Molecular Genetics Bio 30 Practice Test
Yufei Labbe
Molecular diagnostics encompasses the analysis of human, viral, and microbial genomes and the products they encode. Molecular genetics utilizes molecular biology's laboratory tools to relate genetic structure to protein function and, ultimately, health and disease. Variants identified during genetic testing are classified based on diverse evidence types, as the American College of Medical Genetics and Genomics recommends, emphasizing the need for board-certified geneticists to interpret the results. Integrating genetic testing methodologies with clinical expertise is crucial in translating molecular genetics advancements to better patient care.
The field of molecular genetic and genomic testing is undergoing rapid change due to improvements in our understanding of the molecular causes of uncommon and common illnesses and DNA analysis technologies. The advent of molecular genetics has revolutionized healthcare by offering unprecedented insights into the genetic basis of diseases, enabling personalized diagnostics, treatment strategies, and risk assessments. However, this progress brings with it the responsibility for healthcare providers to stay updated with the latest advancements and best practices in genetic testing.
This activity for healthcare professionals is designed to enhance learners' proficiency in identifying patients with indications for molecular genetics testing and interpreting genetic test results. Participants acquire a broader grasp of specimen collection, procedures, indications, potential diagnosis, normal and critical findings, interfering factors, and complications. Learners gain insights into the complexities of molecular genetics, preparing them to collaborate with an interprofessional team that aims to improve outcomes for patients who need molecular genetics testing.
Objectives:
- Identify clinical encounters appropriate for genetic molecular testing, distinguishing cases where such testing can contribute to diagnosis, prognosis, or treatment decisions.Evaluate genetic test results accurately, discerning their clinical significance and relevance to patient management.Differentiate between genetic testing methodologies, understanding their strengths, limitations, and optimal applications to diagnose patients.Implement best interprofessional collaboration and communication practices to ensure that patients who need molecular genetics testing receive comprehensive care that considers their medical, psychological, and social needs, thus improving outcomes.
Molecular genetics testing is fundamental in evaluating inherited disorders, somatic or acquired diseases with genetic associations, and pharmacogenetic responses. Genotyping can provide valuable disease diagnosis, prognosis, and progression indicators, guide treatment selection and response, and identify gene-specific therapeutic targets.[1] Human genetic material primarily consists of double-stranded, helical DNA. This molecule has a backbone composed of alternating sugar (deoxyribose) and phosphate groups, with hydrogen bonds linking nitrogenous base pairs. Specifically, adenine (purine) pairs with thymine (pyrimidine), while guanine (purine) pairs with cytosine (pyrimidine), forming the complementary base pairs within the DNA double helix.[2][3]
The complete decoding of the human genome sequence and the development of powerful identification and cloning methods for genes linked to inherited diseases have transformed the practice of molecular genetics and molecular pathology. Advanced molecular analysis methods can now determine presymptomatic individuals' illness risk, detect asymptomatic recessive trait carriers, and prenatally diagnose conditions not yet evident in pregnancy.[8] Molecular genetics techniques are often the only approaches to these puzzles. Thus, genetic tests are powerful tools for diagnosis, genetic consultation, and prevention of heritable diseases.[9]
Many genetic tests can analyze gene, chromosome, and protein alterations. A clinician often considers several factors when selecting the appropriate test, including suspected conditions and their possible genetic variations. A broad genetic test is employed when a diagnosis is uncertain, while a targeted test is preferred for suspected specific conditions.[10] Molecular tests look for changes in 1 or more genes. These tests analyze the sequence of DNA building blocks (nucleotides) in an individual's genetic code, a process known as DNA sequencing, which can vary in scope.[11]
The targeted single variant test identifies a specific variant in a single gene known to cause a disorder, eg, the HBB gene variant causing β-globin abnormalities that give rise to sickle cell disease. This test assesses the family members of an individual with the known variant to ascertain if they have the familial condition.[12] Single-gene tests examine genetic alterations in 1 gene to confirm or rule out a specific diagnosis, notably when many variants in the gene can cause the suspected condition. Gene panel tests look for variants in multiple genes to pinpoint a diagnosis when a person has symptoms that may fit various conditions or when many gene variants can cause the suspected condition.[13][14]
Whole-exome sequencing or whole-genome sequencing tests analyze the bulk of an individual's DNA to find genetic variations. This approach is useful when a single-gene or panel testing has not provided a diagnosis or when the suspected condition or genetic cause is unclear.[15] This sequencing method is often more cost- and time-effective than performing multiple single gene or panel tests.[16]
Chromosomal tests analyze whole chromosomes or long DNA lengths to identify significant alterations, including extra or missing chromosome copies (trisomy or monosomy), large chromosomal segment duplications or deletions, and segment rearrangements (translocations) (see Image. Trisomy 21 on G-Banded Chromosomal Studies).[17] Chromosomal tests are employed when specific genetic conditions linked to chromosomal changes are suspected. For instance, Williams syndrome results from deleting a chromosome 7 segment.
Gene expression tests assess gene activation status in cells, indicating whether genes are active or inactive, with activated genes producing mRNA molecules that serve as templates for protein synthesis.[18] The mRNA produced helps determine which genes are highly active. Too much activity (overexpression) or too little activity (underexpression) of specific genes may suggest particular genetic disorders, including various cancer types.[19] Biochemical tests assess protein or enzyme levels and activity rather than directly analyzing DNA.[20] Abnormalities in these substances may indicate DNA changes underlying a genetic disorder.
Heritable mutations are detectable in all nucleated cells and are thus considered germline or constitutional genetic changes. Somatic genetic changes are characteristic of acquired or sporadic diseases like cancer.[21] Both scenarios are investigated using similar molecular biology methods to detect DNA and RNA variations, although the interpretation and utility of the laboratory results often differ significantly.[22]
Fluorescent in situ hybridization (FISH), chromosomal microarray analysis (CMA), and cytogenetic analysis (karyotyping) can be used to detect gross mutations like whole- and large-scale gene deletions, duplications, or rearrangements. Conventional karyotyping identifies rearrangements over 5 DNA megabases.[23] FISH has a resolution of 100 kilobases to 1 megabase. Minor alterations, such as single-base substitutions, insertions, and deletions, are detectable with single-strand conformation polymorphism (SSCP) and sequence analysis through next-generation sequencing (NGS). NGS uses genomic DNA (gDNA) or complementary DNA (cDNA) and has 3 modalities: whole genomic DNA, targeted, and exome sequencing.[24]
Denaturing high-performance liquid chromatography (DHPLC) can detect small deletions and duplications. Multiplex ligation-dependent probe amplification (MLPA) extends the range of deletions and duplications detected, bridging the gap between FISH or cytogenetic analysis and HPLC. MLPA is particularly useful in identifying complete or single and multiexon deletions or duplications.[25][26]
Peripheral blood is the specimen required for FISH, MLPA, DHPLC, and sequencing. Amniotic fluid cells and, more recently, cell-free fetal DNA may be used for noninvasive prenatal testing.[27] Ethylenediaminetetraacetic acid is the most commonly used anticoagulant for molecular-based testing. However, acid citrate dextrose (ACD) is an acceptable alternative in cases where cellular form and function must be preserved.
ACD A and ACD B are the only ACD tube designations recognized, differing only by their additive concentrations.[28] Both enhance white blood cell viability and recovery for several days after specimen collection, making them suitable for molecular diagnostic and cytogenetic testing.
FISH utilizes fluorescent DNA probes to target specific gene sequences in interphase or metaphase cells, enabling their visualization and detection. Housekeeping gene probes always serve as positive internal controls. The probe must be large enough to hybridize specifically with the target without impeding the hybridization process. Conventional FISH involves pipetting the hybridization mix onto the cytological sample and incubating them together.
The technique can be applied to suspended cells, cultured cells, and frozen or formalin-fixed paraffin-embedded tissue sections, with subsequent cell sorting for fluorescence signal separation.[29] Preserving nucleic acid integrity and cell morphology is necessary during sample fixation. The experimental FISH procedure includes several preparatory steps, the hybridization reaction itself, and the removal of unbound probes.[30] The probe may be directly labeled with fluorophores or targeted for fluorescent detection using labeled antibodies or similar substrates. Different tags may be used, and different targets may be detected in the same sample simultaneously (multi-color FISH). Tagging is performed in various ways, including nick translation or polymerase chain reaction (PCR) using tagged nucleotides (see Image. Polymerase Chain Reaction). Probes can vary from 20 to 30 nucleotides to much longer sequences.
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