Molecular Biology - Lecture - 13 - Applications of Molecular Techniques in Genetic Testing and Forensics - Introduction to Key Concepts - Part 1 - Complete

Multiplexing strategies

Strategies for simultaneous mutation detection of multiple targets that have been devised for practicality and cost-effectiveness

Whole-genome sequencing

Gold standard for identifying all possible mutations

Whole genome sequencing

Carrier screening, Newborn screening, Diagnostic testing, Presymptomatic DNA testing, Prenatal testing

Applications of molecular genetics

Carrier

Refers to an individual who has a copy of the disease-causing genetic mutation but does not show symptoms of the disease

Carrier screening

Application of molecular genetics that detects recessive mutations in healthy individuals for the purposes of genetic counseling and family planning.

Individuals with family history of the disorder, Population-based screening

2 testing strategies in Carrier screening

Individuals with family history of the disorder

Type of carrier screening

A person who has a sibling or relative having the disorder is at much higher risk of being a carrier than someone in the general population. Because of this higher risk, they may need more thorough testing compared to what is typically done for the general population.

Population-based screening

Type of carrier screening

For large numbers of individuals who have a negative family history but who may be at risk for the disorder because of its prevalence within their ethnic group or in the population.

Population-based screening

Type of carrier screening

It typically strives to keep the testing procedures as rapid and inexpensive as possible, focusing on the more prevalent mutations.

Newborn screening

Application of molecular genetics that aims to identify relatively prevalent inherited defects in otherwise asymptomatic newborns.

Phenylketonuria, Galactosemia, sickle cell disease, Cystic fibrosis

Common conditions screened in Newborn screening

Autosomal recessive

Inheritance pattern of the common conditions screened in Newborn screening

(Phenylketonuria, Galactosemia, sickle cell disease, Cystic fibrosis)

Newborn screening

Application of molecular genetics.

Its goal is to identify affected babies in early life who are still asymptomatic so that dietary or pharmaceutical treatment can be initiated before irreversible damage occurs

T

T/F

Currently, less expensive and more comprehensive biochemical or enzymatic methods are used as a general screening method for newborns.

Newborn screening

Application of molecular genetics.

Molecular genetic methods in this setting are employed mainly as a backup for confirmation of positive results from these tests.

Diagnostic genetic testing

Application of molecular genetics performed on symptomatic individuals

Diagnostic genetic testing

Application of molecular genetics

Since single-gene DNA tests are highly specific and the conditions being tested for are usually rare, they are not suitable for broad differential diagnosis. For these reasons, the patient's clinical presentation must strongly point toward a particular disease before the test is ordered. For some conditions, more traditional methods also exist. Thus, the decision to DNA test must also consider cost, convenience, and utility. Molecular testing may be more advantageous for early or atypical clinical presentations.

Presymptomatic DNA testing

Application of molecular genetics mainly used for late-onset autosomal dominant disorders where individuals with an affected parent know that they have a 50% risk of inheriting the disease-causing gene.

Presymptomatic DNA testing

Application of molecular genetics that allows them to determine their genetic status before symptoms appear so that they can make informed choices about reproduction, work, lifestyle, or begin early monitoring and preventive care.

Huntington disease, Heritable cancer syndromes

Presymptomatic DNA testing

Prototypic disorders in this group

Prenatal testing

Application of molecular genetics that detects genetic disease in the fetus

Prenatal testing

Application of molecular genetics whose primary objective in prenatal diagnosis is the identification of an affected fetus in a timely manner so that a practical option of pregnancy termination can be offered to the couple.

Prenatal testing

Application of molecular genetics that is not legal in the Philippines, even if it is an option.

Prenatal testing

Application of molecular genetics that can help the couple and healthcare team prepare for the birth of an affected child.

Prenatal testing

Application of molecular genetics that includes planning the delivery in an appropriate facility, such as a tertiary hospital with neonatal specialists, and ensuring the necessary treatments, medications, or dietary interventions are available immediately after birth. It also enables coordination among obstetricians, geneticists, and pediatricians for proper continuity of care.

Prenatal testing

Application of molecular genetics where knowing the diagnosis ahead of time gives parents psychological preparation and time to adjust emotionally

Prenatal testing

Application of molecular genetics

Advantages are allowing immediate treatment at birth or providing reassurance if the fetus is unaffected. However, it can be difficult to justify exposing the pregnancy to even a small risk of miscarriage from procedures such as amniocentesis or chorionic villus sampling when done for these reasons.

Molecular heterogeneity

Concept in Genetic Disorders

Genetic disorders can be caused by more than one mutation within the disease gene.

Molecular heterogeneity

Concept in Genetic Disorders

Example: Cystic fibrosis

CFTR gene

Gene mutated in Cystic fibrosis

TSC1, TSC2 genes

Mutated genes that can cause tuberous sclerosis

BRCA1, BRCA2 genes

Mutated genes that can cause family breast or ovarian cancer

Molecular heterogeneity

Concept in Genetic Disorders that implies that a single disease phenotype can result from multiple distinct mutations either within the same gene or across different genes

Variable penetrance

Concept in Genetic Disorders that refers to the proportion of individuals with a disease-causing mutation who actually show the clinical symptoms

Variable penetrance

Concept in Genetic Disorders that is a feature of such relatively common genetic disorders such as Marfan syndrome and neurofibromatosis.

Variable expressivity

Concept in Genetic Disorders that refers to the appearance of different signs, symptoms, and severity of a disorder in individuals inheriting the same mutation.

Variable expressivity

Concept in Genetic Disorders that can be due to modulation of phenotypic expression by other non-allelic or modifier genes.

Uniparental disomy

Concept in Genetic Disorders wherein an individual inherits both copies of a chromosome from one parent only, instead of one copy from each parent.

T

T/F

Normally, an individual should inherit one chromosome from the father and one chromosome from the mother.

Uniparental disomy

Concept in Genetic Disorders where the individual only inherits or inherits two copies of the chromosome from a single parent only.

Cystic fibrosis

Uniparental disomy was first discovered in a patient with this condition

Prader-Willi syndrome, Angelman syndrome

Apart from Cystic fibrosis, Uniparental disomy can also be seen in these conditions

Imprinting

Concept in Genetic Disorders which refers to the differential expression of a gene in an offspring depending on whether it was inherited from the mother or the father or sometimes on other epigenetic influences

Imprinting

Concept in Genetic Disorders where only one parental allele is active. The other is epigenetically silenced or imprinted.

Methyl groups

Imprinting happens because during sperm or egg formation, chemical tags such as ___ may be added to specific genes to silence them. These marks are maintained throughout the cell division and is passed to the offspring.

F

T/F

Imprinting changes the DNA sequence itself

1

Amount of parental allele active in Imprinting disorders

T

T/F

In imprinting disorders, the disease depends on entirely on which parent contributes the functional gene

T

T/F

Imprinting.

If the person inherits one normal copy of the gene that is properly expressed and sufficient for normal function, it can compensate for a mutant copy inherited from the other parent

T

T/F

Imprinting.

If the person receives the normal gene from the parent whose copy is silenced or imprinted, so such as in here in this one, this cannot compensate for a mutant gene inherited from the other parent. Therefore, the disease still occurs.

Anticipation

Concept in Genetic Disorders that refers to a progressive increase in severity and or an earlier age of onset of a genetic disorder in subsequent generations of a family.

Anticipation

Concept in Genetic Disorders that is typically associated with the trinucleotide repeat disorders such as myotonic dystrophy and Huntington disease.

T

T/F

In trinucleotide repeat disorders, the increasing severity can be correlated with further expansion of the repeat region.

Anticipation

Concept in Genetic Disorders

For example, imagine a family across three generations with Huntington disease where the disease anticipates by appearing earlier and more severely. For the first generation or the grandfather, at age 65, he begins showing mild clumsiness and personality changes. Genetic testing reveals 40 CAG repeats. For the second generation, the father, or the grandfather's son, he starts showing clearer motor symptoms such as chorea and cognitive decline at age 42. His genetic test show 48 CAG repeats. For the third generation or the grandson, or the father's son, he begins having significant trouble in school, seizures, and severe muscle rigidity at age 14. Genetic testing reveals over 60 CAG repeats.

Epigenetic influences

Concept in Genetic Disorders that are heritable but potentially reversible changes in gene expression that do not represent a change in the actual DNA sequence.

Epigenetic influences

Concept in Genetic Disorders

The most striking examples are genomic imprinting and mammalian X-chromosome inactivation, both involving transcriptional silencing of genes by methylation.