Questions and Answers
What type of disorders are determined by the interaction of multiple genetic factors and environmental and lifestyle factors?
What is the characteristic of polygenic disorders?
Which of the following is a common cause of infertility, miscarriage, congenital malformations, and intellectual disability?
What is the prevalence of chromosomal disorders in live births?
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What is an example of a structural abnormality in chromosomal disorders?
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Which of the following disorders is always genetic, but sometimes hereditary with a mendelian inheritance pattern?
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What is the primary purpose of newborn screening in Italy?
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What is the significance of Articolo 6, legge 104, 05/02/1992 in the context of newborn screening in Italy?
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What is the purpose of reconstructing the pedigree in the genetic counseling process?
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What is the role of genetic testing in the genetic counseling process?
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What is the purpose of gathering information about personal and family clinical history in the genetic counseling process?
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What is the significance of the publication by Stark and Scott in Nat Rev Genet (2023)?
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What is the purpose of specialist visits in the genetic counseling process?
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What is the role of biochemical assays in newborn screening?
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What is the primary methodology used to test for multiple variants in the CHD7 gene, which is associated with CHARGE syndrome?
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What is the characteristic of the clinical features of RASopathies?
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What is the advantage of NGS gene panels?
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What is the primary purpose of newborn screening for hereditary diseases?
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What is the number of pathogenic variants identified in the CHD7 gene associated with CHARGE syndrome?
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What is a reason for referral to genetic counseling in the preconceptional period?
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What is the name of the signaling pathway involved in RASopathies?
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What is the clinical feature of CHARGE syndrome that affects the eye?
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What is a key aspect of genetic counseling for patients or relatives at risk of an inherited disorder?
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In which scenario is genetic counseling typically referred to?
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What is a reason for referral to genetic counseling in the postnatal period?
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What is discussed during genetic counseling for patients or relatives at risk of an inherited disorder?
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In which scenario is genetic counseling not typically referred to?
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What is a reason for referral to genetic counseling in the oncologic period?
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What is the purpose of the ACMG Standards and Guidelines for variant classification in mendelian diseases?
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What is the classification of a variant that is likely causative of a disease with 90% certainty?
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What is the purpose of the Online Mendelian Inheritance in Man (OMIM) catalog?
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How would you search for inherited diseases of the thyroid gland in OMIM?
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What is the purpose of the ORPHANET portal?
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What is the purpose of GeneReviews?
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What does the symbol '%' indicate in OMIM?
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What is the classification of a variant that is not causative of a disease?
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What is the key characteristic of autosomal dominant inheritance?
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What is the name of the inheritance pattern where the disease is transmitted through the maternal line?
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What is the risk of disease in offspring of an affected parent in autosomal dominant inheritance?
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What is the characteristic of X-linked dominant inheritance?
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What is the type of inheritance where the disease is transmitted by either sex to offspring of either sex?
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What is the name of the inheritance pattern where the disease is transmitted through the paternal line?
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What is the risk of disease for an offspring when both parents are affected heterozygotes?
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In autosomal recessive inheritance, what is the role of the parents of affected individuals?
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What is the risk of disease for an offspring when both parents are carriers (heterozygotes)?
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What is the characteristic of homozygotes compared to heterozygotes in autosomal recessive inheritance?
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What type of mating results in a 50% risk of disease for an offspring?
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In autosomal recessive inheritance, what is the genotype of an unaffected parent?
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What is the characteristic of autosomal recessive inheritance?
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What is the pattern of inheritance when an affected individual has unaffected parents?
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In X-linked dominant inheritance, what is the risk of disease for daughters of an affected mother?
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What is the carrier risk for an individual with an affected parent and a healthy carrier?
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In a family with a history of X-linked dominant inheritance, what is the risk of disease for sons of an affected father?
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What is the characteristic of affected individuals in a family with X-linked dominant inheritance?
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What is the risk of disease for offspring of a healthy carrier and an affected parent?
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What is the characteristic of affected individuals in a family with autosomal dominant inheritance?
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What is the carrier risk for an individual with two healthy parents?
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What is the risk of recurrence for offspring of two healthy carriers?
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What is the risk of disease transmission from an affected father to his sons?
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What is the risk of a daughter being a carrier if her mother is a carrier?
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Why are females more frequently affected than males in X-linked recessive inheritance?
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What is the risk of a son being affected if his mother is a carrier?
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What is the characteristic of X-linked recessive inheritance?
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What is the chance of a daughter being an obligate carrier if her father is affected?
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What is the characteristic of male-to-male transmission in X-linked recessive inheritance?
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What is the role of the mother in X-linked recessive inheritance?
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What is a characteristic of X-linked recessive inheritance?
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What is a characteristic of mitochondrial/maternal inheritance?
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What is a common feature of autosomal recessive and X-linked recessive inheritance?
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What is the characteristic of Y-linked inheritance?
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What is the characteristic of apparent Y-linked inheritance?
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What is a characteristic of carrier risk for an X-linked recessive disorder?
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What is the significance of consanguinity in autosomal recessive inheritance?
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What is a characteristic of affected females in X-linked recessive inheritance?
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What percentage of patients with severe developmental disorders have a pathogenic DNM in a protein-coding gene?
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What is the estimated number of SNVs that arise per cell division in early human embryogenesis?
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What type of mosaicism occurs when a DNM arises in the first few cell divisions after fertilization?
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What is a characteristic of somatic mosaicism?
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What is the average prevalence of developmental disorders caused by DNMs?
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How many genes were enriched in damaging DNMs in the DDD Study published in Nature in 2017?
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What is a consequence of postzygotic DNMs?
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What determines the degree of mosaicism caused by a DNM?
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What is germline mosaicism?
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What is gonosomal mosaicism?
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What is the characteristic of germline mosaicism in monogenic disorders?
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What is the challenge in detecting mosaicism?
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What is an example of a rare genetic disorder caused by postzygotic somatic mutations?
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What is the characteristic of somatic mosaicism in segmental neurofibromatosis type 1?
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What is the significance of analyzing various tissues, even single cells, in detecting mosaicism?
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What is the purpose of comparing affected vs. unaffected tissues in detecting mosaicism?
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What is the main advantage of using NGS and droplet digital PCR for detecting paternal germline mosaicism?
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What is the unit of measurement used to express the frequency of a variant in a specimen?
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What is the purpose of dividing samples into thousands of water-in-oil droplets in ddPCR technology?
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What is the mechanism of FRET in Taqman probes?
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What is the main application of real-time quantitative PCR assay?
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What is the main advantage of using droplet digital PCR over traditional PCR methods?
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What is the purpose of using allele-specific fluorescently-labelled probes in the Taqman assay?
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What is the main limitation of using non-affected tissues for detecting paternal germline mosaicism?
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What is the effect of the exonuclease activity of Taq polymerase on the annealed probe during PCR amplification?
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What is the purpose of using two parallel assays with allele-specific probes in genotyping?
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What is the characteristic of droplet digital PCR (ddPCR) workflow?
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What is the characteristic of Alport syndrome?
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What is the purpose of partitioning PCR reagents into picoliter to nanoliter droplets in ddPCR?
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What is the significance of the fluorescence intensity of each individual droplet in ddPCR?
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What is the advantage of ddPCR in detecting very low-level somatic mosaic variants?
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What is the purpose of using Taqman assay in ddPCR?
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What is the purpose of using unique molecular identifiers (UMIs) in sequencing?
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Why do de novo mutations (DNMs) occur more often in paternal germ cells than in maternal germ cells?
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According to the figure illustrating the paternal age effect, what happens to the number of genome replications in the male germline as paternal age increases?
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What is the approximate number of cell divisions that sperm undergo by the age of 40 years?
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Why is the paternal age effect observed in de novo mutations?
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What is the approximate ratio of paternally derived DNMs to maternally derived DNMs?
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What is the purpose of long sequencing reads in the context of DNMs?
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What is the significance of the difference in germline biology between males and females?
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What is the term for the probability that a genotype will manifest itself in a particular phenotype?
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What is the reason why a child with an autosomal dominant disease is born to unaffected parents?
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What is the term for the degree to which a phenotype is expressed by a particular genotype?
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What is the example of a congenital limb malformation caused by a dominant mutation in the DLX5 gene?
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What is the term for the phenomenon where a single genotype can cause a range of different phenotypes?
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What is the term for the correlation between a particular genotype and the resulting phenotype?
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Why is the utility of genotype-phenotype correlation hampered?
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What is the term for the phenomenon where a disease-causing genotype is not expressed in every individual?
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What was the aim of the analysis by Chen et al. in their 2016 study?
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What percentage of the ExAC data set had genotypes implicated in rare Mendelian disorders?
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How many ExAC individuals had genotypes implicated in severe Mendelian early onset disorders?
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What was the number of unrelated individuals analyzed in the ExAC data set?
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What was excluded from the analysis of the ExAC data set?
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What was the source of the WES data analyzed in the study?
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What is an example of a condition that exhibits variable expressivity?
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What is the factor that affects the penetrance and expressivity of Hemochromatosis?
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What is the inheritance pattern of Familial male limited precocious puberty?
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What is the result of a gain-of-function variant in the LHCGR gene?
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What is the characteristic of Type 1 Waardenburg syndrome?
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What is the main function of the G6PD enzyme in red blood cells?
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What is the reason for the lower penetrance of Hemochromatosis in women?
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What is the type of inheritance pattern of glucose-6-phosphate dehydrogenase (G6PD) deficiency?
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What is the term for the phenomenon where individuals with the same mutation exhibit different degrees of severity or different features of a condition?
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What is the factor that affects the severity of Hemochromatosis?
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What is a trigger for hemolytic anemia in G6PD deficiency?
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What is genetic compensation, also known as genetic buffering?
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What is the approximate number of people worldwide affected by G6PD deficiency?
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What is the effect of mosaicism on disease severity in genetic disorders?
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What is an example of an epigenetic mechanism that affects penetrance and expressivity?
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What is a physiological factor that can affect penetrance and expressivity of genetic disorders?
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What is the purpose of the 'Resilience Project'?
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What is the significance of the study by Chen et al. (2016) in the context of Mendelian diseases?
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What is the name of the characteristic that can cause Mediterranean G6PD deficiency?
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What is the primary methodology used in the 'Resilience Project' to identify healthy individuals resilient to Mendelian diseases?
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What is the number of genes believed to cause 584 distinct severe Mendelian childhood disorders in the 'Resilience Project'?
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What is the significance of the identification of resilient individuals in the context of Mendelian diseases?
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What is the primary goal of the bioinformatic analysis of existing genomic data in the 'Resilience Project'?
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What is the significance of the study by Tarailo-Graovac et al. (2017) in the context of Mendelian diseases?
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Study Notes
Multifactorial Disorders
- Determined by the interaction of multiple genetic and environmental factors
- Polygenic, meaning the interplay of variants in many genes, each exerting a small additive effect
- Individuals inherit an increased risk of developing a disease (susceptibility)
- Familial aggregation without a clear-cut inheritance pattern
- Include pediatric and adult disorders, such as:
- Congenital defects (e.g. cleft lip/palate, neural tube defects)
- Acquired diseases (e.g. diabetes, Crohn's disease, asthma, autism, hypertension)
Chromosomal Disorders
- Due to chromosomal abnormalities occurring during gametogenesis or following fertilization
- Numerical abnormalities:
- Polyploidy
- Aneuploidy
- Structural abnormalities:
- Deletion
- Insertion
- Inversion
- Translocation
- Common cause of infertility, miscarriage, congenital malformations, and intellectual disability
- Present in 1 in 200 live births
Genetic Counselling
- Patients or relatives, at risk of an inherited disorder, are advised of:
- The nature and consequences of the disorder
- The risks and benefits of genetic testing
- The probability of developing or transmitting the disorder
- Options for the management of the disorder
Reasons for Referral
- Preconceptional:
- Genetic disorder present or suspected in family history
- History of repeated miscarriages
- Assisted reproduction
- Prenatal:
- History of previous child with birth defect, developmental delay, or other genetic condition
- Carriers of recessive condition
- Abnormal ultrasound or maternal serum screening results
- Postnatal (infancy to adulthood):
- Newborn with multiple birth defects or positive neonatal screening results
- Child with developmental delay, sensory impairment, intellectual disability, or other suspected genetic condition
- Adult with suspected adult-onset disease
- Oncologic:
- Familial aggregation of cancer
Newborn Screening
- Purpose: early identification of diseases that cause permanent disability or death if not identified and treated early
- Biochemical assays for specific serum analytes:
- Tandem mass spectrometry
- Immunoassays
- Enzyme assays
- Identifies newborns at risk, confirmed with diagnostic molecular tests
- In Italy, 49 diseases are screened nationally, including:
- Cystic fibrosis
- Congenital hypothyroidism
- Phenylketonuria (PKU)
- Other metabolic diseases
Large-Scale Genomic Newborn Screening
- Studies launching internationally
- Cohort sizes and proposed screening approaches are shown
- TBD (to be determined)
- WGS (whole-genome sequencing)
Genetic Counselling Process
-
- Establishing the diagnosis:
- Gathering information (personal and family clinical history, even of those deceased)
- Reconstructing the pedigree (diagram showing family relationships)
- Specialist visits (e.g. ophthalmology, cardiology, neurology)
- Genetic testing (informed consent, cytogenetic and/or molecular testing)
Testing for Multiple Variants in a Single Gene
- Example: CHARGE syndrome
- Rare congenital disorder
- Clinical features: Coloboma of the eye, Heart defects, Atresia of the choanae, Retardation of growth and development, Genitourinary abnormalities, and Ear anomalies and deafness
- Causative gene: CHD7
- > 500 pathogenic variants
- Methodology: Targeted NGS of CHD7 gene
Testing for Variants in a Panel of Genes
- Example: RASopathies
- Group of developmental syndromes
- Caused by variants in genes of RAS-MAPK signalling pathway
- Clinical features: distinct facial features, developmental delays, cardiac defects, growth delays, neurologic issues, and gastrointestinal difficulties
- Methodology: NGS gene panel targeting coding sequences of 32 RAS-MAPK genes
Exome Sequencing
- Useful when:
- Clinical classification of variants
- ACMG Standards and Guidelines (2015)
- Five tiers of variant classification for mendelian diseases based on weight of evidence
Online Resources
- Online Mendelian Inheritance in Man (OMIM)
- Catalog of inherited disorders
- McKusick-Nathans Department of Genetic Medicine, Johns Hopkins Medicine (USA)
- https://www.omim.org
- ORPHANET: the portal of rare diseases and orphan drugs
- Languages: English, French, Spanish, German, Italian, Portuguese, Dutch
- http://www.orpha.net
- GeneReviews – NCBI Bookshelf
- Example: achondroplasia
- https://www.ncbi.nlm.nih.gov/books/NBK1116/
Monogenic Inheritance Patterns
- Autosomal dominant inheritance:
- Each affected person has an affected parent (vertical transmission)
- Each child of an affected parent has a 50% risk of being affected
- Not transmitted by unaffected individuals
- Males and females are equally likely to be affected
- Transmitted by either sex to offspring of either sex
- Autosomal recessive inheritance:
- Affected children born to unaffected parents
- Parents of affected individuals are asymptomatic carriers (heterozygotes)
- Each child of a mating between carrier parents has a 25% risk of being affected
- Affected individuals in a family are usually siblings (horizontal transmission)
- Males and females are equally likely to be affected
- X-linked dominant inheritance:
- Affected individuals have an affected parent (vertical transmission)
- Affected males transmit the disease phenotype to all daughters and no sons
- Both female and male offspring of affected females have a 50% risk of inheriting the phenotype
- Females are more frequently affected than males
- X-linked recessive inheritance:
- Predominantly affects males
- Affected males born to unaffected parents; mother is asymptomatic obligate carrier
- Each son of a female carrier has a 50% chance of being affected; each daughter of a female carrier has a 50% chance of being a carrier
- No male-to-male transmission of disease phenotype; all daughters of an affected male are obligate carriers
- Y-linked inheritance:
- Affects only males
- Affected males always have an affected father (vertical transmission)
- All sons of affected males are affected
- Mitochondrial/maternal inheritance:
- Affects both sexes
- Transmitted by affected mother (maternal inheritance) to all offspring (with frequent exceptions)
- Not transmitted by affected father to any of his offspring (with extremely rare exceptions)
Risk of Recurrence and Carrier Risk
- Autosomal dominant inheritance:
- Risk of disease = 50% (Aa)
- Risk of disease = 75% (homozygotes)
- Autosomal recessive inheritance:
- Risk of disease = 25% (aa)
- Carrier risk = 1/2 (a priori) or 2/3 (a posteriori)
- X-linked dominant inheritance:
- Risk of disease: daughters = 100%, sons = 0% (affected mother); daughters = 50%, sons = 50% (affected father)
- X-linked recessive inheritance:
- Risk of disease: sons = 50%, daughters = 0% (carrier mother); sons = 0%, daughters = 0% (non-carrier mother)
- Carrier risk = 100% (mother), 50% (daughter)
Consanguinity and Y-Linked Inheritance
- Consanguinity increases the risk of inheriting a recessive condition
- Apparent Y-linked inheritance:
- Due to a Y chromosome structural abnormality, not a normal Y-linked trait
Using Random Tags or Unique Molecular Identifiers (UMIs)
- UMIs decrease the number of false positives by making consensus calls from all reads with the same UMI
- UMIs can be used to remove PCR-derived duplicate reads to determine accurately the allelic ratio
Long Sequencing Reads
- Long sequencing reads improve mappability, even across difficult genomic regions such as those containing repeats
- Long reads can be used to phase mutations and generate haplotypes, to help identify the parent of origin of a mutation
Parental Origin of Germline DNMs
- >80% of DNMs are of paternal origin, arising during spermatogenesis
- DNMs increase in number with parental age – more pronounced with paternal ageing (paternal age effect)
- The sperm produced by a 20-year-old male has gone through ∼190 cell divisions (mitoses), and this number increases to ∼650 by the age of 40 years
Paternal Age Effect
- Estimated genome replications in the male germline versus number of de novo mutations (DNMs) in offspring
- >40% of patients with a severe developmental disorder have a pathogenic DNM in a protein-coding gene
- Developmental disorders caused by DNMs have an average prevalence of 1/448 to 1/213 births, increasing with parental age
Postzygotic DNMs can lead to Mosaicism
- Presence in an individual of two or more genetically different cell lines, all derived from one original zygote
- 3 SNVs estimated to arise per cell division in early human embryogenesis
- Degree of mosaicism depends on timing of DNM: high-level mosaicism present in many different tissues, including germline, or low-level mosaicism restricted to a small number of somatic cells or a single tissue
Mosaicism
- Three types of mosaicism depending on timing and cell type (somatic vs. germline):
- Somatic mosaicism: mutation occurs in somatic cells at any stage of life, cannot be transmitted to offspring
- Germline/gonadal mosaicism: mutation occurs in germline during early embryonic development, can be transmitted to offspring
- Gonosomal mosaicism: mutation occurs during early embryogenesis involving both germ cell and somatic cell lineages, can be transmitted to offspring
Detection of Mosaicism
- Difficult to detect, especially low-level mosaicism
- May be tissue-specific, may or may not be detectable in blood
- Ideally requires: (i) analysis of various tissues, even single cells, and (ii) comparison of affected vs. nonaffected tissues
- Paternal germline mosaicism quantified in sperm using NGS and droplet digital PCR, expressed as variant allele frequency (VAF)
Detection of Mosaicism by NGS
- NGS can detect mosaic variants using reads sorted by base, with positive reads grouped together
- Mosaic variants are located within parallel vertical lines and are indicated by the letter of the substituted nucleotide
Detection of Mosaicism by Droplet Digital PCR (ddPCR)
- ddPCR technology uses microfluidics and surfactant chemistries to divide samples into thousands of water-in-oil droplets to run multiple PCR amplifications in parallel
- ddPCR uses a combination of modification of the Taqman qPCR assay using allele-specific fluorescently-labelled probes
Taqman Assay
- Real-time quantitative PCR assay
- TaqMan probes consist of sequence-specific oligonucleotides with a reporter (R) fluorophore covalently attached to the 5’-end and a quencher (Q) at the 3’-end
- During PCR amplification, the exonuclease activity of Taq polymerase cleaves the annealed probe, releasing the fluorophore which will no longer be subject to quenching → increase in fluorescence signal proportional to quantity of template DNA
ddPCR Workflow
- A mixture of PCR reagents including fluorescent probes, primers, and genomic DNA is partitioned into picoliter to nanoliter droplets where theoretically less than one DNA molecule is distributed in one droplet
- Each droplet represents a microreactor for PCR amplification using the Taqman assay
- The fluorescence intensity of each individual droplet is detected and analysed
Complications of Mendelian Inheritance Patterns
- Incomplete penetrance: when individuals with a disease-causing genotype never develop any clinical signs or symptoms
- Variable expressivity: affected individuals within a pedigree show different degrees of severity or different features of the condition
- Genotype-phenotype correlation: clinically useful to guide diagnosis, prognosis, and management of genetic disorders, but utility is hampered by heterogeneity in phenotypic expression
Incomplete Penetrance
- Definition: probability that a genotype manifests itself in a given phenotype
- Example: split hand/foot malformation I (SHFM1) - penetrance 70%
- May cause occasional skipping of generations in dominant inheritance
Variable Expressivity
- Example: Type 1 Waardenburg syndrome - congenital hearing loss, pigment disturbances of iris, hair, and skin; different features of syndrome despite having the same mutation in PAX3
- Factors affecting penetrance and expressivity: physiological factors (age, sex, hormonal factors), genetic mechanisms (repeat expansion, mosaicism, genetic modifiers, genetic compensation), epigenetic mechanisms, and environmental factors
Factors Affecting Penetrance and Expressivity
- Physiological factors: age, sex, hormonal factors (e.g., hemochromatosis - adult onset, increasing severity with age, variable expressivity between individuals with same genotype)
- Sex-limited phenotypes: defect is autosomally transmitted but expressed in only one sex (e.g., familial male limited precocious puberty - AD inheritance, affected boys develop secondary sexual characteristics in early childhood)
Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency
- Inborn error of metabolism, most common hereditary enzyme defect (400 million people worldwide)
- X-linked recessive inheritance
- G6PD enzyme participates in pentose phosphate pathway to generate NADPH, critical role in RBCs for detoxification of reactive oxygen species (ROS)
- Most subjects with G6PD deficiency are asymptomatic, but may develop hemolytic anemia when exposed to oxidative stress
- Triggers for hemolytic anemia: bacterial and viral infections, fava beans and other legumes containing toxic alkaloid glycosides (favism)
Data Mining of Genome Databases
- Identifies healthy individuals with "pathogenic" genotypes for Mendelian disorders
- "Resilience Project" aims to identify healthy individuals resilient to highly penetrant forms of genetic childhood disorders
- Bioinformatic analysis of existing genomic data from 12 cohorts worldwide
- Results: 13 adults with variants for 8 severe Mendelian conditions, with no reported clinical manifestation of the indicated disease
- Conclusion: incomplete penetrance for Mendelian diseases is likely more common than previously believed
- Identification of resilient individuals may provide a first step toward uncovering protective genetic variants that could help elucidate the mechanisms of Mendelian diseases and new therapeutic strategies
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Description
Learn about complex disorders caused by the interaction of multiple genetic and environmental factors, including polygenic inheritance and susceptibility to disease. This quiz covers pediatric and adult disorders.