| First Author | Cattanach BM | Year | 1995 |
| Journal | Mouse Genome | Volume | 93 |
| Issue | 4 | Pages | 1026-1029 |
| Mgi Jnum | J:30568 | Mgi Id | MGI:78690 |
| Citation | Cattanach BM (1995) A new at mutation/ A dominant polycythaemia. Mouse Genome 93(4):1026-1029 |
| abstractText | Full text of Mouse Genome contribution: Restructuring of the Harwell Mouse Genetics Division. It may be noted from the address shown above that we are no longer a Division of the MRC Radiobiology Unit, but now comprise an independent new MRC Mammalian Genetics Unit. The change heralds a major MRC initiative in which mouse genetics research at Harwell is to be substantially expanded with a number of new posts in molecular genetics being introduced. The development is related to the creation of the new MRC Mouse Genome Centre which is to be located in an adjacent building on the same Harwell site. The Centre, to be headed by Prof. Steve Brown, is expected to open on 1st January 1996 and will house approximately 20 staff. It is expected that the work will be closely integrated with that of the main Genetics Unit and the service facility for storing mutant and transgenic stocks as frozen embryos to build up a large genetics base. Prof. Brown will also join the MGU in January as Deputy Director and will be largely responsible for the expansion of molecular work in the Unit. Dr Cattanach is to serve as Acting Director of the new Unit until close to his retirement when Prof. Brown will take over. Research News: 1. New mutations: a) A new a(t) mutation. A new black and tan (a(t)) mutation has been detected in a non-agouti stock of mixed origin in which the Is(7;X)Ct translocation was segregating. It appears to resemble the original a(t) mutation (Dunn, Proc. Natl. Acad. Sci. USA 14: 816-819, 1928) in all respects. As an a(t) mutation has previously been found at Harwell (Lyon, pers. comm.) the present mutation has been named a(t2H) (Cattanach and Clements). b) A dominant polycythaemia. Recent specific locus mutation experiments have included a screen for cytogenetically visible deletions and duplications (Cattanach and Evans, In, Radiation Protection: Molecular mechanisms in radiation mutagenesis and carcinogenesis. Eds K H Chadwick, R Cox, H P Leenhouts and J Thacker. European Comm. pp 93-100, 1994) among the growth retarded progeny of spermatogonially X-irradiated wild type C3H/HeH x 101/H Fl (3H1) males mated with tester (T) stock females. Among these, one male was seen to have notably red ears and feet as an adult, closely resembling homozygotes for the polycythaemic mutant, Hbb(d4), described by Peters et a1 (Genetics 110: 709-721, 1985). On crossing the male with 3H1 females the character was clearly inherited by a proportion of the offspring (13/34), with the phenotype again only being seen in adults. On further crossing the affected young to 3H1 it became apparent that classification was unreliable. The redness of the feet and ears was variable such that regular evaluation was necessary to aid identification of the mutant class. Haematocrits upon adults also proved to be of limited help. Based on 63 progeny screened from the first litters, values over 60% and up to 75% could be obtained but, again, when repeat readings were taken every two weeks over a period of three months, these fluctuated widely and overlapped with control 3H1 levels of 45 - 55% (mean = 51.30 +/- 0.37). No affected animal had consistently high haematocrit values. Apart from the variability, the degree of affect was similar to that observed by Peters et al (Genetics ibid) in Hbb(d4) homozygotes. With further backcrossing to the 3H1, and classification of adults on the basis of any single haematocrit value above 60% (in up to six repeat tests taken two weeks apart), better evidence on the segregation was obtained. Thus, 30 mutants could be recognised among 76 animals scored. The shortage relative to the 50% expectation still suggested some level of mis-classification or reduced penetrance, however. To determine if the Hbb locus was involved, a single mutant male that had shown haematocrit values greater than 60% in three tests was crossed to females of the inbred albino strain, JU/Ct and four affected daughters were then backcrosed to JU/Ct males. In the first cross, 6 animals were deduced to be affected (with single or multiple haematocrits of >60%, ranging in repeat test up to 78%) and 4 appeared normal (with maximum values of 58%, ranging down to 50%). In the backcross there were 14 affected (single diagnostic values, 60% - 81%) and 16 normal (single values, 42% - 58%). As found in the 3H1 crosses, no animal consistently had high haematocrits; and the external phenotype (ears/feet) was also no clearer. Significantly, among the 19 c progeny and 11 + progeny tested from the backcross, there were 10 c affected, 7 + unaffected, 9 c unaffected, and 4 + affected, this clearly failing to provide any evidence of linkage of the condition with c. As the Hbb locus lies only 6 cM from c (Lyon and Kirby, Mouse Genome 93:23-66, 1995 ) it is highly unlikely that the polycythaemia involves Hbb. It may be concluded that the mutation differs both from the mouse Hbb(d4) and also the recessive polycythaemias associated with haemoglobin locus changes in humans (McKusick, In, Mendelian Inheritance in Man, Ed. 10. The John Hopkins University Press, Baltimore, 1992). Homology with dominant human polycythaemias, several different mechanisms for which have been suggested (McKusick, ibid), is clearly a better possibility. The mutant has been given the provisional basic name of polycythaemia (Pcm), and is available to anyone interested in researching it further (Cattanach, Clements and Rasberry). 2. Time of death of Td males. Tattered (Td) is a semi-dominant mutation located close to spf on the proximal part of the X-chromosome (Cattanach, MNL 67:19). Td+ females are viable but exhibit considerable variation in expression. Moderately affected adults show a slight striping of the coat similar to some of the Ta alleles. More severely affected animals are often small in size with scarring of the skin resulting in noticeable bald patches on the coat and tail. Some individuals also have abnormally shaped heads, bent tails and twisted toes, possibly indicating mild skeletal abnormalities. About 15% of these extreme heterozygotes die pre-weaning and it was suspected that there was also some pre-natal loss (Cattanach, MNL 66:61). Not surprisingly, Td is a pre-natal lethal in the male but the time and cause of death has not been established. In order to estimate the time of death of Td males, Td+ females were mated to wild-type (++) males and opened at 12.5-13.5d gestation. The results are shown in Table 1 together with those of wild-type controls. Type of Cross: Td+ffx++mm; No. ff: 8; No. implants: 71; Normal live embryos: 45; Dead embryos or retarded: 17; Embryonic remains/large moles: 3; Small moles: 6; Corpora lutea: 74. Type of Cross: ++ffx++mm; No. ff: 4; No. implants: 38; Normal live embryos: 37; Dead embryos or retarded: 0; Embryonic remains/large moles: 0; Small moles: 1; Corpora lutea: 38. The low incidence of pre-implantation loss (74-71/74) clearly suggests that the loss of Td males occurs after implantation. The incidences of small moles and large moles/ embryonic remains are relatively low (approximately 8% and 4% respectively). However there were significant numbers of dead embryos and severely retarded live embryos (24%) which suggests that these comprise the Td male class. To investigate this further, chromosome preparations were made from the embryonic membranes of a sample of 36 normal live embryos, 7 dead embryos and 4 severely retarded embryos using the method of Evans. (In Mammalian Development, IRL Press 93-114, 1975). These were then C-banded to determine the presence /absence of a Y chromosome. Of the 36 normal live embryos, 24 were chromosomally XX and 12 were XY. Of the 7 dead embryos, 2 were XX and 5 XY, and of the 4 live retarded embryos 2 were XX and 2 were XY. The presence of XX dead embryos suggests that there is a small pre-natal loss of Td+ females before 12.5d and continuing beyond 13.5d gestation, when XX live retarded embryos are still found. This overlaps with the loss of Td males which are found both as dead and as live retarded embryos at 12.5-13.5d gestation. Whether further Td male losses occur before 12.5d or after 13.5d is uncertain (Rasbeny, Beechey and Cattanach). 3. T(1;12)52H Correction of breakpoints. Recently it has been noted that the breakpoints in this reciprocal translocation are in 1D and 12S3 and not in 1F and 12D3, as previously documented (Mouse Genome 89:551, 1991). The mistake was thought to result from transcriptional error (Evans). Linkage data: Location of the T(5:11)57H breakpoint on chromosome 11. The cytological breakpoints of T(5;11)57H have been located at the F/G border (or in G2) on Chr 5 and at the A/B border on Chr 11 (Burtenshaw et al, Mouse Genome 91:856, 1993). Linkage studies have now been carried out to determine the position of the T57H breakpoint on the Chr 11 genetic map, using the markers wa2 and vt. Progeny from backcrosses of wa2 + vt /+ T57H + males to homozygous wa2 vt females were classified for the marker genes and, by fertility tests for the presence/absence of the translocation. The following phenotypic classes were produced :- 29 + T57H +, 24 wa2 + vt, 12 wa2 T57H +, 6 + + vt, 2 + T57H vt, 4 wa2 + +, 2 wa2 T57H vt and 2 + + + , total 81. The probable order, judging by the numbers of double recombinants is likely to be wa2-T57H-vt (4) but could also be wa2-vt-T57H (6), with an RF of 12.3 +/- 3.6% between the breakpoint and vt. If T57H is indeed between wa2 and vt then the RF of 27.0 +/- 4.9% between the breakpoint and wa2 is remarkably high considering that the consensus map distance between wa2 and vt is 23 cM (Lyon and Kirby, Mouse Genome 93:23-66, 1995). and 29.6 +/- 5.1% from the present data. This suggests that T57H enhances crossing over in the region of its breakpoint. Further work is in progress to determine the true order of T57H with regard to wa2 and vt (Beechey, Harrison, Cattanach). |
