Somatic mosaicism -- the presence of genetically distinct populations of somatic cells in a given organism -- is frequently masked, but it can also result in major phenotypic changes and reveal the expression of otherwise lethal genetic mutations. Mosaicism can be caused by DNA mutations, epigenetic alterations of DNA, chromosomal abnormalities and the spontaneous reversion of inherited mutations. In this review, we discuss the human disorders that result from somatic mosaicism, as well as the molecular genetic mechanisms by which they arise. Specifically, we emphasize the role of selection in the phenotypic manifestations of mosaicism.

Mechanisms and consequences of somatic mosaicism in humans

Timely molecular diagnosis of RB1 mutations enables earlier treatment, lower risk, and better health outcomes for patients with retinoblastoma; empowers families to make informed family-planning decisions; and costs less than conventional surveillance. However, complexity has hindered clinical implementation of molecular diagnosis. The majority of RB1 mutations are unique and distributed throughout the RB1 gene, with no real hot spots. We devised a sensitive and efficient strategy to identify RB1 mutations that combines quantitative multiplex polymerase chain reaction (QM-PCR), double-exon sequencing, and promoter-targeted methylation-sensitive PCR. Optimization of test order by stochastic dynamic programming and the development of allele-specific PCR for four recurrent point mutations decreased the estimated turnaround time to <3 wk and decreased direct costs by one-third. The multistep method reported here detected 89% (199/224) of mutations in bilaterally affected probands and both mutant alleles in 84% (112/134) of tumors from unilaterally affected probands. For 23 of 27 exons and the promoter region, QM-PCR was a highly accurate measure of deletions and insertions (accuracy 95%). By revealing those family members who did not carry the mutation found in the related proband, molecular analysis enabled 97 at-risk children from 20 representative families to avoid 313 surveillance examinations under anesthetic and 852 clinic visits. The average savings in direct costs from clinical examinations avoided by children in these families substantially exceeded the cost of molecular testing. Moreover, health care savings continue to accrue, as children in succeeding generations avoid unnecessary repeated anaesthetics and examinations.

Sensitive and Efficient Detection of RB1 Gene Mutations Enhances Care for Families with Retinoblastoma

Somatic mosaicism is well known in disorders where the manifestations are readily seen, e.g. the skin in neurofibromatosis I. In single gene disorders of higher frequency, especially X-linked ones, the frequency of combined germ-line and somatic mosaicism is increasingly being appreciated, e.g. Duchenne Muscular Dystrophy. Cell separation techniques; such as the fluorescence-activated cell sorter (FACS) also detect much somatic mosaicism among blood cells in disorders such as paroxysmal nocturnal hemoglobinuria. Depending on the disorder and the class of mutation, in genes for which there are sufficient numbers of patients studied, 6-20% of cases are due to somatic mutation. This update of my previous review is stimulated by the rapid application of new technologies for the study of DNA variation in disease. The results of these studies implicate somatic mutation in a greater variety of genetic diseases and a wider spectrum of tissues than have previously been shown, including heart and kidney. The classes of mutation have also expanded beyond base pair changes, insertions/deletion (indels), and short tandem repeat mutations to include copy number variants and transposon-mediated mutations. I also briefly discuss previously well-known mosaicism for chromosomal mutations. Genomic sequencing, performed on DNA from blood, shows many mutations which are conclusively somatic in origin. It is still too early to see if there is a different pattern of somatic mutation compared to germ-line mutation. Though the parameters to allow careful quantification are not yet available, it seems that the frequency of gene mutation in embryonic cells is not markedly different than that in the germ line.

Somatic gene mutation and human disease other than cancer: An update

There are increasing reports of multiple different types of somatic mosaicism detected in patients with inherited and non-inherited disorders. The characteristics of several of the major types of mosaicism will be outlined, and contrasted with somatic mosaicism, which is the focus of this article. This review examines examples of somatic mosaicism due to differences in DNA sequence arising from in vivo site specific reversion to normal of inherited mutations in humans. While several known mechanisms of reversion are evident in a number of these examples, they are not in some others. The possible significance of the role of selection, particularly in view of recent results of gene therapy, is discussed.

https://jmg.bmj.com/content/jmedgenet/40/10/721.full.pdf

In vivo reversion to normal of inherited mutations in humans

Mutations in both alleles of the RB1 gene are causal for the development of retinoblastoma, a childhood tumor of the eye. The spectrum of somatic and germline mutations in this gene is dominated by small mutations. Data on small mutations are listed in a locus specific database available at http://www.d-lohmann.de/Rb/mutations.html. Analysis of 368 reported small mutations reveals considerable heterogeneity. A notable recurrence of transitions is observed at 13 CpG-dinucleotides that are part of CGA codons or splice donor sites. Most mutations create a premature termination codon. With few exceptions, patients heterozygous for mutations of this kind develop bilateral retinoblastoma. Missense mutations and inframe deletions are rare. Some of these mutations are associated with a distinct phenotype marked by incomplete penetrance and reduced expressivity.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/%28SICI%291098-1004%28199910%2914%3A4%3C283%3A%3AAID-HUMU2%3E3.0.CO%3B2-J

RB1 gene mutations in retinoblastoma

Although mosaicism can have important implications for genetic counseling of families with hereditary disorders, information regarding the incidence of mosaicism is available for only a few genetic diseases. Here we describe an evaluation of 156 families with retinoblastoma; the initial oncogenic mutation in the retinoblastoma gene had been identified in these families. In 15 ( approximately 10%) families, we were able to document mosaicism for the initial mutation in the retinoblastoma gene, either in the proband or in one of the proband's parents. The true incidence of mosaicism in this group of 156 families is probably higher than our findings indicate; in some additional families beyond the 15 we identified, mosaicism was likely but could not be proven, because somatic or germ-line DNA from key family members was unavailable. Germ-line DNA from two mosaic fathers was analyzed: in one of these, the mutation was detected in both sperm and leukocyte DNA; in the other, the mutation was detected only in sperm DNA. Our data suggest that mosaicism is more common than is generally appreciated, especially in disorders such as retinoblastoma, in which a high proportion of cases represent new mutations. The possibility of mosaicism should always be considered during the genetic counseling of newly identified families with retinoblastoma. As demonstrated here, genetic tests of germ-line DNA can provide valuable information that is not available through analysis of somatic (leukocyte) DNA.

Rebecca E. Fraioli

Papers

Frequency of Somatic and Germ-Line Mosaicism in Retinoblastoma: Implications for Genetic Counseling