Download Cytogenetics

Cytogenetics
Dr Alice S Chau
Cytogenetics
= Study of chromosomes and their abnormalities
Karyotypes
 Display of chromosomes in the order according to
length and shape
 The latter depends on the position of the centromere
at the metaphase:
1. Metacentric
2. Submetacentric
3. acrocentric
Different chromosome karyotypes
Metacentric
Submetacentric
Acrocentric
Short
Arm p
satellite
Long
Arm q
A normal female karyotype – 46, XX
Disorders of chromosomes
Normal female karyotype = 46, XX
Normal male karyoptye = 46, XY
(1) Numerical abnormalities
Aneuploids – failure of separation of paired
chromosomes at cell division
(a) Trisomy e.g. (+21)
(b) Monosomy e.g. (-X)
(c) Triploidy, Tetraploidy e.g. (69, 92 chromosomes)
Disorders of chromosomes
(2) Structural Disorders
(a)Translocation – (t) : reciprocal exchange of
chromosome segment
(b) Deletion – (del) : loss of genetic material
(c) Duplication – (dup) : extra copy of
chromosome region
(d) Isochromosome – (i) : duplication of one arm
and lack of others
(e) Ring chromosome – (r) : abnormal repair at
distal segment to form a ring
Chromosome abnormalities
(1) Autosomal syndromes
 Trisomy 21 (Down syndrome) 47, XY, +21
 Trisomy 18 (Edwards syndrome) 47, XX, +18
 Trisomy 13 (Patau syndrome) 47, XX, +13
 Deletion #15 (Prader Willi syndrome) 46, XX, del(15q)
(2) Sex chromosome abnormalities
 Turner syndrome (45, X)
 Klinefelter syndrome (47, XXY)
 Fragile-X syndrome
Down’s syndrome - face
Down’s syndrome – cyanotic TOF
Down’s syndrome – 47, XY, +21
Down’s syndrome (Trisomy G)
1. General - Hypotonia; incidence 1 in 668 (or 1.5
per 1,000)
2. Eyes - Upward lateral slanting, inner epicanthic
folds, speckling of iris, nystagmus, strabismus
3. CNS - Mental deficiency, I.Q. 25 to 50
4. Cranium - Relatively flat occiput
5. Ears – small, overlapping upper helices, low-set
6. Nose – low nasal bridge, small nose
Down’s syndrome (Trisomy G)
7. Mouth – hypoplastic maxilla, protruding tongue
8. Hands – short hands and fingers, Simian crease,
dermal: distal triradii,  no. of ulnar loops
9. Feet – wide gap at first and second toes
10. Genitalia – small penis, cryptorchidism
11. CVS – A-V communis, VSD, tetralogy of Fallot
12. Related to maternal age –
 1 in 2,300 at 15-19 years
 1 in 46 at 45 years
Relationship of maternal age on
risk of Down’s syndrome
Maternal age
risk
20 years
1 in 1,500
30 years
1 in 900
35 years
1 in 400
40 years
1 in 100
45 years
1 in 30
Down’s inheritance
Down’s 45, XX, t 14/21
Down’s 46, XY, -21, +t(21/21)
Recurrent risk of Down’s syndrome
patient
mother
father
Chance of
recurrence (%)
Carrier
N
Carrier
N
Carrier
N
Normal
C
Normal
C
Normal
C
10-15
5
10-15
5
100
100
Trisomy 21
N
N
1
Translocation or
mosaic
N
N
small
D/G translocation
21/22 translocation
21/21 translocation
Edward’s syndrome
Edward’s syndrome - hand
Edward’s syndrome –
rocker bottom feet
Edward’s syndrome –
rocker bottom feet
Edward’s syndrome –
11 ribs & TOF
Edward’s syndrome – 47, XX, +18
Edwards syndrome (Trisomy E –
47, +18)







Mental retardation
Failure to thrive
Clenched fist
Single palmer crease
“rocket bottom” foot
Deformed ears
Congenital heart disease
Patau’s syndrome (lateral view)
Patau’s syndrome (cleft lip & palate)
Patau’s syndrome scalp defect)
Patau’s syndrome (polydactyly)
Patau’s syndrome (polydactyly)
Patau’s syndrome (cryptorchidism)
Patau’s syndrome 47, XX, +13
Patau’s syndrome (Trisomy D)
1. General – severe developmental retardation,
apnoeic spells, death within 3 months
2. CNS – MD, deafness, minor motor seizures
3. Cranium – sloping of forehead, microcephaly
4. Eyes – microphthalmus and/or colobomata
5. Ears – abnormal helices and/or low-set
6. Mouth – cleft lip and/or palate
7. Hands – polydactyly, trisomy hands, Simian
crease
Patau’s syndrome (Trisomy D)
8. Feet – polydactyly, posterior prominence of heel
9. Cardiac – VSD, rotational anomaly
10. Abdomen – acessory spleen, large gall-bladder,
incomplete rotation of colon
11. Renal – hydronephrosis, polycystic kidney
12. Genitalia : cryptorchism
: partially bicornate uterus
13. Incidence – 1 in 5,000 live births, mean maternal
age 31.6 years
Prader Willi syndrome
Prader Willi syndrome
Prader Willi syndrome - hands
Prader Willi – undescended testes
Prader Willi – undescended testes
Prader Willi – del 15q- (q11-q13)
Prader Willi – del 15q-
Prader-Willi syndrome



One of the short stature conditions
Due to deletion of chromosome 15 (q11-13) and
loss of some genes
Symptoms:
– Short and obese, hypotonic
– Face is normal with narrow forehead
– Eyes are small and almond-shaped
– Hands and feet are fat and small
– Hypogonadism
– Mental retardation, I.Q. 36 to 85
Turner’s syndrome
Turner’s syndrome – low hair line
Turner’s syndrome –
ankle oedema in neonates
Turner’s syndrome – 45, XO
Turner syndrome (45, XO)





Ovarian dysgenesis
Short stature
Webbing of neck
Primary amenorrhoea
Failure of development of secondary sexual
characteristics
Klinefelter’s syndrome
Klinefelter’s syndrome
Klinefelter’s syndrome
Klinefelter’s syndrome –
undescended testes, hypospadia
Klinefelter’s syndrome – 47, XXY
Klinefelter’s syndrome (47, XXY)
1. External genitalia – delayed puberty or small
gonads’
2. Growth & physical measurement, with gross
mental retardation
 In childhood – normal until puberty, or even
taller than control
 After puberty – with “eunuchoid” propertions,
I.e. greater pubis-sole measurement
3. Pubic & facial hair – delayed growing, feminine
in distribution
Klinefelter’s syndrome (47, XXY)
4. Gynaecomastia – between 14 and 16 years
5. Testes – small or thickened cord-like structure
 Histology : large clumps of Leydig cells,
irregularly arranged seminiferous tubules, which
showed hyalinization or sclerosis  “ghost
tubules”, some may have spermatogenesis
6. Incidence –
 In newborn: 1.98 per 1,000  births
 In mental subnormals: 6 to 9 per 1,000
 Mean maternal age: 28.7 years
 Mean paternal age: 30.7 years
Fragile X syndrome
Fragile X syndrome
Familial fragile X syndrome
Familial fragile X syndrome
Familial fragile X syndrome
Fragile X syndrome - ear
Fragile X syndrome
Fragile X syndrome
Fragile X syndrome
 During
harvest, add KCl before staining
with Leishman stain, at Xq:- distal end
shows 1 or 2 satellite appearance = Fragile
site
 50-100 cells are examined, a 4% or higher
frequency is significant
Fragile X syndrome
Fragile X syndrome
Fragile-X syndrome
1. Mental handicap – moderate to severe MD
2. Macro-orchidism – normal testicular volume is 23
to 25 ml :
V = (length) x (width)2 x 
6
The testicular functions (hormone level, sperm
count and morphology) are normal
3. Facies – high forehead, hypoplasia of middle 
of face, large mouth and thick lips, large jaws and
prominent chin, large ears poorly formed
Fragile-X syndrome
4. Speech & language – delayed speech and
defective speech, weak auditory reception and
sequential memory
5. Behaviour – autistic, shy or fearful
6. Increased birth weight, obese or short
Multifactorial Inheritance
Dr Alice S Chau
Multifactorial inheritance
 Caused
by combination effects of many “genes” and
“environmental” factors
Environmental
Genetic
Infection
Radiological
Chemical
Cystic fibrosis
Haemophilia
Marfan syndrome
Examples: body height, intelligence, cleft lip and/or
palate, congenital heart disease, coronary heart
disease, diabetes mellitus, cancers, schizophrenia &
neural tube defect
Incidence of genetic disorders
Chromosomal abnormalities
Single gene mutation
Familial
Multifactorial inheritance
Others & unknown cause
10%
3%
14%
23%
50%
Recurrent risks in genetic disorders
 Single
gene disorders
– Autosomal dominant
50%
– Autosomal recessive
25%
 Multifactorial inheritance
– “empirical risk” – based on direct observation of
data, on a large series of affected families, thus
calculating the percentage
e.g. cleft lip & palate
General population = 1/1,000
First degree relative = 4%
Second degree relative = 7 per thousand
Genes in common
Genes in common
Monozygotic twins
Parent-child, sibling, dizygotic twins
Grandparents, uncle, aunt
First cousin
Second cousin
1
1/2
1/4
1/8
1/32
Genes in common
——
I
II
III
½

¼




¼

/8
1
Neural tube defect
 Incidence
1/1,000 live births
 Due to failure of closure of brain or spinal cord in
foetal life
 Symptoms may vary with size, location and nature
of defect
 Generally about 3% of recurrent risk
  FP during pregnancy
 To reduce chance of occurrence – by giving folic
acid to woman before and early pregnancy
Encephalocele
Meningocele
Meningo-myelocele
Hydrocephalus
Cleft lip & palate
Cleft lip & palate




Incidence 1/600 to 700 births
Due to failure of fusion of 3 upper lip parts to form
one continuous lip & closure of the opening in roof
of mouth, the latter causing cleft palate
Gives rise to feeding problem, teeth development,
speech defect & frequent ear infection
Risk of recurrence:
1. In unilateral cleft lip – 2.7%
In bilateral cleft lip – 5.4%
2. When father affected – 3% of offspring
When mother affected – 14% of offspring
When one sib affected – 4% of future sibling
When two sibs affected – 10% of future sibling
Coronary Heart Disease (1)


1.
2.
The major cause of morbidity and mortality
Genetic factors:
Lipid metabolism
(a) Familial hypercholesterolaemia (A.D.)
1/500 population; due to defect in LDL receptors on surface
of cells
(b) Familiam combined hyperlipidaemia (A.D.)
1/200 population; associated with triglyceride & cholesterol
Lipoprotein – involved in transport of lipid
(a) Apolipoprotein E
It binds with LDL receptors on cell surface and responds for
cholesterol uptake
Coronary Heart Disease (2)
(b) Lipoprotein A
A glycoprotein & binds apolipoprotein B; highly
polymorphic
(c) Angiotensin I converting enzyme
It converts angiotensin I into II; inactivates bradykinin

Environmental factors:
diabetes mellitus, hypertension, obesity,
smoking, exercise, dietary, menopausal women
Diabetes Mellitus
 Incidence
– Year 2000 – 1,700 million world population
– Year 2030 – 3,700 million world population
– In Hong Kong – 1 in 10 adults, tendency to be younger in type II
 Maternal
diabetes will cause “caudal dysplasia
syndrome” in offspring (16% cases)
Major features in type I & type II
DM
> 40 years
Insulin production
Type I DM (insulin
dependent)
< 40 years old (HLADR3 or DR4 antigens)
None
Insulin resistance
-
+
Auto-immunity
+
-
Sibs recurrence
1-6%
10-15%
M-Z twin concordance
0.35-0.5
0.9
Obesity
Uncommon
Common, with
family history
Onset
Type II DM
partial
Cancers
 Second
leading cause of death, following heart
diseases
 Some cancers may run in family e.g. colon, breast,
ovarian, prostate & retinoblastoma
 “Oncogene” – a gene affecting cell growth or
development which can cause cancer
 “Carcinogen” – cancer-causing agents e.g.
– aflatoxin B1 – can induce specific p53 mutation in liver cancer
– Benzopyrene – in cigarettes; produces p53 codon mutation in
lung cancer
(i) Colo-rectal cancer

Risk of this cancer with one affected first degree
relative is 2x or 3x higher than in general
population
a) Adenomatous polyposis coli (APC, familial polyposes)
a tumour suppressor gene, at 5q21 deletion, related to 85% of
C-R cancer, due to mutation
b) Hereditary nonpolyposis colon cancer (HNPCC)
genes in DNA mis-match repair, at 2q15-21, related to 1-5% of
C-R cancer, due to genomic instability

Periodic screening of family members on polyps;
sigmoidoscopy is advisable
(ii) Breast cancer
 Most
common cancer among females
 When one affected first-degree relative, the risk of
developing breast cancer doubles
 Several gene mutations are known in DNA repair
e.g. BRCA1 at 17q21 and BRCA2 at 13q12
 However, >90% breast cancers are not hereditary
 Environmental factors – nulliparity, high fat diet,
alcohol, oestrogen replacement therapy
 Frequent screening by mammography is useful
(iii) Ovarian cancer
 About
5% of females with ovarian cancer have a
family history of this disorder
 Incidence 1/70 women
 Mutation gene of BRCA1 at 17q21
 1% is autosomal dominant gene
(iv) Prostate cancer
 Most
common diagnosed cancer in males
 Second to lung cancer causing death
 Identified linked polymorphism on 1q
 When an affected first degree relative, the risk
increases by 2 to 3x
 About 5-10% of prostate cancer are result of inherited
mutation
 Environmental factor - a high-fat diet
 Readily diagnosed by digital examination and PSA
(prostatic specific antigen) test
 Tumour progresses slowly thus early diagnosis is useful
(v) Retinoblastoma
 Most
common childhood eye tumour
 Incidence 1 in 20,000
 40% are familial and inherited as autosomal
dominant
 Multifactorial and bilateral involvement
 Deletion gene at RB1 at 13q14
 15% cases will develop osteosarcoma
 Treatment – radiation, cryotherapy or enucleation
(vi) Cancer cytogenetics
 “Philadelphia
chromosome” in chronic
myeloid leukaemia (ph1)
 Reciprocal translocation between chromosome
9 and 22 or t(9;22)
Normal
9
Normal
22
der(9)
der(22)
Schizophrenia
 Onset
at late adolescent or early adult period
 Incidence 1%
 A serious psychotic illness, presented by behaviour
problem with emotional disorder showing delusion,
withdrawal and hallucination
 Males & females are equally affected
 No specific genes have been cloned but the gene
“SCZD2” at 11q might predispose to schizophrenia
& marked familial aggregation has been observed
Schizophrenia
 When
one affected parent is diagnosed, the risk for
offspring is 8 to 10% (i.e. 10 times higher than general
population)
 If one affected parent and one sib affected, the risk for
next child is 14% recurrence
 Environmental factors – poor socio-economic status, child
born in winter season
 Poorer prognosis occur in early onset of age and in males
 Several genes for neurotransmitters (serotonin), N-T
receptors and neurotransmitter-related enzymes (e.g.
monoamine oxidase, tyrosine hydroxylase, & dopamine-hydroxylase) are studied in families, as well as linkage
studies with several markers – the result is inconclusive