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Genetics, Mosaicism
Introduction
Genetic mosaicism is characterized by the presence of 2 or more genetically distinct cell populations within an individual, all originating from a single zygote. Mosaicism results from postzygotic genetic alterations that occur after fertilization. In contrast, chimerism arises from the coexistence of cell populations derived from 2 or more distinct zygotes.
The human body undergoes an estimated 1016 cell divisions during a lifetime. Because each mitotic event carries a risk of genetic error, virtually all individuals harbor genetically distinct cell populations and are therefore mosaic to some degree. Although most postzygotic mutations are clinically silent or are eliminated by mechanisms such as apoptosis, some persist and contribute to disease. Somatic mosaicism has been implicated in a wide range of human pathologies, including chromosomal disorders such as Turner Syndrome and the development of benign and malignant neoplasms, which are themselves manifestations of mosaicism.[1][2][3][4][5]
Development
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Development
Human development begins with the formation of a zygote through the fusion of a haploid sperm and ovum, each containing 23 chromosomes. As the zygote undergoes repeated mitotic divisions to generate the cells of the body, postzygotic genetic alterations may occur. Consequently, mosaicism can develop at any stage after zygote formation, resulting in 2 or more genetically distinct cell populations within a single individual.[1][2]
Mosaicism is broadly classified into 2 categories:
- Somatic mosaicism: This form occurs when 2 or more genetically distinct cell lineages are present within somatic tissues. Because germ cells (sperm and oocytes) are generally unaffected, somatic mosaicism is not typically transmitted to offspring. However, recent evidence suggests that somatic mosaicism arising during the preimplantation stage of embryonic development may involve both somatic and germline cell lineages, creating the potential for transmission to future generations.[3][4]
- Germline mosaicism: This form occurs when 2 or more genetically distinct cell lineages are present within germ cells. Unlike somatic mosaicism, germline mosaicism can be transmitted to offspring. Individuals with germline mosaicism may be clinically unaffected if the pathogenic variant is confined to germ cells; however, the mutation may be inherited by offspring, who may then manifest the associated genetic disorder.[5][6][7]
Constitutional mosaicism occurs during the preimplantation or embryonic stages of development and becomes an integral part of the organism. In contrast, somatic mosaicism results from postzygotic genetic alterations that occur after embryogenesis and is generally restricted to specific tissues or cell populations.[8][9] Constitutional mosaicism results from postzygotic errors in chromosome segregation during mitotic cell division early in embryonic development.[10]
Mosaic embryos may be classified into 3 categories (Image 1):
- Aneuploid mosaic: Aneuploid mosaicism consists of multiple aneuploid cell lines within the same embryo, with all affected cell populations exhibiting abnormal chromosome numbers.[11] IIn chromosomal aneuploid mosaicism, cells may lack an entire chromosome (monosomy; 45 chromosomes) or contain an extra chromosome (trisomy; 47 chromosomes).[12][13]
- Euploid-aneuploid mosaic: Euploid-aneuploid mosaicism is characterized by the coexistence of euploid and aneuploid cell populations within the same embryo. Some cells contain the normal complement of 46 chromosomes, whereas others are aneuploid (eg, 45 or 47 chromosomes). Clinical severity is influenced by the proportion and distribution of euploid and aneuploid cells. This form of mosaicism is relatively common and is associated with a greater likelihood of embryonic survival.[6][14]
- Ploidy mosaics: Ploidy mosaicism, also known as mixoploidy, involves the presence of cell populations with different multiples of the haploid chromosome number.[15] In these cases, an additional complete set of chromosomes is present in affected cells, as seen in triploidy (69 chromosomes) or tetraploidy (92 chromosomes). Embryos with predominantly triploid or tetraploid cell populations are generally nonviable. However, triploid and tetraploid cells may occur physiologically in certain tissues, including the liver and bone marrow.[16][17][15]
The distribution of mosaicism and its phenotypic manifestations depend largely on the timing of the mutation during embryonic development. If a mutation occurs during the first mitotic division after fertilization, approximately half of the individual's cells will carry the mutation, potentially producing a distinct midline demarcation between affected and unaffected tissues. Although the precise timing of left-right axis separation in humans remains uncertain, mutations arising after this event are more likely to be confined to one side of the body and less likely to cross the midline.
Mutations that occur before primordial germ cell differentiation (approximately within the first 15 mitotic divisions) may be present in both somatic and germline tissues. In contrast, mutations arising after this stage are generally restricted to either somatic or germline cell lineages. Consequently, estimating the timing of a mutation can help predict the proportion and distribution of mutant cells and may provide insight into the risk of transmitting the mutation to future offspring.[18]
Cellular
Based on the distribution of affected cells, mosaicism can be classified into 2 broad categories:
- Generalized mosaicism: In generalized mosaicism, 2 or more genetically distinct cell populations are distributed throughout the organism. This form of mosaicism typically results from a mutation occurring before cellular differentiation, allowing the abnormal cell line to contribute to multiple tissues and organ systems. Generalized mosaicism may also arise from mutations present in paternal or maternal germ cells that are transmitted to the zygote.[13][19][20]
- Confined mosaicism: In confined mosaicism, genetically distinct cell populations are restricted to specific tissues, organs, or regions of the body rather than being distributed throughout the organism. Examples include mosaicism limited to the brain, heart, or liver. A specialized form, confined placental mosaicism, occurs when chromosomal abnormalities are present in placental tissues but absent from the fetus and may arise during early embryonic development or placental growth.[21][22]
The consequences of mosaicism depend on the specific genetic alteration involved. Genetically distinct cell populations may be phenotypically indistinguishable when the variant does not alter gene expression or cellular function. Such cases may occur when the mutation affects a noncoding genomic region or when a recessive variant is not expressed in the presence of a normal allele.[23][24]
Mosaicism arises from postzygotic genetic alterations, including single-nucleotide variants, chromosomal abnormalities, and copy number variants, that result in genetically distinct cell populations within an individual. The extent and clinical impact of mosaicism depend on the developmental timing of the mutation and the proportion and distribution of affected cells. Mosaicism may be limited to somatic tissues, confined to the germline, or involve both somatic and germline lineages. In some cases, germline mosaicism can result in transmission of the mutation to offspring despite the absence of clinical manifestations in the parent.[4][25]
The phenotypic expression of mosaicism depends on the proportion and distribution of affected cells. Clinical manifestations may arise during intrauterine development, at birth, or later in life, depending on the specific genetic alteration and other modifying factors.[26][27]
Biochemical
Mosaicism may result from gene-level mutations arising through the following mechanisms:
- Single-nucleotide variants
- Small insertions or deletions
- Trinucleotide repeat expansions and contractions
- Autonomous mobile element insertions [18]
Chromosomal and gene-level mutations may alter the biochemical activity of affected cells by changing the quantity, structure, or function of expressed gene products. For example, expression of myelin basic protein in the optic nerve and spinal cord is altered in heterozygous female mice carrying the rumpshaker mutation, an X-linked genetic defect.[28] Similarly, chromosomal abnormalities such as tetrasomy 3q26.32-q29 have been associated with cutaneous hyperpigmentation.[29] In X-linked Agammaglobulinemia, affected individuals fail to develop mature B lymphocytes, resulting in an absence or severe deficiency of plasma cells and markedly reduced antibody production.[30]
Some studies have reported an association between mosaic Trisomy 13 and hypomelanosis of Ito, a condition characterized by localized areas of hypopigmentation resulting from abnormal melanocyte function or distribution.[31]
Molecular Level
The early embryo is particularly vulnerable to mitotic errors because embryonic genome activation occurs only after the first several cell divisions. During this period, development depends primarily on maternally derived transcripts and proteins within the oocyte. Delayed expression of genes involved in cell division and chromosome segregation contributes to the high frequency of chromosomal mosaicism observed during early embryogenesis. Accordingly, mosaicism has been reported in up to 70% of cleavage-stage embryos and 90% of blastocyst-stage embryos generated through in vitro fertilization.[32][13]
Function
The functional consequences of mosaicism depend largely on the proportion, distribution, and type of affected cells. Clinical manifestations may vary considerably, as illustrated by the following examples:
- Mosaic loss of the Y chromosome in hematopoietic cells has been associated with increased morbidity and mortality in older adults.[33]
- Mosaic Klinefelter Syndrome (46,XY/47,XXY) is associated with reduced testicular volume and impaired testosterone production.[34]
- In sporadic retinoblastoma, both the degree and distribution of mosaicism influence disease severity and age at onset.[35]
- Mosaic variants in the SMC1A gene have been identified in buccal mucosal cells of individuals with clinically diagnosed Cornelia de Lange Syndrome.[36]
In some cases, mosaicism has little or no functional consequence. This may occur when the mutation is phenotypically silent, affects a recessive allele, or is present in too few cells to produce a detectable clinical effect. A classic example involves X-linked disorders.
In X-linked conditions, the clinical phenotype may be modified by mosaicism. For example, pathogenic variants in the MECP2 gene, which cause Rett Syndrome, are typically lethal in hemizygous males because they possess only a single X chromosome. However, if a male is mosaic for a pathogenic MECP2 variant, a proportion of cells retain a normal copy of the gene. The presence of these unaffected cells may mitigate disease severity and increase the likelihood of survival.[37]
Some evidence suggests that mosaicism may serve as a protective mechanism, allowing survival in the presence of chromosomal abnormalities that would otherwise be incompatible with life. Only a limited number of full autosomal trisomies, most notably trisomies 13, 18, and 21, are compatible with live birth. In contrast, a broader spectrum of chromosomal abnormalities has been documented in the mosaic state. Reported examples include mosaic trisomies involving chromosomes 8 (Warkany syndrome), 14, 16, and 17. Likewise, complete autosomal monosomies are generally considered incompatible with human survival, with only rare reports of mosaic monosomy 21. Another notable example is mosaic tetrasomy 12p, which causes Pallister-Killian Syndrome. In such conditions, survival is thought to be possible because a sufficient proportion of cells retain a normal chromosomal complement, thereby mitigating the effects of the chromosomal abnormality.[38][18][39]
Mechanism
Somatic mosaicism may arise from a wide range of genetic alterations, including chromosomal abnormalities, copy number variants, and single-nucleotide mutations. Mosaic aneuploidies are generated primarily through 2 mechanisms (see Image 2): postzygotic mitotic nondisjunction and postzygotic trisomy rescue following meiotic nondisjunction. The latter mechanism represents an attempt to restore a normal chromosomal complement through the loss of a supernumerary chromosome, a process known as chromosomal rescue. Although chromosomal rescue may reestablish euploidy, it can also result in uniparental disomy, which may disrupt genomic imprinting and increase the risk of homozygosity for pathogenic recessive variants.[38][18][40] Additional mechanisms that can give rise to mosaicism include anaphase lag and endoreduplication.[13]
Testing
The detection of embryonic and fetal mosaicism has become increasingly feasible through prenatal diagnostic testing. The most commonly used invasive prenatal diagnostic procedures are chorionic villus sampling and amniocentesis, each with distinct advantages and limitations.[8]
- Amniocentesis: Amniotic fluid is aspirated from the uterine cavity, typically at or after 15 weeks of gestation, for cytogenetic and molecular genetic analysis. Amniocentesis performed before 15 weeks of gestation is associated with an increased risk of fetal complications and is generally not recommended.[41][42]
- Chorionic villus sampling: Chorionic villus tissue is obtained for genetic analysis, typically beginning at approximately 10 weeks of gestation. Because the sampled tissue is of placental origin, results may occasionally reflect confined placental mosaicism rather than the fetal genotype.[41]
Neither procedure directly samples embryonic or fetal tissue; instead, genetic analysis is performed on amniotic fluid cells or placental tissue obtained through chorionic villus sampling.
Multiple laboratory techniques have been developed to detect mosaicism and other chromosomal abnormalities.
Karyotyping
Karyotyping is a conventional cytogenetic technique that can identify different chromosomal complements within a single individual. It remains widely used in clinical practice and enables detection of numerical and large structural chromosomal abnormalities, including aneuploidies, translocations, and large deletions or duplications, typically greater than 5 Mb. However, its resolution is limited, and low-level mosaicism may be difficult to detect.[43]
Fluorescent In Situ Hybridization
Fluorescence in situ hybridization (FISH) enables targeted analysis of specific chromosomal regions and can detect smaller copy number changes than conventional karyotyping, often at resolutions approaching 50 kb. Because FISH can be performed on interphase nuclei, it allows evaluation of large numbers of cells and may improve the detection of mosaicism. Nevertheless, accurate identification of low-level mosaicism often requires analysis of a substantial number of cells.[44][45]
Sanger Sequencing
Sanger sequencing permits nucleotide-level analysis and remains useful for targeted evaluation of specific genes and variants. However, because it is limited to relatively small genomic regions, it has largely been supplanted by high-throughput genomic technologies that can simultaneously assess large portions of the genome.[38][18][46]
Chromosomal Microarray
Chromosomal microarray (CMA) detects copy number variations across the genome without requiring cell culture. CMA includes both comparative genomic hybridization (CGH) arrays and single-nucleotide polymorphism (SNP) arrays. Compared with conventional cytogenetic techniques, CMA offers improved sensitivity for mosaicism, with reported detection thresholds of approximately 10% to 20% for CGH arrays and as low as 5% for SNP arrays.[43][47][48]
Next-Generation Sequencing
Next-generation sequencing (NGS) provides high-resolution, genome-wide analysis and can identify single-nucleotide variants, small insertions and deletions, copy-number alterations, and other genomic abnormalities. Its high sequencing depth enables detection of low-level mosaicism that may be missed by other methods. NGS is particularly useful for identifying nucleotide-level variants and has become an important tool in clinical genetics, assisted reproductive technology, and preimplantation genetic testing.[38][49]
Pathophysiology
Gene-level and chromosomal alterations may result in genomic instability, producing genetically distinct cell populations within the same individual. Mosaic chromosomal abnormalities can lead to altered cellular function through changes in gene dosage and gene expression, resulting in abnormal protein production and diverse clinical phenotypes.[50]
The clinical consequences of mosaicism are illustrated by several examples:
- Somatic chromosomal abnormalities may influence expression of the amyloid precursor protein and have been implicated in neurodegenerative disorders such as Alzheimer Disease.[51]
- In Down Syndrome, individuals with mosaic trisomy 21 generally exhibit milder cognitive impairment than those with nonmosaic trisomy 21, reflecting the presence of both trisomic (47,XX,+21 or 47,XY,+21) and euploid (46,XX or 46,XY) cell populations.[52]
- In mosaic Turner Syndrome, loss of an X chromosome during early embryonic cell division results in both 45,X and 46,XX cell lines. Individuals with mosaic Turner syndrome often have a milder phenotype than those with complete monosomy X.[53]
The biological consequences of somatic mosaicism can be conceptualized as a sequence of events:
- A postzygotic genetic alteration occurs during mitosis.
- Two or more genetically distinct cell populations develop.
- Cellular function may be altered through changes in gene expression, protein production, or hormone synthesis.
- If a sufficient proportion of cells is affected, tissue and organ function may become impaired.
- Clinical manifestations emerge, with severity depending on the distribution and proportion of affected cells.
- In some cases, expansion of the mutant cell population may contribute to neoplastic transformation and malignancy.[54][55][56][57][58][59][6]
The consequences of germline mosaicism differ because the mutation is confined to germ cells:
- A genetic alteration arises in a subset of germ cells during gametogenesis.
- The affected individual is often clinically unaffected because somatic tissues do not carry the mutation.
- An affected germ cell participates in fertilization, transmitting the variant to the offspring.
- The offspring may inherit the mutation in all cells or as a mosaic state, depending on the timing and mechanism of the genetic alteration.
- Clinical manifestations in the offspring depend on the specific mutation and the proportion and distribution of affected cells.[60][61][62][6][63]
Clinical Significance
Mosaicism has important implications for genetic counseling, particularly when identified during prenatal testing.[64] Because chromosomal nondisjunction occurs more frequently during oogenesis than spermatogenesis, counseling should consider the origin and type of chromosomal abnormality. Although the recurrence risk associated with mosaicism is often difficult to predict, germline (gonadal) mosaicism should be considered when a child is affected by a seemingly de novo genetic disorder or chromosomal abnormality. Germline mosaicism may explain familial recurrence of rare pathogenic variants despite unaffected parents.[65] Because the clinical consequences vary according to the specific chromosome or gene involved, genetic counseling should be individualized based on the nature and extent of the mosaicism.[8]
Mosaicism can influence survival in chromosomal disorders that are typically incompatible with life. Although complete autosomal monosomies are generally lethal in humans, mosaic forms may be compatible with survival when a sufficient proportion of cells retain a normal chromosomal complement.[66][67]
Mosaicism has also been implicated in oncogenesis. Age-related accumulation of somatic genetic alterations contributes to tissue mosaicism and may increase the risk of malignant transformation in susceptible cell populations.[56][57]
Several single-gene disorders demonstrate clinically significant mosaicism:
- Hereditary tyrosinemia type 1: Pathogenic variants in the FAH gene cause hereditary tyrosinemia type 1. Somatic mosaicism may occur in hepatocytes, resulting in populations of both affected and genetically corrected cells.[68][64]
- Bloom syndrome: Pathogenic variants in the BLM gene, which encodes a DNA helicase involved in DNA replication and repair, cause Bloom syndrome, a disorder characterized by growth impairment, immunodeficiency, genomic instability, and increased cancer risk.[69][70]
- Duchenne muscular dystrophy: Mosaicism involving pathogenic variants in the dystrophin gene has been reported in affected individuals and carrier mothers.[71]
Mosaicism is also an important consideration in assisted reproductive technologies. Embryonic mosaicism is relatively common in embryos generated through in vitro fertilization, and many fertility centers incorporate preimplantation genetic testing to identify chromosomal abnormalities and reduce the risk of transmitting genetic disorders.[6][49][19]
Media
(Click Image to Enlarge)
Image showing inheritance of X-linked dominant mutation from an affected father, The sons of a man with an X-linked dominant disorder will not be affected, but his daughters will all inherit the condition Contributed by National Institute of Health ( http://ghr.nlm.nih.gov/handbook/illustrations/xlinkdominantfather )
(Click Image to Enlarge)
Image 1: Pedigree of a Prevalent Autosomal Recessive Disease Contributed by Chishti, Muhammad S et al. “Splice-site mutations in the TRIC gene underlie autosomal recessive nonsyndromic hearing impairment in Pakistani families.” Journal of human genetics vol. 53,2 (2008): 101-5. doi:10.1007/s10038-007-0209-3 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2757049/)
(Click Image to Enlarge)
Types of Mosaicism Contributed by Dr. Pradip Chauhan. MS, P.D.U. Government Medical College, Rajkot, Gujarat, India
(Click Image to Enlarge)
Mechanism of Mosaicism (Image 2a: Shows mosaicism due to disconjugation, Image 2b: Shows mosaicism due to chromosomal rescue mechanism) Contributed by Dr. Pradip Chauhan, MS (Anatomy); P.D.U. Government Medical College, Rajkot, India.
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