Heritable fragile sites on human chromosomes

Summary The chromosomal gaps associated with fragile sites at 2q11, 10q23, 11q13, 16q22, 20p11 and Xq27 do not stain with silver nitrate as do NOR regions of the acrocentric chromosomes.     

Heritable fragile sites on human chromosomes

The fragile site at Xq27 which is associated with X-linked retardation with macroorchidism has been studied in 21 retarded males. These males were from 12 families, and studies of nine of the familes were possible. Detection of carrier females is difficult, especially with increasing age. The fragile site was demonstrated in only five of 13 obligate carrier females. It is concluded that using present methods, cytogenetic detection of carriers is fairly reliable in females aged less than 20--25 years but unreliable in older females.     

Heritable fragile sites on human chromosomes

Summary A kindred is described in which six members have a fragile site at 12q13. This fragile site was found to be suppressed by folic acid and thymidine in lymphocyte culture. An updated classification of known fragile sites is presented.     

Heritable fragile sites on human chromosomes. XI. Factors affecting expression of fragile sites at 10q25, 16q22, and 17p12.

The fragile sites at 10q25, 16q22, and 17p12 can all be induced in lymphocyte culture by BrdU or BrdC added 6-12 hrs prior to harvest. Without induction, fra(10)(q25) is rarely expressed spontaneously, whereas fra(16)(q22) is frequently expressed spontaneously. Fra(17)(p12) is frequently expressed spontaneously but is probably expressed only after induction in some individuals. Distamycin A, netropsin, and Hoechst 33258 induced high levels of expression of fra(16)(q22) and fra(17)(p12) but did not enhance expression of fra(10)(q25). The mechanisms of induction of fra(16)(q22) by BrdU and distamycin A appear to be different, since the time of induction by BrdU reaches a maximum about 12 hrs prior to harvest whereas induction by distamycin A requires much longer exposure. The fragile sites at 10q25 and 16q22 were both induced in fibroblast culture by BrdU. Fra(17)(p12) is accepted as a fragile site because preliminary studies show that it behaves similarly in lymphocyte culture to fra(16)(q22); however, there is only limited evidence for fragility at 17p12.     

Heritable fragile sites on human chromosomes. VIII. Preliminary population cytogenetic data on the folic-acid-sensitive fragile sites.

The incidence of the autosomal folic-acid-sensitive fragile sites in 524 institutionalized retardates (.0095) was found to be significantly higher than in 1,019 unselected neonates (.00098), suggesting that heterozygosity for these fragile sites may not be as harmless as previously thought. When one of the parents of an index case was found to carry the fragile site, that parent was always the mother. The fragile site at Xq27 was not found among the neonates studied, but was present in 1.6% of the institutionalized retarded males examined; if this fragile site occurs in normal males, then it does so rarely. Further cytogenetic studies of fragile sites are required on both normal and abnormal populations.     

Heritable fragile sites on human chromosomes. V. A new class of fragile site requiring BrdU for expression.

A new fragile site at 10q25 is described, representing a new class of fragile site that requires bromodeoxyuridine (BrdU) in the culture medium for expression. This new fragile site is present in approximately one in 30 of the Australian population; it has only been observed in heterozygotes.     

Heritable fragile sites on human chromosomes II. Distribution, phenotypic effects, and cytogenetics.

Individuals and families have been documented in which there are a number of fragile sites on chromosomes. These include sites at 2q11, 10q23, 11q13, 16p124, 16q22, 20p11, and Xq27 or 28. Fragile sites reported in the literature are compiled. The cytogenetics of the sites is discussed. The phenotypic effects of the sites are considered, and it is speculated that homozygosity of the autosomal sites might be deleterious as is hemizygosity of the site on Xq. These sites are used in the previous report which documents the effect of tissue medium components on their expression.     

Expression of fragile sites in human sperm and lymphocyte chromosomes

Summary Sperm and lymphocyte chromosome studies in a normal, fertile male have shown a high degree of coincidence between chromosome lesions and fragile sites in both types of cells. In this donor we also found that some fragile sites expressed in sperm chromosomes coincided with those expressed in lymphocyte chromosomes. These results indicate that the chromosome lesions expressed in sperm do not occur at random and that they are not technical artifacts. The fragility expression in sperm chromosomes could reflect in vivo conditions. The presence in some sperm metaphases of acentric fragments suggests that chromosome fragility can result in the loss of chromosome fragments or give rise to de novo structural rearrangements. However, the incidence of sperm with chromosomal abnormalities observed in this man was within the normal range.     

Expression of folate-sensitive fragile sites in lymphocyte chromosomes

Summary The expression of folate-sensitive fragile sites (FS) was analyzed using MTX as a fragility inducer in seven normal subjects [four unrelated persons and three members of one family (father, mother, and son)]; a woman heterozygous for fra Xq27.3 with a 47,XXX karyotype; and her son, affected by the fra-X syndrome. The mean expression of chromosome lesions (CL) other than Xq27.3 was 70.1% (686CL in 978 metaphases), and the coincidence between CL and FS was 68.9%. We propose six new c-fra sites: bands 4q33 and 11q22 because they were found in two members of the same family; band 13q32 because it had a frequency of expression of 3% of metaphases; and bands 3p13, 8q21, and Xq21 because they were observed in four of the nine individuals studied.     

Heritable fragile sites on human chromosomes. IX. Population cytogenetics and segregation analysis of the BrdU-requiring fragile site at 10q25

The frequencies of the bromodeoxyuridine (BrdU)-requiring fragile site at 10q25 in 1,026 unselected neonates, 901 patients referred for chromosome studies, and 87 institutionalized retardates were not significantly different from each other. The gene frequency was .013, and the population was in Hardy-Weinberg equilibrium. Segregation analysis confirmed that the fragile site followed codominant inheritance. This fragile site and its nonfragile allelomorph can be considered to constitute the first true chromosomal polymorphism to be described in man.     

Heritable fragile sites on human chromosomes. X. New folate-sensitive fragile sites: 6p23, 9p21, 9q32, and 11q23

Four new folate-sensitive fragile sites are documented at 6p23, 9p21, 9q32, and 11q23. These have all been shown to be heritable except for the one at 9p21, which has been seen only in a single individual. As with the other autosomal fragile sites, these appear to be innocuous in heterozygotes.     

Variation in the expression of aphidicolin-induced fragile sites in human lymphocyte cultures

Summary A correlation between specific fragile sites and cancer breakpoints has been suggested raising the question of fragile site expression as a predisposing factor in the occurrence of cancer in some persons. Before addressing the question of increased fragility among patients at high risk for cancer, we analyzed the variability of aphidicolin-induced fragile sites among nine normal persons and also among repeated samples from three of these individuals. Considerable variation in both the frequency and location of these fragile sites was observed and the data strongly suggest the significant variation of 6 of the 16 selected sites to be primarily due to sampling differences. These findings indicate that the use of fragile sites as a screening tool for patients at high risk of cancer should be carefully monitored relative to the variation inherent in both culture and individual expression.     

Heritable fragile sites on human chromosomes. VII. Children homozygous for the BrdU-requiring fra(10)q25 are phenotypically normal

A brother and sister have been detected who are homozygous for the bromodeoxyuridine (BrdU)-requiring fragile site at 10q25. The children are phenotypically normal, indicating that homozygosity for this fragile site is harmless, at least during childhood.     

The distribution of MspI-induced breaks in human lymphocyte chromosomes and its relationship to common fragile sites

The restriction endonuclease MspI (cleavage site C/CGG) induces chromosomal breaks in human lymphocytes. The breakpoints are distributed non-randomly along the chromosomes and the pattern of MspI-induced breakage depends on the recovery time (20 h or 6 h). Chromosomal bands preferentially involved in breakage are likely to coincide with bands where common fragile sites are located.     

Factors affecting lymphocyte culture for chromosome studies

Factors affecting the transformation of brown trout, Salmo trutta L. , and carp, Cyprinus carpio L. , lymphocytes in culture for subsequent karyotyping are examined. Optimum conditions for mitosis are established in respect of medium, mitogen, atmosphere, buffering system and serum.     

Neither age nor sex influence the expression of folate sensitive common fragile sites on human chromosomes

Summary The expression of folate sensitive common fragile sites was investigated in 82 normal healthy males and females of various ages. In 100 studied metaphases of each of these controls, between 0 and 56 lesions were detected (mean 18.3 ± 10.3 SD). No significant difference was found between the mean number of expressed lesions in females and males. No age-effects were observed. Two new common fragile sites were discovered at 6p21 and 17q21. Their fragile site status, however, needs to be confirmed.     

Giemsa staining of the sites replicating DNA early in human lymphocyte chromosomes.

A timetable for the initiation of DNA replication in human lymphocyte chromosomes has been established by a technique which allows detection of areas of chromosomes replicating at a given interval of the S-phase. The resolution of the method, using 33258 Hoechst-Giemsa staining, is more refined than that obtained with 3H-thymidine autoradiography. Early replicating regions coincide with R-bands. The timetable is rather coarse since replication may start asynchronously in the same region of homologous autosomes of the same metaphase and since even the sequence of bands appearing on individual chromosomes sometimes deviates from the rule.