Download Mech63-RvwGeneticDisordersPt1

MOD #63
Wed, 05/07/03, 9am
Dr. Eisenberg
Jennifer Uxer for Adam Ackerman
Page 1 of 6
Chapter 6: Genetic Disorders—continued
Exam 2
As: 54 – 58 As; 2 were 100s
Cs: 15 – 20 If you’re 1 of these you might want to modify your study habits.—Dr.
Putthoff
Fs: 3 Same case as the Cs.
Dr. Eisenberg’s Introduction
13 years ago David Richards & Gib Lewis of the Texas legislature wanted to set up a
DNA testing lab. This was established here at TCOM. Initially the lab just did paternity
testing. Now they’re also involved in the TX missing person’s database, forensic testing,
TX Lyme Disease research project. The DNA lab is the only university in the FBIs
CODEX system and has just recently come to oversee the Ft. Worth police department’s
DNA testing. Dr. Eisenberg is the Chairman of this lab.
Dr. Eisenberg was the Chairman of the DNA advisory committee to the FBI. He’s also
the advisor to the European economic community and works to standardize DNA testing
procedures in Europe and in South America.
I.
Introduction
 This finished the initial power points from yesterday
 The 3rd set of power points has been posted now.
 Study the transmission patterns, predominant Medelian disorders,
pedigree analysis, examples of karyotype analysis.
 Go through the practice questions at the end of the 2nd lecture.
 Tomorrow will be a Q & A session with the quiz early.
 Friday’s exam is 25 points and 25 questions. It will have pedigrees,
karyotypes, vignettes where you will determine the disease, the risk
associated with having additional kids.
II.
Chromosomal Abnormalities
A. Numerical chromosomal abnormalities
 Know that the major root cause of these is nondisjunction
 Results from the failure of paired chromosomes or chromatids to separate
and move to opposite poles of the spindle at anaphase either during mitosis
or meiosis
 When nondisjunction occurs during gametogensis, the gametes formed have
either an extra chromosome (n + 1) or one less chromosome (n - 1)
 If there are another number of chromosomes other than 23 or a
multiple of 23, it is referred to as aneuploid. Kleinfelter’s 47, XXY is an
example.
B. Syndromes of autosomal chromosomes
MOD #63
Wed, 05/07/03, 9am
Dr. Eisenberg
Jennifer Uxer for Adam Ackerman
Page 2 of 6
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Trisomy 21 Down Syndrome - single most common cause of mental
retardation
 Fragile X is also a major cause of mental retardation
 Results primarily from nondisjunction during the first meiotic division
(maternal)
 4% of cases result from a Robertsonian translocation of an extra long arm
of chromosome 21 to an acrocentric chromosome
o Robertsonian translocation: 2 large chromosomes rejoin and a small
portion of the small chromosome may be lost
o These people are phenotypically normal depending on how much
material is lost
o These are not necessarily a balanced translocation.
o The initial problem occurs during formation of the egg/sperm (meiosis)
o The next problem occurs when a person with Robertsonain
transformation mates: fertilization → mitosis → improperly paired
chromosomes → causes “funky” things
 1% of Down syndrome patients are mosaics, having a mixture of cells with
46 and 47 chromosomes
C. Down Syndrome
 Mosaicism is caused by nondisjunction during mitosis of a somatic cell in
the early stages of embryogenesis
 Maternal age has a strong influence on the incidence of trisomy 21. It
occurs 1 in 1550 live births to women under the age of 20 years, and 1 in 25
in mothers over 45 years of age
 Only a small region band 22q22.1 is required to be trisomic for the
syndrome
 Doesn’t require a complete extra chromosome 21—just need q (long arm)
band 22.1 duplication. Don’t have to know the bands!
 Be aware of the clinical manifestations: mental retardations, facial
features—epicanthic folds and flat facial profile, congenital heart defects,
abundant neck skin, intestinal stenosis, gap between the 1st & 2nd toe,
hypotonia, umbilical hernia, Simian crease in the hand,
D. Trisomy 18: Edwards Syndrome
 Incidence: 1:8000. 10x less often than Down’s Syndrome
 Karyotype 47 (total number of chromosomes), XX (means female), +18
(additional chromosome is chromosome 18)
 People with this can be mosaics.
 Clinical manifestations: mental retardation, prominent occiput, low set
ears, micrognathia, short neck, overlapping fingers, congenital heart defects,
renal malformations, limited hip abduction, rocker bottom feet.
E. Trisomy 13: Patau Syndrome
 3rd most common trisomy
 Karyotype 47, XX, +13
MOD #63
Wed, 05/07/03, 9am
Dr. Eisenberg
Jennifer Uxer for Adam Ackerman
Page 3 of 6
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III.
If there’s a translocation, the karyotype is 46, XX, +13. q10 on 13
translocates to q10 on 14.
There doesn’t have to be complete extra bond on chromosome 13. Just
have to have a trisomy for that region.
Clinical: microcephaly, mental retardation, microphthalmia, cleft lip and
palate, polydactlyly, cardiac defect, umbilical hernia, rocker bottom feet,
renal defects.
Syndromes of the Sex Chromosomes
A. General
 Y chromosome - The testis determining gene is located in a 230-kb region
near the end of the short arm of the Y chromosome. (This is the signal
coding region and determines maleness.)
 Y chromosome coding < X chromosome coding
 X chromosome - inactivation of either the maternal or paternal X
chromosome occurs at random among all the cells of the blastocyst at about
16th day of embryonic life, Lyon hypothesis, Barr body
 Lyon hypothesis = lionization: responsible for X-linked disorders.
 Review: for a recessive disease to be expressed, you need a double dose of
the genes—need 2 of that gene for expression.
 Very rarely, a female can have a presentation of a recessive disease with
only 1 of the recessive genes due to inactivation of the functional X gene.
B. Klinefelter Syndrome
 Very common
 Klinefelter Syndrome (47, XXY) - male hypogonadism that occurs when
there are two or more X chromosomes and one or more Y chromosomes
 Another case of aneuploidy because there’s an X.
 Side note: any time there’s a Y, the person is male (despite the 2 Xs here)
 Incidence is approximately 1 in 850 male births
 Principle cause of reduced spermatogenesis and male infertility
 Clinical manifestations
o Most patients have a distinctive body habitus with an increase in length
between the soles and pubic bone, which creates the appearance of an
elongated body
o Small atrophic testes, often associated with a small penis
o Don’t enter puberty: Lack of secondary male characteristics such as
deep voice, beard, and male distribution of pubic hair
C. Turner Syndrome
 Turner Syndrome results from complete or partial monosomy of the X
chromosome and is characterized primarily by hypogonadism in phenotypic
females
 57% of patients are missing an entire X chromosome, resulting in a (45,X )
karyotype
MOD #63
Wed, 05/07/03, 9am
Dr. Eisenberg
Jennifer Uxer for Adam Ackerman
Page 4 of 6
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This, too, is aneuploidy where there’s 1 less chromosome due to a deletion
of 1 X. The nondisjunction occurs in meiosis.
 There are a number of karyotypes for this, but (45,X) is the most common.
 Most common sex chromosome abnormality in females
 Clinical manifestations: infertility, streak ovaries, small breasts, short
stature, ammenhorrea.
D. X-Linked Disorders
 Gene responsible for the disease resides on the X chromosome
 X-linked traits can be dominant or recessive
 Lack of transmission from father to son because dad passes his Y to the son.
 Most X-linked traits are recessive so that heterozygous females do not
exhibit the disease
E. Fragile X Syndrome
 Most common form of inherited mental retardation
 The fragile site at Xq27.3 represents a specific locus where chromosomal
breakage can readily occur
 The fragile site is characterized by an amplification of a CGG repeat within
the FMR-1 gene
 FMR-1 gene has about 29 – 30 CGG repeats in it in a normal individual.
 These repeats are amplified in meiosis. As the number of repeats increases,
so does the manifestation of the disease.
 Has a premutation: the number of repeats is in an intermediate area. These
folks are carriers.
 If there are > 230 repeats, the person has a full-blown manifestation.
 The presence of clinical symptoms and a cytogenetically detectable fragile
site is related to the extant of amplification of the CGG repeats
 Normal transmitting males and carrier females carry 50 to 230 CGG repeats,
expansions of this size are called premutations
 Affected individuals have the full mutation which is characterized by a large
expansion of 230 to 4,000 repeats
The following slide in the notes is not shown
here.
 It’s a pedigree of a family with Fragile X.
 It tells how a carrier has a premuatation
and how you can get offspring with
premutations and others with full
mutations.
 Take Home Message:
o A greater # of copies means a greater
risk of disease manifestation.
o If there are >230 copies, the person will
present with the disease.
Here you can see the breakage. The region is
susceptible to breaking due to the repeats.
MOD #63
Wed, 05/07/03, 9am
Dr. Eisenberg
Jennifer Uxer for Adam Ackerman
Page 5 of 6
F. Other diseases associated with triplet expansion
 Taken from the picture in the power points
 Myoclonus epilepsy
 Friedreich ataxia
 Huntington disease
 Myotonci dystrophy
 The triplet doesn’t have to be on an X chromosome or in an exon.
 It can be in an intron, promoter region, noncoding area (causing mRNA
instability)
IV. Mitochondrial DNA mutations
 2 types of DNA in a cell: genomic DNA & mitochondrial DNA
 Mito DNA codes for tRNA genes, electron transport genes, ATP formation,
among others. Due to these, there can be diseases associated with
mitochondrial DNA.
 Mitochondria are maternally inherited. So, if a female expresses a disease,
her kids will, too.
 All progeny of affected males are normal.
 If a male has the disease, none of his kids will if his wife is normal.
 Females and males are affected equally.
 In 1 egg, there are hundreds of mitochondria.
 The example provided was of Leber Optic Neuropathy.
V.
Genetic molecular diagnostic techniques
A. PCR
 Primary method employed today
 Amplify the gene to look for a specific mutation or irregularity in that gene
 Slide picture shows a mutation affecting the presence of a restriction site
seen in Factor V mutation.
 After PCR, the results on the slide show a normal, a mutant, and a
heterozygote. The normal has 3 bands. The mutant has 1 of these bands
and a completely different band. The heterozygote demonstrates all 4
bands; if it’s an autosomal recessive disease, the person’s a carrier.
B. Hybridization Assay
 This is done with a Southern Blot or a Dot Blot.
 A probe is used to detect the presence or absence of a gene. The probe
hybridizes to the area if it is normal.
MOD #63
Wed, 05/07/03, 9am
Dr. Eisenberg
Jennifer Uxer for Adam Ackerman
Page 6 of 6
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Due to potential problems with the act of hybridizing, a probe for the mutant
is also used to ensure that hybridization did in fact occur.
 In the Dot Blot, the normal condition using the normal probe shows up as a
dark blot; the heterozygote is a lighter color, and the mutant does not show a
dot. The reverse is true with the probe for the mutant: the normal does not
have a dot; heterozygote is a light color, and the mutant has a dark dot.
C. Fragile X—an amplified triple repeat.
 PCR limits the size of the DNA to a couple thousand base pairs.
 With a premuation, the person has a larger band.
 If the person expresses Fragile X, you won’t see a band on the PCR
because the number of repeats is too high so PCR can’t amplify it.
 Therefore, you confirm with a Southern Blot hybridization to
corroborate your results of a large amplification triplet.
D. Association Linkage analysis
 Rarely used now because most mutant genes have been identified.
 If we didn’t know a mutant gene, we know that there’s a marker that’s
associated with that mutant gene.
 This creates a restriction site change allowing us to determine the gene.
 You can see a smaller fragment with S. Blot.
E. Last Slide
 Polycystic kidney disease is caused by a dinucleotide repeat.
 DNA polymorphisms from a variable number of CA repeats. Allele C is
linked to a mutation in the gene for polycystic kidney disease.