phytohemagglutinin] stimulated T lymphocytes).
2/ Accumulation in the metaphase: aided by colchicine treatment.
3/ Swelling in hypotonic solution (0.075 M KCl).
4/ Fixing.
5/ Spreading on specimen slides.
6/ Staining: Giemsa’s solution.
7/ Light microscope: metaphase plate.
8/ Photography.
9/ Print. Cut out images of individual chromosomes and arrange them according to the principles of the ideogram (Denver nomenclature).
10/ The result is an ordered (arranged) karyogram . Evaluate and 11/ establish karyotype .
Some comments on terminology
English (mainly USA) (printed photo of a) metaphase spread karyotype (in photographic presentation) karyotype (symbols) European not ordered karyogram ordered karyogram karyotype (symbols)
SHAPES OF CHROMOSOMES, I
A: metacentric, B: submetacentric, C: acrocentric.
1: sister chromatids, 2: centromere, 3: short arm, 4: long arm, 5: satellite, 6: secondary constriction.
A (not ordered) karyogram
Shapes of chromosomes, II
(for instance in mouse)
PRINCIPLES OF KARYOGRAM ARRANGEMENTS (diploid human cells)
The Denver system .
Order the chromosome images according size: the biggest is the first. In the case of same size pairs the more metacentric comes first.
The homologous pairs , which are identical in both sexes, are numbered from 1 to 22. These are called autosome s .
Sex chromosome s are denoted with X and Y . They are not homologs, contain different genes, but in meiosis of the male they behave as homologs in the reduction division.
Chromosomes also have been grouped: A (1-3) B (4-5) = big submetacentrics, C (6-12) = big metacentrics, = medium size submetacentrics, D (13-15) = big acro-centrics, E (16-18) = small submetacentrics, F (19-20) = small metacentrics, G (21-22) = small acrocentrics.
On that basis X belongs to group C, Y to group G.
ARRANGED KARYOGRAMS
FEMALE MALE
Description of karyotypes
with conventional symbols: 46,XX
=
altogether 46 chromosomes, among them two X, normal female karyotype.
46,XY
=
altogether 46 chromosomes, among them one X and one Y, normal male karyotype
.
In English speaking countries (and in general: in English publications) usually the image of a–representative, characteristic– ordered karyogram is given as a karyotype . (The use of the term karyogram is only occasional.) (See further details below.)
LIGHT MICROSCOPIC INVESTIGATION OF HUMAN METAPAHSE CHROMOSOMES
II. BANDING TECHNIQUES 1/ Dividing cells (predominantly: in vitro cell culture, e.g. T cells).
2/ Accumulation in the metaphase: aided by colchicine treatment.
3/ Swelling in hypotonic solution (0.075 M KCl).
4/ Fixing.
5/ Spreading on specimen slides. (Pretreatment for banding.) 6/ Staining: Giemsa’s solution or quinacrine (a fluorescent dye) like C, R and T, developed by special staining procedures.) . These give G bands and Q bands, respectively. (There are also other bands, 7/ Conventional light or fluorescent microscope.
8/ Photography.
9/ Print. Cut out images of individual chromosomes and arrange them according to the principles of the banded ideogram (Paris nomen clature).
10/ The result is an ordered (arranged) karyogram . Evaluate and 11/ describe karyotype .
Q-bands and G bands on chromosomes,
and (Paris) ideogram
There are other banding procedures, too, resulting in C , R and T bands, respectively. Today the Giemsa ( G ) banding technique is mostly used (G + = dark staining, G – = light staining). Chromosome arms are divided in regions and these are numbered 1, 2, 3, and so on in both p and q directions, starting from the centromere . The bands within the regions are numbered according to the same rule.
In quinacrine ( Q ) banding Q + fluorescence, Q – = intense = faint fluorescence. Q + Q – = G – .
= G + , A number of new high resolution banding techniques have also been developed which allow better longitudinal resolution within a chromosome.
Human chromosome #1, standard (left) and results of two high resolution bandings.
NUMERICAL ABERRATIONS OF HUMAN CHROMOSOMES
NUMERICAL ABERRATIONS,
POLYPLOIDIE
S
triploidy (3n = 69) tetraploidy (4n = 92)
Human numerical chromosome aberrations,
POLYPLOIDIES All these are mentioned as euploidy , the actual chromosome numbers are exact multiples of n, i.e. of the haploid set.
Please note. Triploidy as well as tetraploidy are lethal conditions in humans , they result in spontaneous abortion rather than in stillbirth.
The traditional nomenclature defines these conditions, however, as euploidy : they are exact multiples of n. (The haploid chromosome set is represented by n. In Homo sapiens n = 23. Triploidy = 3n, tetraploidy = 4n.) “Eu” as first syllable in some compound words means “real”, “right”, “good”, “regular”, “self evident”, and so on.
In plants polyploidy may contribute to the development of desirable traits in agriculture (higher yield, better resistance, and so on). The majority of wheat (corn) sorts in production are hexaploid (6n = 42). In that case we speak about allopolyploidy , where the three originally diploid chromosome sets come from three different (closely related) ancestors.
In the animal world, in vertebrates polyploidy is compatible with life–if at all–in salamanders (amphibian); in higher order animals that is lethal, like in humans.
Human numerical autosome aberrations (anomalies), aneuploidies
(Euploidy: see earlier. “an-” = privative prefix. Aneuploid = non-euploid.
The chromosome number is not exact multiple of n.)
CAUSES OF ANEUPLOIDIES
1) Nondisjunction . Homologous chromosomes or sister-chromatids fail to separate from each other. It results in hyper or hypohaploid gamete, or in hyper and/or hypodiploid somatic cells.
Occurs either in a/ meiosis , when all the cells of the offspring are aneuploid, or in b/ mitosis , where it creates (organs) involved.
chromosome mosaicism , that is, in one individual two (or more) cell types (cell lines) can be ascertained with different chromosome numbers. (For instance 46,XX / 47,XX,+21.) The extent and severity of the symptoms depend on the ratio of normal / aneuploid cells, on one hand, on the other, on the tissues 2) Delay (late arrival) of a chromosome division a chromosome cannot reach the newly forming nucleus, remains in the cytoplasm, where it will be destroyed. This phenomenon creates monosomy, i.e. . In the telophase of the cell hypodiploidy . May occur in mitosis and in meiosis as well.
>2n = hyperdiploid, <2n = hypodiploid. The latter is rare because of the low viability of monosomic cells.
NONDISJUNCTION…
…of X chromosomes in the first (left) or in the second (right) female meiotic division . Please observe. If the first meiotic division is abnormal, the resulting oocyte (mature egg cell) contains the two X homo logs. If a nondisjunction occurs in the second meiotic division, sister chromatids remain together, which results in the presence of two identical X chromosomes in the egg cell (apart from the effects of crossing over).
MITOTIC NONDISJUNCTION
The monosomic cell is non-viable, at least in the majority of the cases.
MOSAICISM An individual organism ( from one zygote ) is composed of two (or more) genetically different types of cells.
In this case: chromosome mosaicism, e.g. 46,XY/47,XY,+21 .
CHIMERISM An individual organism contains genetically different cells which derive from two zygotes .
In our example: chromosomal chimerism, e.g. 46,XX/46,XY .
Possible sources of chimeras Two egg cells fertilized by two sperm cells, and the two zygotes form one preembryo . Essentially the same, one cell, however, is a fer tilized polocyte (fed by the fertilized egg cell).
Blue = nucleus Dizygotic twins may exchange cells (e.g. bone marrow stem cells) via their common placenta. Green signal = X chromosome .
Organ transplantation: 46,XY ► 46,XX. Red signal = Y chromosome .
DOWN SYNDROME, I
What is a syndrome?
A group of symptoms and signs, which, when considered together, characterize a disease or lesion.
It is not necessary to see all diagnostic signs in one patient, some 3-4 very typical leading symptoms may already define a syndrome.
In this cytogenetics lecture you will see: Down syndrome, Pätau syndrome, Edwards syndrome, Turner syndrome, Klinefelter syndrome, and Cri du chat syndrome (cat cry disease).
You need not to learn the symptoms and signs of the syndromes listed above (those are dealt with in detail by Clinical Genetics), but you are supposed to mention the most important characteristic(s), one or two, in order to show that you know what you are speaking/writing about.
Down syndrome, II
Down syndrome as trisomy 21
Karyotype: 47,XX,+21 or 47,XY,+21, that is trisomy 21 .
If the supernumerary chromosome #21 is present in all the cells of a Down syndromic individual, we deal with a form of meiotic origin. (See the characteristic symptoms and signs in the next figure.) In the case of mosaicism (which is very rare), the symptoms are strikingly variable and diverse, e.g. no mental retardation in the presence of well visible facial signs (or vice versa), and so on.
Symptoms and signs in Down syndrome
In creased liability to leukemia Increased liability to infections
Incidence birth.
of meiotic trisomy 21 in relation to maternal age at Pregnant women above 35 are at increased risk to have a Down syndromic baby, they should be offered a pre natal diagnosis. (The risk of other aneuploi dies is also higher.) Don’t forget: the majority (about 80%) of Down syndromic child ren are born to mothers <35 years.
Down syndrome and maternal age
Maternal age and risk of Down syndromic birth (international statistical data) Maternal age Risk of Down syndr.
Maternal age Risk of Down syndr.
Maternal age Risk of Down syndr.
<25 1:1500 33 1:570 42 1:65 25 26 27 28 29 30 31 32 1:1350 1:1300 1:1200 1:1100 1:1000 1:910 1:800 1:680 34 35 36 37 38 39 40 41 1:470 1:380 1:310 1:240 1:190 1:150 1:110 1:85 43 44 45 46 47 48 49 50 1:50 1:35 1:30 1:20 1:15 1:11 1:8 1:6
Do we have any means to prevent birth of Down syndromic babies?
Yes, termination of pregnancy with a 21 trisomic embryo/fetus. Not legal in all countries. Where permitted on the basis of medical indication, one needs well established diagnosis .
How can a reliable diagnosis be made? By karyotyping only . Sampling for that, however, is an invasive method .
Based on the statistical data cited above, is the maternal age >35 yr the only indication of a prenatal diagnostics?
And why 35?
The latter is a compromise. Risk of “spontaneous” abortion after sampling vs.
risk of birth of Down syndromic child. Above 35 the latter is higher.
Are there some non-invasive methods at hand indicative of Down syndromic pregnancy, if present, in maternal age groups <35 yr?
First: in utero ultrasound investigation of the embryo .
Second: determination of biochemical markers in maternal blood , which might be different in normal vs. Down syndromic pregnancies.
If these investigations disclose some elevated risk of the birth of a Down syn dromic child, chromosome diagnosis has to be made.
Down syndrome diagnosis and screening Assesment of risk in utero Praenatal chromosome analysis – reliable diagnosis
(1)
Chorionic villus sampling (CVS) /
(2)
Amniocentesis Screening in (early) pregnancy Ultrasound .
Determination of the size of nuchal translucency (NT).
Biochemical marker investigations in maternal blood. They are non-invasive methods (taking venous blood is generally assumed harmless). The evaluation is statistical, if more markers are investigated, they can be indicative of the eventual presence of a Down syndromic embryo/fetus in the maternal womb. A combina tion and comparison of the first and second trimester findings are evaluated.
Main biochemical markers used in screening protocols: Alpha-fetoprotein (AFP)* Non conjugated (unconjugated) oestriol (uE3) Inhibin A (INH-A), a peptide hormone (of two subunits), inhibits FSH ** excretion.
Pregnancy-associated plasma protein (PAPP-A) Free and/or total human choriogonadotrophin (hCG) * Very important in prenatal diagnostics of neural tube defects (NTD), see there.
** Follicle stimulating hormone.
Chorionic villus sampling, sketch of CVS ► Can be performed on week 9-10.
Delivers trophoblast cells (instead of embryonic), they come from the same zygote, but higher risk of diagnostic error .
◄ Sketch of amniocentesis Fetal cells in the amniotic fluid are obtained. (These cells as well as the amniotic fluid are/can be subjects of different analyses.) Traditional pre natal karyotyping only after in vitro culture of the cells. Amniocentesis earliest in week 16 of the pregnancy. Plus 2-3-4 weeks in culture. Too late. Prefer interphase cytogenetics.
In utero ultrasound investigation: determination of nuchal translucency A vizsgála-tot a 10-12. (másutt: a 11 13.) terhességi héten vég-zik.
Ha 3 mm- nél vasta-gabbat mérnek, az Down szindróma gyanúját alapozza meg, külö-nösen, ha az orrcsont hiányával társul.
▼ ▲
During week 10-12 (other auth ors prefer 11-13) of the pregnancy, if the indi cated dis tance >3 mm, that speaks for risk of trisomy 21, especially if the nasal bone cannot be seen.
Down syn dromic pregnancies = red curve.
MoM = mul tiple of the median.
Distribution of some relevant biochemical markers Some se lected combina tions are informa tive.
Cross-trimester marker ratios in prenatal screening (2007) Mehod First trimester {NT, PAPP-A, free beta-hCG} Second trimester Detection rate % {AFP, uE3, total beta-hCG, INH-A} Cross trimester ratios {NT, PAPP-A, free beta-hCG} + {AFP, uE3, total beta-hCG, INH-A} Cross trimester ratios (another survey) 83,7 84,4 90,8 False-positive rate % 5,1 6,6 3,1 {NT, PAPP-A, free beta-hCG} + {AFP, uE3, total beta-hCG, INH-A} Detection rate % = retrospective analysis.
90,2 3,9 Some comments.
The results of these investigations may be indicative of other chromosomal anomalies as well as malformations, too. E.g. AFP is elevated in the case of an open neural tube (and also in the case of twins).
CT = cross tri mester.
Integ rated = NT result included.
Serum integ rated = serum values only.
What can we learn from this figure?: (1) Screening without karyotyping does not give reliable diagnosis.
(2) Results of second trimester included ► rather late arrival.
(includes NT) Cost per Down syndromic pregnancy diagnosed At 95% detection rate the cost can vary between GBP 16,500 and 31,400. ( British data, 2007.
) # # # Financing national health care If a bigger population is investi gated, the cost of screening 100,000 pregnant women can vary between GBP 3,540,000 and 6,740,000, de pending on the extent of biochemi cal markers involved.
(Average = GBP 51.4 per case.)
PÄTAU SYNDROME
Patau * syndrome: trisomy 13
Multiple developmental anomalies, very limited lifetime.
Trisomy 13 newborns usually die before age of 12 months.
* Remember: there is no umlaut in the English, thus no ä for Klaus Pätau of a German immigrant ancestry.
Trisomy 13, symptoms and signs
EDWARDS SYNDROME: trisomy 18
Multiple developmental anomalies, very limited lifetime.
Trisomy 18 newborns usually die before age of 12 months.
Trisomy 18, symptoms and signs
NUMBERS of GENES IDENTIFIED ON HUMAN CHROMOSOMES by the HUMAN GENOME PROJECT (HGP)
,
as by February, 2007*
# 1 = 2782
2610
# 9 = 1148
1076
# 17 = 1469
1394
# 2 = 1888
1748
# 3 = 1469
1381
# 4 = 1154
1024
# 10 = 1106 # 11 = 1848 # 12 = 1370
983 1692 1268
# 18 = 432 # 19 = 1695 # 20 = 737
368 1592 710
# 5 = 1268
1190
# 6 = 1505
1394
# 7 = 1452
1378
# 8 = 984
927
# 13 = 551
496
# 14 = 1275
1173
# 15 = 945
906
# 16 = 1109
1032
# 21 = 352
337
# 22 = 742
701
X = 1336
1141
Y = 307
255
Σ human genes (in this table) = 28,924 Remember: a gene here = a given protein coding sequence of the DNA.
Small characters: data from January, 2006.
* The figures are the same in January, 2008.
SEXUAL DIMORPHISM IN HUMANS
Is there any relationship between the dimorphism of sex chromosomes and the sex determination and sex development (resulting in the well known human sexual dimorphism)? – Yes.
Is it possible to compensate for the gene dosis differences (XX vs. XY) of females and males in the phenotype? (X = 1336, Y = 307 genes.) – Yes. See M. Lyon’s hypothesis of X inactivation.
SEX DETERMINATION IN HUMANS
a genetic / chromosomal approach
Sexual development and some aspects of its determination
1) Chromosomal sex XX vs. XY karyotype,
SRY
gene on the short arm of Y.
2) Gonadal sex Testosterone (androgene hormone) production by the fetal testes .
Hormone receptor for testosterone encoded in the X chromosome (female development is automatic, this is suppressed in the case of the presence and action of Y chromosome and testosterone).
3) Genital sex Legal sex (as registered at birth).
4) Secondary sexual characteristics (developing mainly after puberty).
5) Psychosomatic sex (also determined by environmental effects).
Fate of the indifferent gonadal anlage
In the presence of Y chromosome, and es pecially by the activity of SRY gene in it = development of testes.
In the absence of SRY effect, the ovary development is automatic.
Important early steps in sexual differentiation
(see precise details in embryology)
MALE: XY, SRY (sex determining region of the Y chromosome) activity. ► Indifferent genital ridge ► testes ► testosterone. ► Wolffian duct further develops (gives rise to prostate, ductus epidydimidis, vesicula seminalis etc.) ► while Müllerian duct disappears. ► Sinus urogenitalis closes. ► Tuberculum genitale develops into penis. ► Genital ridges close to form scrotum.
FEMALE: XX automatism. No SRY effect, ► effect, ► no development of Wolffian duct. ► culum genitale develops into clitoris. ► no testes, ► no testosterone Müllerian duct further develops and gives rise to tuba uterina, uterus, and vagina. ► Sinus urogenitalis remains open (labia minora, vestibulum vaginae). ► Tuber Genital ridges do not close, form labia maiora.
Genes on the Y and X chromosomes
Pseudoautosomal region (PSAR)*.
SRY
= sex determining region of the Y chromosome (previously: TDF testis determining factor, this term is still in use today).
= Localization: Yp11.3.
ZFY
= zinc finger protein Y (Yp11.32), a transcription regulator**.
Y X Pseudoautosomal region (PSAR)*.
TFM
= gene of the androgene (dihydrotestosterone) receptor.
ZFX = zinc finger protein X (Xp22.3-p21), a transcription regulator, and so on… 99 other genes (including **), which code for proteins expressed in the testes (or in tumors).
* PSAR on the short arm. Those in the Y and X are homologs to each other. (There are some pseudoautosomal genes in other locations, too.) ** There is a number of other transcription regulators.
SRY on the Y chromosome TFM on the X chromosome
The gene codes for a transription regulator protein. Mutation ► 46,XY female.
Translocation ► 46,XX male (Klinefelter syndrome).
Gene for dihydrotestosterone receptor (Xq11-q12). ║ TFM = testicular feminisation, ║ Other name: AIS = androgen insensitivity syndrome. ║ Complete form: Morris syndrome, ║ incomplete form: Reifenstein syndrome. ║ Karyotype: 46,XY. Testes in the abdominal cavity. ║ No ovaries, no uterus.
║
Genes on autosomes
AMH
, gene coding for anti-mullerian hormone. (Other name: Mullerian Inhibiting Factor, MIF, or MIS) Responsible for the physiological de generation of the Müller duct (which gives rise to internal female organs).
Location: 19p13.3-p13.2. Expression: the encoded protein (560 amino acids) is produced in the testis, in the Sertoli cells. (In the same gonad the Leydig cells produce the testosterone, which is involved in the differentiation of the Wolff duct into prostate, seminal vesicle, spermatic duct, respectively. The Amh gene becomes silent after puberty.
AMHR
= gene coding for the receptor of AMH (MIF) protein. PMDS = persistent Mullerian duct syndrome. (For instance uterus in male, etc.) Type I: defective Amh. Type II: defective Amhr.
GENE DOSAGE COMPENSATION
Although there is a number of characteristic phenotypic differences, there is no phenotypic sign of the more genes in females than in males.
Hypothesis of Mary Lyon: one out of the two X xchromosomes becomes inactive. (It forms a heterochromatic Barr-body attached to the internal surface of the nuclear envelope. If there are more than two X chromosomes, one remains active and all the others become inactivated: dosage compensation .) X-inactivation is random , takes place in the 3 rd embyonic week.
If an X chromosome– by chance, been inactivated in a cell , either of paternal or maternal origin–, has in all descendants of that very cell the same X will be inactivated and forms Barr-body . This can be expressed as “inactivation is imprinted”.
Because of that random inactivation females are functional mosaics .
Normally there is no preference which X to remain active. We have quite a big number of data indicating the validity of the random type inactivation resulting in a closely fifty – fifty ratio. This is, however, a statistical value, which is a subject of considerable deviations, too.
One, two, and four Barr bodies, respectively, in nuclei of buccal epithelial cells. Barr body in polymorphonuclear granulocyte nucleus (drumstick)
SEX CHROMOSOMAL ANEUPLOIDIES in HUMANS KARYOTYPES and CONSEQUENCES
X MONOSOMY
Turner syndrome: 45,X.
No Barr body.
Leading symptoms: Sexual infantilism = immaturity of secondray sexual characteristics.
Short stature.
Cubitus valgus, pterygium colli, and so on.
Sterility (infertility) = No maturation and production of egg cells. Instead of ovarian tissue so called gonadal streak , which is made of connective tissue. Not only germ cells are not produced, neither estrogen, nor progesterone hormones. Consequently the level of gonadotropin is high.
Main symptoms and signs of Turner syndrome
pterygium colli
Turner syndrome
gonadal streak and its histology No Grafian follicles, no meiotic divisions are seen in the histological picture.
No maturation of egg cells. No production of ovarial hormones.
Turner syndrome lymphatic edema on the neck and on the feet Karyotype: 45,X
Symptoms of Turner syndrome can be partially prevented Early diagnosis (verified in the neonatal period), including immediate chromosome analysis (e.g. already from cord blood).
Hormonal treatment: (i) growth hormone, (ii) estorgen and progesterone.
Psychical/psychological support to the parents as well as to the Turner syndromic patient herself.
49,XXXXX X pentasomy
X chromosomal hyperdiploidies
47,XXX trisomy X
X chromosomal hyperdiploidies
47,XXX ( trisomy X ) is not considered a syndrome: there are no characteristic signs and symptoms of this aneuploidy, the phe notype is–in the majority of cases–normal*. It can result, how ever, in the birth of a 47,XXY (Klinefelter syndromic) baby.
(Theoretically the chance would be 1/4; 1/4 XX, 1/4 XXX, 1/4 XY, and 1/4 XXY. In fact the real risk is much lower.) In the case of triplo-X, prenatal chromosome analysis is strongly rec ommended.
X tetrasomy (48,XXXX) is characterized by mental retardation; X pentasomy (49,XXXXX) patients show severe mental and corporal retardation in all cases, although in these aneuploidies only one X remains active, all others form Barr bodies.
* See table of spontaneous abortions and chromosome anomalies.
Sex chromosomal hyperdiploidies
Klinefelter sydrome : sex chromosome trisomy = 47,XXY.
Barr body is present in cell nuclei.
Leading symptoms: eunuchoid type tall stature, gynecomastia, no body hairs or very sparse, no beard, small testes with no Leydig and Sertoli cell functions.
Sterility (infertility) : no spermiogenesis.
High gonadotrope hormone level in blood.
Klinefelter syndrome (47,XXY)
Symptoms and signs in Klinefelter syndrome
Klinefelter syndrome variants (48,XXXY, 49,XXXXY, 48,XXYY)
In the case of 47,XXY Klinefelter syndrome mental abilities are of average in majority of patients.
If more than one extra X chromosome is present in the karyotype (48,XXXY, 49,XXXXY) mental retardation or even severe mental retardation is one of the leading symptoms, although only one X chromosome remains active, all the others form Barr body.
47,XYY
No symptoms, therefore this is not a syndrome.
Risk: 47,XXY karyotype offspring.
Prenatal diagnosis stongly recommended.
STRUCTURAL ABERRATIONS OF HUMAN CHROMOSOMES
One should not forget the fact: there are a number of genes in one–light microscopically visible–band. Any deletion, duplication or rearrangement involves minimum 50 genes. (In high resolution banding approximately 25.)
AUTOMATED INVESTIGATION OF HUMAN METAPAHSE CHROMOSOMES
1/ Dividing cells (predominantly: in vitro cell culture, e.g. T cells).
2/ Accumulation in the metaphase: aided by colchicine treatment.
3/ Swelling in hypotonic solution (0.075 M KCl).
4/ Fixing.
5/ Spreading on specimen slides. ( Pretreatment for banding .) 6/ Staining: Giemsa’s solution or quinacrine (a fluorescent dye) .
7/ Conventional light or fluorescent exit port.
microscope. TV/CCD camera on its 8/ Images to computer which has been programmed for automatic chromosome analysis. Arranged karyogram on the monitor.
9/ The operator is allowed to correct the arrangement offered by the computer.
10/ The result is an ordered (arranged) karyogram . Evaluate and 11/ establish karyotype . Save as a karyotype file.
STRUCTURAL CHROMOSOME ABERRATIONS
– Deletion Ring chromosome formation Microdeletion – Duplication Microduplication (visible within the chromosome), microchromosome – – Isochromosome Translocation (deletion and duplication simultaneously) Unidirectional traslocation Reciproc translocation Robertsonian translocation (centric fusion) – Inversion Pericentric Paracentric – Break and gap – and some other forms (e.g. homogeneously staining region, double minutes)
Deletion
any chromosome fragment without a centromere will be lost
Ring chromosome the two telomeric fragments are lost
Cri du chat syndrome (cat cry disease): 46,XX,5p–
Isochromosome formation
Instead of the regular separation of sister chromatids, the long and short arms, respectively, remain together, because of misdivision of the centromere. Consequently, in one daughter cell the short arm, in the other the long arm will be present in duplicated (trisomic) form, while the long arm and the short arm, respectively, will be monosomic in that very cell.
X long arm isochromosome has been described in Turner syndrome. Do not miss the point: that is X short arm monosomy at the same time.
Robertsonian translocation = centric fusion:
46,XX,–14,t(14;21), Down syndrome
The Robertsonian translocation = centric fusion… …may be the result of a new chromosomal abnormality = de novo mutation, but also can be inherited from a balanced carrier parent: 45,XX,–14,–21,t(14;21) . ► Very important. A careful chromosomal diagnosis to be established in any has case of Down syndromic birth, and if translocation is discovered, the parents have to be investigated, too. Eventually some other balanced carrier(s) might be present in the family of the affected.
Theoretical gametic combinations in balanced t(14;21) translocation carriers
The theoretical risk in a family where one parent is a balanced carrier of a Robertsonian 14/21 translocation is calculated as follows: 1/3 normal, 1/3 balanced translocation carrier, and 1/3 unbalanced translocation, i.e. Down syndrome.
The empirical statistical risk is, however, somewhat better: 1/10 for Down syndrome if the mother, and 1/20 if the father is balanced carrier.
Even if we calculate with this latter figures, that is high enough to indicate: a prenatal chromosome diagnostics is a must . “Must” means: anyone in the health profession must be aware of the enhanced risk (like in the case of a pregnant woman ≥35), and is obliged to inform the parents of that risk. It is the parents’ decision, however, whether they accept the information obtained at the genetic counseling or not. Please note: no suggestion, no persuasion, no obligation, just information.
Other types of Robertsonian translocations
t 21/22 Down syndrome Pätau syndrome
46,XX,–14,t(13;14) 45,XY,–13,–14,t(13,14) Pätau syndrome balaced carrier parent
Enhanced risk for incorrect separation after meio tic pairing, resulting in severe gametic (and zygotic) chromoso mal aber rations. (See also consider ations on inversions below.) Reciprocal (non Robertsonian) translocation
Inversions
Like in the case of balanced translocations, inversions do not necessarily involve gene loss or gene duplication, i.e. the visible phenotype can be normal, without any indication of the presence of some chromosomal aberration in a balanced carrier.
If, however, during meiosis of a balanced carrier recombination(s) cause severe rearrangements, these latter may result in an unbalanced genome in the offspring, which at one end may be incompatible with life (e.g. infertility, or serial spontaneous abortions), on the other end may result in serious malformations.
Consequences of inversion and recombination in meiosis…
… and gamete formation after crossing over
and so on...
Familiar mental retardation (FMR)…
… fragile X (fraX, FRAXA) syndrome 1) Not a real break on the X chromosome, it can be evoked and made visible by certain cell culture technique (inhibition of DNA synthesis).
2) This abnormality can be made responsible for the fact, that mental retardation is more frequent among males than females (hemizygosity), although also heterozygous females may express the symptoms. The fragile site at Xq27.3 is associated with the
fmr
gene, i.e. patients with fragile X syndrome do not express the
FMR
1 gene.
3) More details in chapter on dynamic (expansive) mutations.
Fragile-X and familial mental retardation
CHROMOSOMAL ABNORMALITIES and SPONTANEOUS ABORTIONS (in 100,000 pregnancies
)
Karyotype Sp. abortion Livebirth Normal 7,500 84,450 Trisomy Karyotype Sp. abortion Livebirth 47,XYY 4 46 1 0 0 47,XXY 4 44 2 159 0 45,X 1,350 8 3 53 0 47,XXX 21 44 4 95 0 5 0 0 Translocations , 6 - 12 561 0 balanced 14 164 13 128 17 unbalanced 225 52 14 275 0 15 318 0 Triploidy 16 1,229 0 17 10 0 Tetraploidy 1,275 0 450 0 18 223 13 19 - 20 52 0 Others 280 49 21 350 11 3 Σ chromosomal 22 424 0 aberrations 7,500 550 –(13, 18 and 21) 3,176 0 Altogether 15,000 85,000
SISTER CHROMATID EXCHANGE (SCE)
Up to a limited number (approximately 5-10
/
metaphase) this phenomenon is considered normal. In certain hereditary diseases the frequency is, however, elevated, and that is indicative of the presence of e.g. Bloom syndrome, Fanconi anemia, xeroderma pigmentosum (all these are one or other kind of DNA repair defects).
SCE frequency is increased e.g. in radiation injury as well as in the presence of some chemical mutagens.
CHROMOSOME ABERRATIONS and CANCER, I
Philadelphia chromosome in CML (chronic myelogeneous leukemia).
Reciproc translocation
►►►
A fusion of
ABL
(proto-oncogene) and
BCR
gene.
Chromosome aberrations and cancer, II
t(8;14) in B-cell lymphoma IGH = immunoglobulin heavy chain gene. MYC = protooncogene, which is activated to oncogene because of translocation
.
Advantages of the FISH technique in general, and in the detection of minor chromosome abnormalities
FISH = fluorescent in situ hybridization.
You know a given DNA sequence (e.g., from the results of the HGP). Synthesize a complementary single standed oligo nucleotid probe. Label the latter with a fluorochrome.
Separate the two strands of the original double helix (e.g. by moderate heating), in situ, in the chromatin material.
Add probe. Let it anneal (hybridize) to the complementary strand (only to that, nowhere else), e.g., by slow cooling.
There are chromosome-specific (region-specific, band specific, etc.) probes as well as gene specific probes, and the like.
UP-TO-DATE DEVELOPMENTS of CYTOGENETICS
1/ Dividing cells are not inevitably necessary, 2/ interphase cell nuclei can also be investigated in some cases.
3/ Fluorescent in situ hybridization = FISH , with complementary single stranded DNA probes (most commonly labeled oligonucleotides).
4/ These probes are: gene-specific, gene segment-specific, chromosome specific, chromosome segment-specific and so on, 5/ i.e. extremely high resolution can be achieved.
6/ Probes are mostly labeled with a fluorochrome, and the preps are 7/ investigated in fluorescence or in laser confocal microscope.
8/ CCD / TV camera ► computer, programmed for karyotypic and metaphase chromosome analysis, or a number of other evaluations.
9/ The investigator has a chance to reconsider and correct the result.
10/ The goal of the investigation can be karyotyping, analysis of individual chromosomes. Even one gene can be identified. And so on.
11/ Another objective: CGH (comparative genomic hybridization) with quantitative evaluation.
FURTHER DEVELOPMENTS in CYTOGENETICS
In situ PCR (polymerase chain reaction, amplification of a well defined, given sequence of a DNA molecule).
Qualitative and quantitative DNA analysis by PCR (with or without DNA sequenation), with DNA chips, and many other recently developed methods.
For instance: one identifies a representative segment on the long arm of chromosome 21 at the DNA level and determines if it is present in 1× dose (normal), or in 1.5× doses (trisomy 21) in isolated DNA of the patient. Cheaper and faster method than traditional karyotyping.
Note, however. This diagnosis does not tell t(14;21) Robertsonian translocation Down syndrome from trisomy 21 “classical” Down syndrome. One has to investigate the presence or absence of 21p, too.
CHROMOSOME PAINTING
with chromosome-specific fluorescent DNA probes ( 46,XY / 47,XY,+8 ) Trisomy 8 might be viable in the chromosome mosaic form, if only few cells are trisomic.
FISH, a way to the interphase cytogenetics
Left: DiGeorge syndrome, del(22q11.21-q11.23), microdeletion.
Right: Williams–Beuron syn drome, del(7q11.2), microdeletion.
Detection of a marker (mar) microchromosome
MCB = multi color banding
UNIPARENTAL DISOMY (UPD)
Some conceptuses have a normal 46,XX or 46,XY karyotype, but have inherited two copies of the same chromosome from just one of the two parents . This may result in abnormal phenotypes which are different according to parental origin of the relevant chromosome
.
Isodisomy . Both homologs of the affected are identical.
Heterodisomy . Both homologs of one parent are present in the individual affected.
UPD is probably a result of an eventual correction of hyperdiploidy. The conceptus may be trisomic, but the zygote finds a way to get rid of the supernumerary chromosome (in order to remain viable). For example
:
15mat 1 + 15mat 2 + 15pat a fter correction 15mat 1 15mat 2 + l5pat , or + 15pat , or 15mat 1 + 15mat 2 =
UPD
.
The first two versions will not be detected.
Prader–Willi and Angelman syndromes 46,XX or 46,XY individuals who inherit both copies of their chromosome #15 from their father , develop Angelman syndrome ; if both copies of #15 are maternally inherited , Prader–Willi syndrome is seen.
Explanation. In non-UPD patients with Prader–Willi syndrome 15q12 is deleted from the paternal #15 chromosome , in Angelman syndrome the maternal #15 suffered a microdeletion of 15q12 . If there is no paternal #15 in the karyotype, it means also the absence of the indicated region. This abnormality shows itself in the Prader–Willi syndrome.
The basic concept is found in the phenomenon of genomic imprinting . Some genes are active only on the maternal or only in the paternal chromosome. If the maternal region should be active, but that is missing, the situation is equivalent with the occurrence of a loss of function mutation. Prader–Willi and Angelam syndromes were identified originally as recessively transmitted human malformations.