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Human Genetics CMMB 413 > Chromosomal and Genomic Basis of Disease > Flashcards

Chromosomal and Genomic Basis of Disease Flashcards

1
Q

What is cytogenetics?

A

The study of the structure, function and evolution of chromosomes and the changes in them which can result in phenotypes of clinical significance.

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2
Q

Name two things which can explain why similar/ identical chromosomal abnormalities may show a range of variable phenotypes.

A
  • Non-genetic environmental triggeres/ effects
  • Genetic variation, either within the site of the abnormality or in other areas which effect them.
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3
Q

What is the most common trisomy in a

  • prenatal context?
  • postnatal context?
A
  • Trisomy 16
  • Trisomy 21 (down syndrome)
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4
Q

What are the three non-mosaic disorders that allow for post-natal survival? What are their incidence of birth?

A
  • Trisomy 21 (Down syndrome) 1/850 births
  • Trisomy 18 (Edward’s syndrome) 1/ 6,000 - 8,000 births
  • Trisomy 13 (Patau syndrome) 1/ 12,000-20,000 births
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5
Q

What are the three chromosomes which contain the least number of genes?

A

most - 13, 18, 21 - least

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6
Q

What kind of trisomy is being described?:

Hypotonia, loose skin on nape of the neck, clinodactyly, high risk of leukemia, flat occiput, palmar crease, congenital heart disease.

What is the life expectancy of this individual?

A

Trisomy 21/ Down Syndrome
55 years

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7
Q

What kind of trisomy is being described?:

Hypertonia, prenatal growth deficiency, Rocker-bottom feet, fist clenching, severe heart malformation.

What is the life expectancy of this individual?

A

Trisomy 18/ Edward’s Syndrome
< a few months

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8
Q

What kind of trisomy is being described:?

Microcephaly sloping forehead, Rocker-bottom feet, fist clenching, severe CNS malformation, polydactyly, low set ears and receding jaw.

What is the life expectancy of this individual?

A

Trisomy 13/ Patau Syndrome
< a year

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9
Q

What is the genetic cause of 95% of trisomy 21? In which parent does this event usually occur?

A

Meiotic nondisjunction during meiosis. In 75% of cases this is in the maternal parent.

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10
Q

Do all down syndrome patients have 47 chromosomes?

A

No.
Translocation Down Syndrome, Mosaic Down Syndrome and Partial Down Syndrome patients can all have only 45 or 46 chromosomes.

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11
Q

What percentage of Down Syndrome is due to Robertsonian translocations?

A

3-4%

pg 1

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12
Q
  • How does mosaicism in trisomy 21 usually occur?

What percentage of down syndrome cases are mosaic?

  • How does a mosaic trisomy 21 phenotype compare to a non-mosaic trisomy 21?
A
  • Generally it occurs through a trisomic conception followed by a loss of the extra chromosome during mitosis in some embryonic cells.
  • 2-4%
  • Mosaic trisomy 21 results in more milder clinical expression, the exact features of which depend on the type of tissue in which the mitotic anomaly occurred.
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13
Q

Why are partial trisomy down syndrome cases of particular interest to researchers?

A

Because partial down syndrome is due to a duplication in only a particular part of the chromosome. The study of patients with this disorder can help researchers understand which regions of the genome are responsible for what symptoms, particularly the cardiac phenotype.

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14
Q

What is the most common cause of trisomy 13 (Patau Syndrome)?

A

Increased maternal age is a risk factor. It is mostly due to a nondisjunction event at meiosis I. However, it can also be caused by an unbalanced translocation.

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15
Q

Depression alert

  • What percentage of trisomy 18 conceptions survive to term?
  • What percentage of trisomy 18 births survive the first few weeks of life?
  • What percentage of trisomy 18 births survive to 1 year?
  • What percentage of trisomy 18 births survive within the
A
  • less than 5 %
  • 50 %
  • 8 %
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16
Q

When is first trimester prenatal screening preformed? What does it entail? What is the goal?

A

11-13 weeks into gestation.

  • Blood test / Serum screening
  • Ultrasounds to assess physical features
  • maternal age
  • maternal blood samples to assess circulating fetal cell-free DNA.

The goal is to inform on the risks for particular birth defects and genetic disorders.

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17
Q

What is a common neural tube defect which can be observed with an ultrasound?

A

Meningomyelocele sac

18
Q

What is a feature that can be observed by week 11 in trisomy 21 fetuses?

A

Nuchal translucency. This leads to the loose skin around the neck.

19
Q

How are free beta human chorionic gonadotropin (L. beta HCG) levels affected by the three viable types of trisomies?

A

Increase in Trisomy 21
Decrease in Trisomy 18
Decrease in Trisomy 13

20
Q

What is some of the information which can be obtained by a Non-Invasive Prenatal Screening (NIPS)?

A
  • Chromosomal aneuploidy detection
  • Subchromosomal deletions/duplications
  • Whole fetal genome sequencing
  • Fetal methylome
  • Fetal transcriptome
21
Q

What are examples of invasive diagnostic tests?

A
  • Chorionic villus sampling (placental tissue)
  • Amniocentesis sampling (amniotic fluid)

These carry a 1 in 200-250 chance of causing a spontaneous abortion.

22
Q

What are the differences and similarities between Chorionic villus sampling (CVS) and Amniocentesis?

A
  • Both are invasive diagnostic tests
  • Both can be Trans-abdominal procedures
  • Both use real time ultrasound to guide the needle and monitor the procedure
  • CVS is done at 10-11 weeks while amniocentesis is done at 15-17 weeks.
  • CVS can be a trans-cervical procedure.
23
Q

with rapid aneuploidy detection, what is a hallmark of trisomy?

A

three spikes at a certain chromosome value.

24
Q

What will be the normalized sequence representation of a normal and a trisomy 21 individual in whole genome sequencing?

A

~1.0 for normal
~1.5 for trisomy 21 at chr21 only.

25
Q

Why can segmental duplications existing in different regions of the genome cause duplication or deletion?

A

If two areas of a stretch of DNA are highly homologous copies of repeated DNA, It is possible for a non-allelic crossing over event to occur which causes unequal crossing over. Therefore leading to deletions or duplications of the sequences between these two homologous repeating regions.

26
Q

What is the name of a syndrome caused by the 22q.11.2 deletion?

A

DiGeorge or velocardiofacial syndrome

27
Q

How many genes are deleted in the 3Mb proximal deletion of 22q11.2 deletions? Name one and the effect it has on phenotype when it is deleted.

A
  • 30 genes within the 3Mb deletion.
  • TXB1 is a gene whose deletion causes heart defects.
28
Q

How many live births have a 22q11.2 deletion?

A

1 in 4,000 live births

29
Q

What are some characteristics of DiGeorge syndrome?

A
  • craniofacial anomalies
  • intellectual disability
  • immunodeficiency
  • heart defects.
30
Q

What methods can be used to detect 22q11.2 deletion syndrome?

A
  • FISH
  • Cell free DNA
31
Q

What are the names of microduplication of 22q11.2? What are some clinical phenotypes?

A

22q11.2 duplication

quadruple tetrasomy inversion 22q11.2 (cat-eye syndrome)

  • ocular coloboma
  • congenital heart defects
  • craniofacial anomalies
32
Q

What locations do segmental duplications predispose a chromosome to higher levels of duplication or deletion disorders?

A

pericentromeric (next to a centromere)

subtelomeric (under a telomere)

33
Q

What are some clinical features of Cri-du-chat syndrome?

A
  • crying sounds like a mewing cat
  • microcephaly
  • hypertelorism
  • epicanthal folds
  • low-set ears

-micrognathia

  • preauricular tags
34
Q

What genetic changes causes Cri-du-chat syndrome?

A

A terminal or interstitial deletion of a region of the 5p (short arm of chromosome 5)

35
Q

How can de novo translocations be detected? How often do they occur?

A

FISH, reverse transcription PCR and capture based target sequencing

36
Q

When are epigenetic imprinting markers established?

A

during gametogenesis

37
Q

What are the symptoms of Prader-Willi Syndrome (PWS)?

A
  • Short stature
  • Hypotonia
  • Small hands and feet

-mild to moderate intellectual capacity, hypogonadism

38
Q

What causes most cases of PWS?

A

paternally-inherited deletion of chromosome 15 (15q11-q13) requiring that all genetic information is derived from the mother.

39
Q

What are some symptoms of Angelman Syndrome (AS)?

A
  • Severe intellectual disability
  • happy demeanor
  • hand flipping, Ataxic gait,
40
Q

What causes AS (Angelman Syndrome) ?

A

maternally-inherited deletion of Chromosome 15. Deletion of chromosomes 15q11-q13 causes father to give all information.

Human Genetics CMMB 413 (7 decks)

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