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Somatic mutations and genome mosaicism in aging and disease.

Age-related genome mosaicism is an inherent feature of multicellularity and genomic instability. It occurs because of DNA mutations, the accumulation of which leads to diverse genomic landscapes across different tissues. DNA mutations in the genome are consequences of DNA damage, changes in the chemical structure of DNA, such as strand breaks or loss of bases. DNA damage is very frequent and normally repaired quickly. However, errors intrinsic to DNA repair or replication can give rise to permanent changes in genome sequence information. Such DNA mutations are diverse and include single-nucleotide variants, small insertions and deletions, and larger genome structural variants. Since the 1950s, somatic mutations have been proposed to be a major cause of aging. Indeed, somatic mutations are the cause of cancer, the risk of which increases exponentially with age, and possibly other age-related diseases, such as neurodegenerative diseases and cardiomyopathies. Somatic mutations vary from cell to cell owing to the innate stochasticity of their occurrence, from error-prone processing of randomly inflicted DNA damage. With the emergence of single-cell and single-molecule sequencing, it has become possible to quantitatively analyze somatic mutations in human cells and tissues. Here, we discuss a possible causal relationship between mutation-driven mosaicism of the somatic genome and aging-related functional decline and disease by exploring several predictions of the somatic mutation theory of aging.

Humans↗

Survey of diagnostic laboratories highlights need for improved standards in somatic genomic testing and reporting.

There is a growing international need to support somatic genomic testing, standardised variant curation and improved patient access to molecular profiling for somatic conditions, including cancer. We conducted a survey of scope, curation, reporting and sharing practices of diagnostic laboratories performing somatic testing in Australia and New Zealand. Laboratories with accreditation (n = 41) were invited in 2023 to complete a semi-structured, 25-question interview. Responses were received for 27 laboratories (66% response rate) offering solid tumour, haematological malignancy and non-cancer services. Only 36% of laboratories offered tests capturing the full breadth of variants, from single-nucleotide variants to gene fusions. Knowledge sharing was rare, with only one laboratory submitting variant classifications to a public knowledge base. Most laboratories (96%) conducted somatic testing in oncology. Of cancer laboratories, 35% offered testing considered capable of comprehensive genomic profiling (CGP). Almost half of cancer laboratories had already adopted the 2022 ClinGen/CGC/VICC oncogenicity guidelines, and 84% were using AMP/ASCO/CAP 2017 clinical significance guidelines. Only 47% of mixed discipline cancer laboratories reported biomarkers such as tumour mutational burden, with wide variation in reporting of matched therapy options. Our study has generated a unique overview of somatic laboratory practices in the region, and areas for global standardisation in somatic molecular testing and reporting. We also provide a model for practice and guideline uptake assessment, for application by other country-wide networks. This is particularly relevant in anticipation of CGP mainstreaming, with the increasing complexity of sequencing interpretation for laboratories and clinicians.

Humans↗

Co-existence of somatic hypermutation and gene conversion in hypervariable regions of single Igkappa clones.

In the rabbit, recent investigations have provided evidence that gene conversion leads to the generation of diversity of heavy chain rearranged VH-DH-JH genes. No data have been published on a similar mechanism for rabbit light chains. In our laboratory, we initially infected rabbits with Trypanosoma brucei, which stimulates B-cell hyperplasia and hypergammaglobulinaemia. The heterozygous rabbits exhibited the Ckappa1 b4 and b9 kappa light chain allotypes. After reverse transcription of mRNA, and cloning and sequencing of cDNA, the Vkappa-Jkappa-Ckappa genes provided evidence for both somatic hypermutation and gene conversion. We saw that in each of the b4 and b9 kappa light chain cDNA, CDR1 and CDR3 carried both point mutation and provisional gene conversion traits. In the CDR2 region, point mutation and gene conversion inserts were observed in the b4 genes, with only gene conversion in two b9 genes. In the CDR regions, although some genes exhibited only somatic hypermutation or gene conversion, others showed linkage of both somatic hypermutation and gene conversion in the same sequence. This also marks the first time that somatic hypermutation and gene conversion in the same cloned CDR region has been observed in Vkappa1 genes; however, it has been seen earlier in rabbit heavy chain VH sequences. Furthermore, the addition of several codons to the CDR3 segment by gene conversion may have provided a mechanism for length variation. In addition, we demonstrated that Jkappa and framework region segments contained examples of somatic hypermutation. Confirmation of gene conversion necessitates that donor sequences be identified as providing the templated inserts. Thus after cloning two pseudogenes we found putative CDR3 donor segments for two CDR3 rearranged genes. The results offer additional mechanisms for the generation of diversity among rearranged rabbit kappa light chain genes. Whether there is a relationship or influence of gene conversion upon somatic hypermutation or vice versa is not discernable at present.

Amino Acid Sequence↗

Patterns of age-dependent changes in the numbers of lymph follicles and germinal centres in somatic and mesenteric lymph nodes in growing C57Bl/6 mice.

The timing of the first appearance of lymph follicles and germinal centres in various lymph nodes, and the ways in which numbers of these and IgM-synthesising cells increase within the nodes, were investigated in male and female C57Bl/6N mice aged from 4 d to 16 wk. The lymphoid organs examined were the Peyer's patches, spleen, somatic (submandibular, deep cervical, brachial, axillary, inguinal and popliteal) and visceral (mesenteric and lumbar) lymph nodes. Primary follicles appeared in most somatic lymph nodes 6 d after birth. The number of follicles per node then increased rather sharply in larger lymph nodes and slowly in smaller nodes, up to 28 d of age, reaching a level which varied according to the location of the node. Thereafter, the number of follicles in the somatic lymph nodes increased only slightly to moderately, reaching a peak or plateau at 8-12 wk. In the mesenteric (ileocaecal) nodes, primary follicles first appeared at 12 d, then increased linearly during the suckling period and after weaning to reach a plateau at 8 wk of age. Germinal centres appeared in the submandibular and mesenteric nodes at 28 d and their numbers increased consistently in the latter, while remaining low in the former. The impact of possible 'natural' exogenous antigen stimulation of the various lymph nodes was estimated from the presence of IgM-synthesising cells and germinal centres. Differences between the patterns of age-dependent changes in the numbers of lymph follicles observed in the somatic and mesenteric lymph nodes during their ontogeny are discussed in relation to differences in the magnitude of the exogenous antigen stimulatory effect. We also found that the variations in the numbers of lymph follicles produced in somatic lymph nodes at different locations during the first 28 d after birth reflected differences in the dimensions of the body regions drained by a particular somatic lymph node at this stage of development.

Aging↗

Rehabilitation of somatic sensation and related deficit of motor control in patients with pure sensory stroke.

OBJECTIVE: To assess the effectiveness of a rehabilitative training program for deficits in somatic sensation and motor control of the hand in patients with pure sensory stroke. DESIGN: Multiple baseline and before-after follow-up trial with behavioral analysis of single cases. SETTING: Rehabilitation unit of a university hospital in Italy. PARTICIPANTS: Four patients were studied: 2 had a unilateral lesion confined to the parietal lobe (patients 1, 2), and 2 had a unilateral lesion of the thalamus (patients 3, 4) that also lapped the posterior limb of the internal capsule. All 4 patients had chronic deficits in somatic sensation and motor control of the contralesional hand. INTERVENTION: Behavioral training consisting of exercises aimed at improving somatic sensation and motor control of the affected, contralesional hand. Thirty treatment sessions, each lasting 50 minutes, were performed. MAIN OUTCOME MEASURES: Somatic deficit was evaluated with 5 tests, and motor control deficit was assessed with 4 tests. One functional test estimated the influence of somatic deficit on daily activities. A visual analog scale (VAS) was also submitted to the patients' relatives to evaluate the amount of use of the affected arm in daily life activities. A baseline was obtained by recording each measure, except for the VAS, 4 times at the first evaluation session. Evaluation sessions were conducted before, after, and 6 months after the end of the experimental treatment. RESULTS: All patients showed a stable baseline in at least 8 of the outcome measures. Patients 1 and 2 significantly improved in 9 and 7 outcome measures, respectively. Patients 3 and 4 improved in 4 and 7 outcome measures, respectively. With the exception of case 3, all patients considerably increased their use of the affected arm during daily life. The improvement was generally stable over a 6-month period, suggesting that the treatment had a long-term effect. CONCLUSIONS: Results suggest the possible effectiveness of our training program for treating somatic and motor control deficits of the hand in patients with cortical or subcortical pure sensory stroke.

Adult↗

Prevalence of somatic alterations in the colorectal cancer cell genome.

Although a small fraction of human cancers have increased rates of somatic mutation because of known deficiencies in DNA repair, little is known about the prevalence of somatic alterations in the vast majority of human cancers. To systematically assess nonsynonymous somatic alterations in colorectal neoplasia, we used DNA sequencing to analyze approximately 3.2 Mb of coding tumor DNA comprising 1,811 exons from 470 genes. In total, we identified only three distinct somatic mutations, comprising two missense changes and one 14-bp deletion, each in a different gene. The accumulation of approximately one nonsynonymous somatic change per Mb of tumor DNA is consistent with a rate of mutation in tumor cells that is similar to that of normal cells. These data suggest that most sporadic colorectal cancers do not display a mutator phenotype at the nucleotide level. They also have significant implications for the interpretation of somatic mutations in candidate tumor-suppressor genes.

Colorectal Neoplasms↗

Relationship between somatic mutation and neoplastic transformation.

Somatic mutation and neoplastic transformation of diploid Syrian hamster embryo cells were examined concomitantly. Mutations induced by benzo[a]pyrene and N-methyl-N'-nitro-N-nitrosoguanidine were quantitated at the hypoxanthine phosphoribosyltransferase and Na(+)/K(+) ATPase loci and compared to phenotypic transformations measured by changes in cellular morphology and colony formation in agar. Both cellular transformations had characteristics distinct from the somatic mutations observed at the two loci. Morphological transformation was observed after a time comparable to that of somatic mutation but at a frequency that was 25- to 540-fold higher. Transformants capable of colony formation in agar were detected at a frequency of 10(-5)-10(-6), but not until 32-75 population doublings after carcinogen treatment. Although this frequency of transformation is comparable to that of somatic mutation, the detection time required is much longer than the optimal expression time of conventionally studied somatic mutations. Neoplastic transformation of hamster embryo cells has been described as a multistep, progressive process. Various phenotypic transformations of cells after carcinogen treatment may represent different stages in this progressive transformation. The results are discussed in this context and the role of mutagenesis in the transition between various stages is considered. Neoplastic transformation may be initiated by a mutational change, but it cannot be described completely by a single gene mutational event involving a dominant, codominant, or X-linked recessive locus. Neoplastic transformation induced by chemical carcinogens is more complex than a single gene mutational process. Thus, this comparative study does not give experimental support to predictions of the carcinogenic potential of chemicals based on a simple extrapolation of the results obtained from conventional somatic mutation assays.

Adenosine Triphosphatases↗

Related 5S RNA transcription factors in Xenopus oocytes and somatic cells.

Xenopus oocytes contain an abundant protein that binds specifically to the center of 5S RNA genes and directs their transcription by RNA polymerase III. This protein also binds to 5S RNA. We show here that transcription of cloned 5S RNA genes in extracts derived from Xenopus tissue culture cells is dependent on the intragenic control region and is inhibited by 5S RNA and by antibodies raised against the previously characterized oocyte transcription factor. Somatic cells contain a protein that is similar to the oocyte factor in charge, affinity for heparin-agarose, and antigenicity but has an apparent molecular mass about 2000 daltons greater than that of the oocyte protein. Our experiments strongly suggest that this larger protein is the transcription factor for 5S RNA genes in somatic cells. The 5S RNA may regulate its own synthesis in somatic cells by binding to this protein, which is present at a low concentration. The presence of two different proteins responsible for 5S RNA synthesis in oocytes and in somatic cells cannot by itself explain the developmental control of oocyte and somatic 5S RNA genes, because somatic cell extracts transcribe both types of gene.

Animals↗

Ranking and frequency of somatic symptoms in residents near composting sites with odor annoyance.

The presented study reports the prevalence of somatic symptoms in three study samples living in the vicinity of composting plants. Microorganisms were measured in the air of the residential areas closest to the plants at the same time an epidemiological investigation was performed in the neighborhood near (150 to 1,500 m) to three plants and in corresponding control residential areas of the same district. Nine hundred and seventy-nine residents were questioned about the odor annoyance in their vicinity. An instrument measuring somatic complaints (SOMS 2 acc. Rief et al., 2001) was used to determine the unexplained somatic symptoms of the past two years and a gender-independent Total Complaint Index (TCI) was calculated. The percentages of study population reporting somatic symptoms were higher in all six samples in comparison with the German population (Rief et al., 2001) and in samples living near composting sites compared to the corresponding control samples. The study sample living close to site A (exposed to bioaerosols and odor annoyance) had the highest rates of complaints. A difference could be seen in comparison to the corresponding control sample (TCI: p[Anear vs. Acontrol] = 0.001; Mann-Whitney). In this group breathlessness was reported more than twice as often as in the other three samples. Out of the five most frequently reported symptoms four corresponded to the five complaints the German population reports most frequently in all groups. Nausea was the fifth most reported symptom in both samples reporting annoying residential odors (Anear and Bnear). The type of somatic symptoms reported most often was influenced little by environmental odors and medically relevant bioaerosol concentrations, except for nausea in context with annoying residential odors. As expected frequency of reporting general somatic symptoms was influenced by the perceived environment near the three composting sites. Concerning the sum of bodily complaints (TCI) though, this was only significant in the group exposed to medically relevant concentrations of residential outdoor bioaerosols, which was accompanied by high rates for breathlessness. The SOMS2 was able to mirror measurable, medically relevant environmental exposures in study groups and showed fewer changes concerning annoying or presumed environmental exposures.

Aerosols↗

Group psychotherapy for women molested in childhood: psychological and somatic symptoms and medical visits.

Molested women who completed a series of 16 weekly group psychotherapy sessions conducted by social workers improved substantially regarding various aspects of psychological functioning, including self-image, coping techniques, relationship issues, and mothering. In addition, there was significant improvement in all psychological symptom scales and all global indices of symptomatic distress measured by the SCL-90-R. Furthermore, the improvement was present immediately after therapy and, with the exception of the hostility score, persisted 1 year later. Although the somatization score was reduced, the number of visits for physical symptoms did not change. The patients studied manifested characteristics typical of previously surveyed women with a history of childhood abuse, including a frequent history of major surgery (Drossman et al., 1996; Longstreth & Wolde-Tsadik, 1993; Springs & Friedrich, 1992) and, in some, a previous problem with alcohol (Springs & Friedrich, 1992; Walker et al., 1995) or drugs (Longstreth & Wolde-Tsadik, 1993; Miller & McCluskey-Fawcett, 1993; Springs & Friedrich, 1992). Also, nearly one half of the subjects had irritable bowel syndrome, the prototypical functional bowel disorder (Drossman et al., 1995; Longstreth & Wolde-Tsadik, 1993; Scarinci et al., 1994; Walker et al., 1995). Most of their baseline SCL-90-R scores were > 1 SD above the nonpatient norms. A problem inherent in assessing the long-term benefit of this study and other group psychotherapy studies is the tendency for some patients to continue similar or different forms of therapy after completing the group sessions. More than one half of patients received subsequent therapy that could have influenced their status at 1-year follow-up. However, most of the symptom dimensions and all global indices were similar 1 year posttherapy in the women who did not receive more treatment as compared to results in the women who did. Patients who received additional therapy had higher somatization scores before, immediately after, and 1 year posttherapy; scores in the other group increased 1 year posttherapy. Although the indications for subsequent therapy were not surveyed, there was an association between additional psychological care seeking and somatization. Furthermore, improvement in psychological status reflected by the phobic-anxiety score immediately posttherapy may have contributed to the decision of some patients to seek subsequent therapy. In the group without additional treatment, the loss of some of the initial somatization improvement at 1 year may have contributed to the lack of reduction in medical care visits in the combined groups. We speculate that provision of additional therapy to more patients might have had a long-term effect on somatization and reduced medical visits. We obtained complete psychological data and nearly complete medical-visit data on our patients, and our survey included 1-year follow-up. Our survey did not meet rigorous methodological standards for an outcome study, however. We surveyed only a small number of patients and did not collect similar data on an untreated control group. It was not possible to distinguish health care visits for organic versus functional disorders, but such a distinction may be artificial, because psychological factors may influence health care seeking for "organic" illness. Because our measurements came from a subset of our patients who were willing to complete the survey questionnaires, we do not know how generalizable the findings are. There is increasing awareness among health care professionals that childhood sexual abuse is common and that it may have serious and long-term psychological and medical sequelae. Our data suggest that group psychotherapy by social workers for women victims may have long-lasting psychological and somatic symptom benefits. Reduction in health care usage was not found, and this outcome may require the identification and treatment of patients who need additi

Adult↗

Anxiety and depression in individuals with somatic health problems. The Nord-Trøndelag Health Study (HUNT).

OBJECTIVE: To examine the relationship between anxiety disorders and depression and various somatic health problems in the general population. DESIGN: Cross-sectional study with survey methods and clinical examinations. SETTING: The Health Study of Nord-Trøndelag, Norway (the HUNT study). PARTICIPANTS: 60869 individuals aged 20-89 years. MAIN OUTCOME MEASURES: Anxiety disorder, depression and their comorbidity are categorized based on scores on the Hospital Anxiety and Depression Scale. All somatic health variables are self-reported, while blood pressure, height and weight are measured. Multivariate nominal logistic regression analyses are used to investigate the relationship between somatic variables and the anxiety/depression categories. RESULTS: Most somatic health variables show a stronger association with comorbid anxiety disorder/depression than with anxiety disorder or depression alone. About one-third of individuals reporting somatic health problems also have anxiety disorder and/or depression. CONCLUSION: Somatic health problems carry a high risk of both anxiety disorder and depression. Active identification and treatment of these co-occurring mental disorders are of practical importance.

Adult↗

Young female survivors of childhood leukaemia do not have increased somatic concerns.

OBJECTIVE: This study examined whether experience of cancer in childhood leaves a hypersensitivity to various somatic symptoms. Further, are self-reported somatic symptoms explained by medical late-effects or a worry of recurrence of the cancer? METHODS: Of the total of 44 female survivors of leukaemia, 42 were compared with 69 age-matched healthy controls. We used a questionnaire to study self reported somatic symptoms and a face-to-face interview to explore worries about recurrence of the illness. Health status and medical late effects were evaluated by a paediatric haematologist. RESULTS: In contrast to our assumptions, young survivors of leukaemia reported fewer somatic symptoms than healthy age-matched comparison subjects (p < 0.001). Late physical sequelae were uncommon except in the survivors of allogeneic bone marrow transplantation. Of the survivors, 52% were afraid of recurrence of the illness. The presence of physical or visible impairment and worry of recurrence were unrelated to frequency of somatic symptoms. CONCLUSIONS: The results suggest that experience of childhood leukaemia and its treatment does not result in increased somatic concerns or hypochondriacal tendencies.

Adaptation, Psychological↗

New multiple somatic mutations in the RET proto-oncogene associated with a sporadic medullary thyroid carcinoma.

Medullary thyroid carcinoma (MTC) occurs mostly as a sporadic tumor or in connection with inherited cancer syndromes-multiple endocrine neoplasia (MEN) types 2A and 2B and familial MTC. Germline mutations in the RET proto-oncogene are found in most of the familial cases. Somatic mutations in the RET proto-oncogene are detected in 23%-69% of patients with sporadic MTC. The most frequent somatic mutation is Met918Thr in exon 16 and only a small percentage of mutations in other RET exons have been observed. In a very few cases double mutations were found. Genetic screening for somatic mutations in RET exons 10, 11, 13, 14, 15, and 16 in Czech patients with sporadic MTC was carried out by DNA sequencing. This study presents a new triplesomatic mutation Gly911Asp, Met918Thr, and Glu921Lys in exon 16 of the RET proto-oncogene detected in an 18-year-old Czech male patient. In the second case, a new double-somatic mutation Val591Ile in exon 10 with a concomitant somatic mutation Met918Thr in exon 16 was found in a 77-year-old Czech female patient. These both newly described somatic multiple mutations were revealed in a hemizygous status, the loss of heterozygosity in tumor tissues in comparison with germline DNA was confirmed.

Adolescent↗

Histocytological analysis of callogenesis and somatic embryogenesis from cell suspensions of date palm (Phoenix dactylifera).

BACKGROUND AND AIMS: The date palm is a dioecious perennial species of the Arecaceae for which in vitro micropropagation is essential to ensure the renewal of palm plantations. This study presents a histocytological analysis of the traditional Mauritanian Amsekhsi cultivar beginning from the initial callogenesis and continuing up to the establishment of the cellular embryogenic cell suspensions. The formation of somatic embryos and their development into rooted plants are also described. METHODS: Foliar segments of seedlings cultured in the presence of 2,4-D produced primary calli that were chopped to produce fine friable granular calli that subsequently produced cellular suspensions when transferred to liquid medium. The somatic proembryos that developed after removal of the 2,4-D were plated on agar medium where they developed into rooted plants. Thin sections of tissue fragments taken at each stage of the process were stained using Periodic Acid Schiff and Naphthol Blue-Black. KEY RESULTS: The first cellular divisions were localized close to the vascular vessels of the leaf. The primary calli were obtained within 2 months. Fine friable granular calli grew quickly after the primary calli were chopped. Individual embryogenic cells were identified that rapidly started to divide and developed into globular proembryos. In addition, in the microcalli, breaking zones appeared in the thick pectocellulosic walls which delimited the pluricellular proembryos. The anatomy of somatic embryos is similar to that of zygotic embryos despite a deficit in the accumulation of intracellular proteins. When rooted with NAA, the vitroplants developed a strong orthotropic taproot. CONCLUSIONS: This study contributes to understanding the whole process of somatic embryogenesis, but two specific questions remain to be answered: what factors are involved in the reactivation of the somatic cells at the beginning of the initial callogenesis, and why do the somatic embryos not accumulate proteins in their tissues during maturation?

Arecaceae↗

Embryonic germ cells induce epigenetic reprogramming of somatic nucleus in hybrid cells.

Genomic reprogramming of primordial germ cells (PGCs), which includes genome-wide demethylation, prevents aberrant epigenetic modifications from being transmitted to subsequent generations. This process also ensures that homologous chromosomes first acquire an identical epigenetic status before an appropriate switch in the imprintable loci in the female and male germ lines. Embryonic germ (EG) cells have a similar epigenotype to PGCs from which they are derived. We used EG cells to investigate the mechanism of epigenetic modifications in the germ line by analysing the effects on a somatic nucleus in the EG-thymic lymphocyte hybrid cells. There were striking changes in methylation of the somatic nucleus, resulting in demethylation of several imprinted and non-imprinted genes. These epigenetic modifications were heritable and affected gene expression as judged by re-activation of the silent maternal allele of Peg1/Mest imprinted gene in the somatic nucleus. This remarkable change in the epigenotype of the somatic nucleus is consistent with the observed pluripotency of the EG-somatic hybrid cells as they differentiated into a variety of tissues in chimeric embryos. The epigenetic modifications observed in EG-somatic cell hybrids in vitro are comparable to the reprogramming events that occur during germ cell development.

Alleles↗

Patterns of somatic mutations in immunoglobulin variable genes.

The mechanism responsible for somatic mutation in the variable genes of antibodies is unknown and may differ from previously described mechanisms that produce mutation in DNA. We have analyzed 421 somatic mutations from the rearranged immunoglobulin variable genes of mice to determine if the nucleotide substitutions differ from those generated during meiosis and if the presence of nearby direct and inverted repeated sequences could template mutations around the variable gene. The results reveal a difference in the pattern of substitutions obtained from somatic mutations vs. meiotic mutations. An increased frequency of T:A to C:G transitions and a decreased frequency of mutations involving a G in the somatic mutants compared to the meiotic mutants is indicated. This suggests that the mutational processes responsible for somatic mutations in antibody genes differs from that responsible for mutation during meiosis. An analysis of the local DNA sequences revealed many direct repeats and palindromic sequences that were capable of templating some of the known mutations. Although additional factors may be involved in targeting mutations to the variable gene, mistemplating by nearby repeats may provide a mechanism for the enhancement of somatic mutation.

Animals↗

Somatic crossing over in Glycine max (L.) Merrill: mutagenicity of sodium azide and lack of synergistic effect with caffeine and mitomycin C.

Glycine max (soybean) is one angiosperm which lends itself to the study of somatic crossing over. This is made possible because some varieties have gene combinations Y(11)Y(11), Y(11)y(11) and y(11)y(11) in the segregating populations from Y(11)y(11) plants. The gene in question is responsible for chlorophyll synthesis. The Y(11)Y(11) plants have dark green leaves, Y(11)y(11) are light green and y(11)y(11) plants are golden yellow. The heterozygous plants have dark green, yellow and dark green-yellow (double) spots on the leaves of the untreated control material, whereas the two homozygotes are almost always devoid of somatic sectoring. Application of caffeine, or mitomycin C, to the seeds increased the frequency of double, dark green and yellow spots on the Y(11)y(11) background. Possibly, some dark green or yellow spots originate by failure of one of the two components of what might start as a double spot due to somatic crossing over. The application of NaN(3) increases the frequency of dark green or yellow spots, almost exclusively. The two spots increase in equal frequency. The y(11)y(11) plants so treated do not have any light green sectors, but dark green, Y(11)Y(11), plants do develop a few light green or very dark green spots. The data indicate that NaN(3) is capable of inducing nondisjunction, but does not cause mutations (at this locus), chromosome fragmentations (segmental losses) or somatic crossing over to an appreciable degree. It has previously been shown that caffeine-induced chromosome rejoining in Vicia faba can be inhibited by treating the roots with NaN(3). In the present experiments NaN(3) did not affect the processes of somatic crossing over as induced by caffeine or mitomycin C. The effect was additive. This system offers advantages for studying chemical mutagens in that somatic crossing over, point mutations, segmental losses through chromosome breakage and nondisjunction can all be studied in a single treatment to the seeds.

Azides↗

Somatic instability of the CTG repeat in mice transgenic for the myotonic dystrophy region is age dependent but not correlated to the relative intertissue transcription levels and proliferative capacities.

A (CTG)nexpansion in the 3'-untranslated region (UTR) of the DM protein kinase gene ( DMPK ) is responsible for causing myotonic dystrophy (DM). Major instability, with very large expansions between generations and high levels of somatic mosaicism, is observed in patients. There is a good correlation between repeat size (at least in leucocytes), clinical severity and age of onset. The trinucleotide repeat instability mechanisms involved in DM and other human genetic diseases are unknown. We studied somatic instability by measuring the CTG repeat length at several ages in various tissues of transgenic mice carrying a (CTG)55expansion surrounded by 45 kb of the human DM region, using small-pool PCR. These mice have been shown to reproduce the intergenerational and somatic instability of the 55 CTG repeat suggesting that surrounding sequences and the chromatin environment are involved in instability mechanisms. As observed in some of the tissues of DM patients, there is a tendency for repeat length and somatic mosaicism to increase with the age of the mouse. Furthermore, we observed no correlation between the somatic mutation rate and tissue proliferation capacity. The somatic mutation rates in different tissues were also not correlated to the relative inter-tissue difference in transcriptional levels of the three genes (DMAHP , DMPK and 59) surrounding the repeat.

Age Factors↗