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Cracking the Code:

A Journey through Genetics

Amanda Cocca & Anna Haasen

Clinical Genetics MLTs

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Learning Objectives

•Describe the basic structure of the genetic code

and how genetic variation relates to disease.

•Differentiate between inherited (constitutional) and

acquired (malignant) genetic changes.

•Give examples of the application of molecular

genetics in the medical community.

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DNA: The Blueprint of Life

• DNA =

deoxyribonucleic acid

• Contains instructions

for living organism

• Most DNA in nucleus

46 chromosomes

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DNA: The Blueprint of Life • 1953: Watson & Crick DNA double helix

• Banister = sugar-phosphate backbone

• Stairs = Hydrogen bonding between base pairs

One nucleotide

“A” “T”

“C” “G”

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What is a “Gene”?

• Defined section of DNA

• Instructions for how to make a protein

• Gene = introns + exons + untranslated regions

T A A T G G G C T A G C G T A T A C G A T G G G C A A T A T T G A C C A C A T T A A T T A

eg. BRCA1 gene has 24 exons coding for one protein

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The Central Dogma

DNA

translation

RNA

PROTEIN

transcription

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• Normal cell division

• Typical of ordinary tissue

growth

• Result: two daughter cells

each with the same number

and kind of chromosomes as

the parent cells

Mitosis

• Creates all cells of the body

(except germ cells; eggs and sperm)

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Mitosis

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Mitosis

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Mitosis

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Mitosis

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Mitosis

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Mitosis

END RESULT:

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Metaphase

8

8

46,XX

20

20

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Meiosis

END RESULT:

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Sex Determination

Zygotes

(offspring)

44+

XX

44+

XY

22+

X

22+

Y

22+

X

Egg

Sperm

♀

♂

44+

XX ♀

44+

XY ♂

Female: 46,XX

Male: 46,XY

END PRODUCT

OF MEIOSIS

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Non-Disjunction

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Non-Disjunction Example: Down Syndrome (Trisomy 21)

Extra chromosomal material

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Non-Disjunction Example: Turner Syndrome (45,X)

•Missing chromosomal material

•Webbed neck, short stature,

delayed growth

•Unable to conceive without

fertility treatments

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Constitutional Genetic Changes • Inherited or de novo

• Blood, tissue, saliva, products of

conception, cheek swabs

• Diagnosis, management, recurrence risk

assessment

• Testing:

• Chromosome analysis (karyotype)

• Chromosome microarray

• Exome sequencing

• Testing for single gene disorders by molecular

techniques

• Next generation sequencing (NGS) panels

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• Mutations in somatic cells

(cells other than sperm and

egg)

• Can occur at any time in a

person’s life

• Caused by environmental

factors

• Changes cannot be passed

along to the next generation

Acquired Genetic Changes

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1. Disease Classification

2. Disease Sub-type

3. Drugs that will target the

genetic mutations

4. Prognosis

5. Long term treatment

Acquired Genetic Changes

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Disease Classification and Subtype

• t(15;17)APL/AML-M3

Drug treatment

• ATRA (all-trans-retinoic acid)

Prognosis

• Remission rates of 80-90%

• Long term survival > 75%

Long term treatment

• Track molecular level of t(15;17) by PCR-based assays

• Eligibility for stem cell transplant

Example: Acute Myeloid Leukemia (AML)

Acquired Genetic Changes

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•Single Nucleotide Polymorphisms (SNPs)

– No effect on health

– ~10 million SNPs in the human genome

Genetic Variation

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•Variant of Uncertain

Significance (VUS) – Suspected benign

– Significance unknown

– Suspected pathogenic

Genetic Variation

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•Cleft palate order CMA

•Small deletion weakly linked

to autism (VUS), unrelated to

original indication for testing

•Pre and post test counselling

Case study of VUS

Genetic Variation

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HGVS nomenclature

• Human Genome Variation Society

• A universal “genetic” language

BRCA1 c.230 C>T (DNA-level)

The nucleotide at base pair 230 in the BRCA1 gene changed

from cytosine to thymine.

BRCA1 p.Pro1430Ser (Protein-level)

The amino acid at position 1430 in the BRCA1 gene changed

from a proline to a serine.

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Modes of Inheritance Autosomal Dominant

•Disease phenotype in

individuals with one

copy of the mutated

allele (gene)

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Modes of Inheritance

•Affected individual

must have two copies

of recessive allele to

show phenotype

•Carriers have one

dominant and one

recessive copy

Autosomal Recessive

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Pedigrees

Eg. Huntington Disease - Autosomal dominant disease

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Single Gene diseases • A change in sequence (“mutation”) in one gene causes

the disease

• Defined pattern of inheritance

Example: Phenylketonuria (PKU) in PAH gene

T A A T G C T C G G C

Normal

protein

Normal

enzyme

function

T A A T G C T A G G C

Altered

protein

Non-functional

enzyme =

phenylalanine

build-up in the brain

NORMAL

ABNORMAL

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Multifactorial diseases

Genetic Variation

• Copy number variation

• Mutations in specific genes

• Polymorphisms

Environment

• Pathogens

• Smoking

• Diet

• Sun exposure

• Combination of genetic variation at multiple loci in the

genome and environmental factors

• No clear pattern of inheritance

eg. Mutation in BRCA1 or BRCA2 gene indicates an

increased risk of developing breast or ovarian cancer

in the individual’s lifetime.

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Applications of Molecular Genetics

1. Forensics

2. Diagnosis and prognosis of disease

3. Pharmacogenetics

4. Monitoring response to treatment

5. Predictive testing for disease risk

6. Prenatal genetic testing

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FORENSICS •DNA evidence important in court of law

•Compare human identity profiles to

identify victims or suspects

DNA FOUND UNDER

VICTIM’S FINGERNAILS

SUSPECT #2

WE FOUND

A MATCH!! SUSPECT #1

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DIAGNOSIS AND PROGNOSIS

OF DISEASE PML-RARa (subtype M3) AML1-ETO (subtype M2)

POSITIVE

FOR

AML1-ETO

AML

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PHARMACOGENETICS

•Variants in an individual’s genome

may affect their response to a

particular drug

– Absorption

– Activation

– Target response

– Catabolism and excretion

• Tailor the dosage based on

patient’s genetic make-up

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MONITORING RESPONSE TO

TREATMENT

• Determine how well patient is responding to treatment

by quantifying abnormal RNA/DNA in body fluid

Decreasing quantity of abnormal nucleic acid

CML

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PREDICTIVE TESTING

•Determine a family member’s susceptibility to

developing a disease (single-gene vs. multifactorial)

• Recurrence risks

Eg. Hereditary Hemochromatosis - Autosomal recessive disease

NORMAL SEQUENCE

ABNORMAL SEQUENCE

A>G

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PRENATAL GENETIC TESTING Before Conception

• To identify reasons for infertility or recurrent miscarriage

(chromosome studies)

• To screen embryo prior to implantation in assisted

reproductive technologies

During Pregnancy

• To identify aneuploidy during pregnancy

• To predict whether baby carries known familial disease

After Delivery

• To investigate unusual phenotypes (microarray)

• To confirm results of Newborn Screening program

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ETHICS & GENETICS Entering uncharted territory…

To claim PEP hours

• Take the quiz found with the webinar

• You will be awarded a printable certificate upon successful completion of the short quiz

• If you need help please contact the CSMLS Learning Services department at coned@csmls.org