Clinical Cytogenetics and Testing for Developmental Disabilities
Merlin G. Butler, F. John Meaney in Genetics of Developmental Disabilities, 2019
Individuals occasionally have more than one cell line (one normal and one abnormal) present in their tissues. If one cell line is derived from the other, the individual is referred to as a mosaic. If the cell lines are derived from different zygotes (i.e., 46,XX/46,XY), the individual is referred to as a chimera. In most instances of mosaicism, the abnormal cell line has a numerical abnormality (i.e., 46,XX/47,XX,+8).Mosaicism is rare for structural abnormalities. The degree to which a mosaic individual is clinically affected usually depends upon the tissues that are affected and to a lesser degree on the frequency of abnormal cells within a specific tissue. A routine cytogenetic analysis of 20 cells in a single tissue sample typically excludes ~20% mosaicism with 99% confidence (41).
Understanding genetics and genetic tests
David M. Luesley, Mark D. Kilby in Obstetrics & Gynaecology, 2016
Another occurrence to be familiar with is that of chromosomal mosaicism. Chromosomal mosaicism refers to the presence of two or more cell lines in an individual, which differ from each other in chromosome number or structure. Although the individual has developed from a single fertilised egg, the two cell lines means that they have two different genotypes. Mosaicism commonly results from errors in cell division during mitosis, but other explanations exist. Mosaicism in chromosomal disorders is well known. For example, about 1 percent of patients with trisomy 21 mosaic for cell lines with two copies of chromosome 21 and cell lines with three copies of chromosome 21 (Fig. 32.2).
Genetic counselling in Mendelian disorders
Angus Clarke, Alex Murray, Julian Sampson in Harper's Practical Genetic Counselling, 2019
Mosaicism denotes the occurrence in a single individual of cells or tissue of more than one genetic constitution. The term is confined to differences that have originated after fertilisation (the term chimaera is used for those rare situations where a single individual originates from more than one fertilised egg). Mosaicism results from mutations occurring in early development so that a significant part of the body cells or germ line (or both) carries the mutation. Visible chromosomal mosaicism has been recognised for many years (see Chapter 4), but only since the development of molecular analysis has the importance of its occurrence for other types of mutation been demonstrated.
Chromosomal microarray analysis detects trisomy 9 mosaicism in a prenatal case not revealed by conventional cytogenetic analysis of cord blood
Published in Journal of Obstetrics and Gynaecology, 2019
Hai-Shen Tang, De-Gang Wang, Lv-Yin Huang, Dong-Zhi Li
Chromosomal mosaicism is a biological phenomenon that is defined by the presence of two or more chromosomally different cell lines in an individual. In cytogenetic prenatal diagnosis, it mainly involves full aneuploidies or, more rarely, structural rearrangements (Grati et al. 2017). The standard cytogenetic technique, or karyotyping, is able to demonstrate the distribution of the abnormal cell lines and the chromosome morphology. This data has important practical implications for counselling management. New methods have recently become available to detect mosaicism. In particular, the chromosomal microarrays (CMA) technique can bypass the need for culturing and the results can be given in a timely manner. This approach has the advantage of detecting the microdeletions/microduplications and low-level mosaicism that are usually beyond the ability of traditional karyotyping (Filges et al. 2011; Karampetsou et al. 2014). We hereby report a prenatal case of trisomy 9 mosaicism in which routine karyotyping and CMA have discordant results based on the testing of foetal blood.
Neuropsychological and Social Characteristics of a 7 Year Old Child with Hypomelanosis of Ito Followed for 11 Years
Published in Developmental Neuropsychology, 2022
George P. Prigatano, Alexandra Novak, Vinodh Narayanan
Hypomelanosis of Ito (HI) “is a neurocutaneous disorder with hypopigmented lesions following the lines of Blaschko” (Ream, 2015, p. 281). The single genetic etiology of this condition has not been defined, but there has been an association with chromosomal mosaicism. “Mosaicism occurs when there are two different populations of cells during embryologic development, often due to mutation or chromosomal nondisjunction in one cell line” (Ream, 2015, p. 286). While skin hypopigmentation is the most prominent feature of HI, this clinical condition is often associated with a variety of musculoskeletal and central nervous system abnormalities. While the neuropsychological and neurobehavioral symptoms of these children can be variable (e.g., attentional disturbances, autism, Asperger’s syndrome, developmental delays, and poorly defined learning disorders (Kumar, Radhakrishnan, Chowdhary, & Giampietro, 2001; Pavone, Praticò, Ruggieri, & Falsaperla, 2015)), delays in speech production and reduced intellectual ability are commonly reported (Ruggieri & Pavone, 2000). Esquivel, Pitt, and Boyd (1991) have also reported that many children with HI have seizures during early childhood without a common pattern of abnormal rhythmic EEG findings.
Clinical diagnosis of presumed SOX2 gonadosomatic mosaicism
Published in Ophthalmic Genetics, 2021
Malena Daich Varela, Robert B. Hufnagel, Bin Guan, Delphine Blain, Julie C. Sapp, Andrea L. Gropman, Ramakrishna Alur, Jennifer J. Johnston, Leslie G. Biesecker, Brian P. Brooks
Mosaicism refers to a phenomenon in which a mutational event occurs post-conceptually, resulting in two or more different cell populations within the same individual (20). Depending on the timing of the post-zygotic mutation, mutant cells can be observed in different patterns. If the mutation occurs before left-right differentiation (in human, exact date unknown, but likely before five weeks of gestation; in mouse, around day 8 post-fertilization) (21), mutant cells populate both sides of the embryo, including the gonads. Mutations that happen after this point are typically unilateral and may show a striking respect for the midline when manifest on the skin. Another embryologic milestone is the differentiation of the primordial germ cells, which occurs by the 24th embryonic day in humans. If a mutation occurs after this point, it usually affects either the somatic or germinal lineage, not both. If the mosaicism hypothesis were correct in the family described here, the maternal variant in SOX2 must have occurred before the differentiation of the primordial germ cells (gonads and eyes affected), and even before left-right differentiation, (both eyes involved), but after or as a cause of the twinning event (only one twin affected) (22).
Related Knowledge Centers
- Chimera
- Egg Cell
- Endoreduplication
- Multicellular Organism
- Mutation
- Genotype
- NONdisjunction
- Lineage
- Fertilisation
- Anaphase Lag