Hum Genet (1989) 81 : 200-202

Letter to the editors

© Springer-Verlag 1989

Recombination between DXS7, DXS84 and a rare form of X-linked retinitis pigmentosa (McK-30320)

Diana Curtis and C. E. Blank

Centre for Human Genetics, 117 Manchester Road, Sheffield, $10 5DN, UK

We report a family with the variant McK-30320 type X-linked retinitis pigmentosa (XLRP). A recombination event in this family supports the evidence for a separate XLRP locus for this variant type, as originally argued by Nussbaum et al. (1985).

McKusick (1983) catalogues three clinically distinct forms of XLRP: the classic form (McK-31260), the choroidoretinal dystrophy form (McK-30330), and the variant with a distinc- tive tapetal reflex in carrier females (McK-30320). Whether these three forms are due to more than one gene location for XLRP, are allelic forms at a single locus, or represent hetero- geneous clinical expression of a single mutant locus has re- cently been discussed.

Bhattacharya et al. (1984) first reported genetic linkage between the D N A probe L1.28, which identifies a restriction fragment length polymorphism (RFLP) at the locus DXS7, and the XLRP locus. Clayton et al. (1986) have calculated the mean estimate of the recombination fraction (0) between DXS7 and XLRP to be 0.09 (maximum lod, 14.01; confidence limits, 0.04-0.17). These workers have convincingly argued for an XLRP location between DXS7 and DXS14. Nussbaum et al. (1985) and Denton et al. (1988) have described kindreds with McK-30320 type XLRP with a different estimate of re- combination between the disease and DXS7. Both workers argue for a more distal location for XLRP (McK-30320) rela- tive to DXS7 on the basis of the different recombination frac- tion and on the basis of observed recombination events. How- ever their data were insufficient to prove or disprove allelism of this rare form with classic XLRP. Clayton et al. (1986), using data pooled from all published families (excluding Denton et al. 1988), failed to detect evidence for heterogeneity.

The family pedigree is shown in Fig. 1. The consultand (IV.4), who has had significant vision problems since early childhood, finally became registered as partially sighted at the age of 24 years. His mother 's maternal grandfather and great uncle were reported to have become blind by their early thir- ties. In 1984 ophthalmological examination of his mother (III.5) showed minimal fundal changes but undoubted elec- troretinogram (ERG) abnormality; his younger sister (IV.5) had undoubted fundal abnormality with a normal ERG; and his aunt (III.1) had the typical signs of the heterozygous female. The consultand's elder sister (IV.3), father (III.4), and his aunt III.3 were normal on fundal examination, and his other male relatives III. 6 and IV. 1 were without significant vi- sion defect. At the end of the investigation retinitis pigmen-

Offprint requests to." D. Curtis

tosa was described as segregating as an X-linked recessive trait in this family.

In 1986 DNA probe studies were initiated with further counselling and possible prenatal diagnosis for the sisters (IV.3, IV.5) in mind. The probe findings were discordant with the clinical observations outlined above. Therefore a second ophthalmological opinion was sought from colleagues at the West Midlands Regional Genetic Service who were aware only that our probe and clinical findings were in conflict. Ex- amination confirmed the earlier findings, but it was noted that III.5 exhibited an unusual tapetal reflex, the macula having "a beaten metal appearance, with glistening patches" which might indicate a distinct form of XLRP. This observation was consis- tent with the family described by Nussbaum et al. (1985) and three further families recently described by Denton et al. (1988), all of whom exhibit this rare, but distinctive, sign of the condition classified as McK-30320 type XLRP.

Ill

IV

1925

1 2

] ] 2 2 2 1

3 1929

1963 1964

T 2--2 1 1 1 2 2 2 2 ] 1

DXS 28

164 84

7 14

1934 1936

Fig.L Pedigree with RFLP genotypes ordered as in Human Gene Mapping 8 (1985) and Table 1. [] Affected males, (?) obligate hetero- zygous females

1956 1962 1968

2 1 1 2 I 2

1 2 2 2 1 I 1 2 I 2 1 2 1 2 2

Table 1. Technical data and sources of probes used in this study (according to Human Gene Mapping 8 1985)

Probe a Locus Band location RE RFLP b Source of probe c

201

C7 DXS28 Xp21.3 EcoRV 7.5 : 8.0 kb J.-L. Mandel

pERT87.15 DXS164 Xp21 Taq 3.1 : 3.3 kb L.M. Kunkel 754 DXS84 Xp21 Pst 12 : 9 kb P. Pearson L1.28 DXS7 Xpll.3 Taq 12 : 9 kb P. Pearson 58.1 DXS14 Xpll Msp 4.2 : 2.5 kb L.M. Kunkel

aOther more distal probes, JXI.1 99.6 (DXS41) and 782 (DXS85), were uninformative bThe commoner allele is given first and referred to as allele 1 in Fig. 1 eWe are greatly indebted to these workers for their generosity in supplying us with these probes

Table 2. Two-point linkage analysis of XLPR with five Xp probes

DXSI4 DXS7 DXS84 DXS164 DXS28

0.0 1.40 0.80 0.82 0.05 -0.54 0.27 1.27 0.74 0.76 0.10 - 0.28 0.44 1.14 0.68 0.67 0.15 -0.14 0.47 1.00 0.61 0.63 0.20 - 0.06 0.45 0.85 0.54 0.55 0.25 -0.01 0.39 0.71 0.47 0.48 0.30 0.02 0.31 0.55 0.39 0.39 0.35 0.03 0.22 0.40 0.30 0.31 0.40 0.03 0.13 0.26 0.21 0.21 0.45 0.02 0.05 0.12 0.11 0.11

0 0.40 0.15 1.40 0.80 0.82 2 0.03 0.47 0.0 0.0 0.0

DNA was prepared from all family members indicated on the pedigree. Standard laboratory procedures were used to prepare restriction enzyme digests, which were blotted onto

32 nylon membranes and hybridized to P-dCTP-labelled probes. The probes used in this study and their sources and technical data are given in Table 1.

The RFLP genotypes are shown in Fig. 1. When phase is known, haplotypes are separated by a vertical line with the male-derived haplotype on the left. The obligate heterozygote III.5 is informative for all five probes studied. Phase in III.5 can be derived from the phase-known haplotypes of IV.3 and IV.5 (assuming legitimacy) and from IV.4. The genotypes ob- served in III.3, III.5, and III.6 give some clues as to the geno- types of the deceased grandparental generation II. A recombi- nation between DXS84 and DXS7 is indicated in the children of III.5, with the least number of recombinants derived by as- signing the cross-over to IV.5. Thus IV.3 and IV.5 have inher- ited the same maternal haplotype below the cross-over, which implies that neither girl is a carrier of the XLRP gene. If XLRP is located (Wright et al. 1987) between DXS7 and DXS14, a further double cross-over event between DXS7 and DXS14 is required to comply with the clinical information that IV.5 is a carrier. In any other haplotype arrangement, cros- sing over is frequent, and several of the possible haplotype ar- rangements would require a double cross-over event or that all three sibs are recombinants. Alternative explanations are that this is not a case of XLRP or that this XLRP gene is lo- cated elsewhere on the X chromosome.

The recombinant events observed in the triply informative meiosis in III.5, her phase-known daughters IV.3 and IV.5, and her affected son IV.4 argues for an XLRP site between

DXS7 and DXS84. Only one cross-over event is required in IV.5 if the gene for XLRP is located in this position. The pat- tern of recombination reported by both Nussbaum et al. (1985) and Denton et al. (1988) is congruent with the recombi- nation observed in our present family. In all cases the par- simonious distribution of cross-over events is best served by placing the XLRP gene distal to DXS7 and proximal to DXS84. It is interesting that none of these families have recombination events that could argue for a proximal position of XLRP rela- tive to DXS7.

The RFLP data was analyzed using the computer program LIPED (Ott 1974), a disease frequency of less than 10 -4 (Bunker et al. 1984), RFLP allele frequencies from Human Gene Mapping 8 (1985), and penetrance for female carriers of 0.08 (Friedrich et al. 1985). The maximum likelihood recom- bination distance (6) between XLRP and the five informative test loci and the probability limits for 6 were calculated ac- cording to Ott (1985). This small family, which includes one recombinant in three phase-known progeny, inevitably pro- duces lod scores that do not reach levels of significance; more- over, calculating confidence limits is unrealistic. The linkage data calculated (Table 2) for this family suggests that this XLRP locus is closest to DXS84, further from DXS7, and farthest from DXS14. This map position is in agreement with the findings of Nussbaum et al. (1985) and Denton et al. (1988) who used linkage data related specifically to families in whom unusual tapetal reflex has been identified. Experience with our family suggests that it may be extremely important to exclude an unusual tapetal reflex in carrier women if possible recombination events are to be interpreted correctly.

Acknowledgements. We thank Dr. Sarah Bundey, West Midlands Re- gional Genetics Services, and Mr. S. J. Crews at Birmingham and Mid- land Eye Hospital for reviewing ophthalmological findings in these patients.

R e f e r e n c e s

Bhattacharya SS, Wright AF, Clayton JF, Price WH, Phillips CI, McKeown CME, Jay M, Bird AC, Pearson PL, Southern EM, Evans HJ (1984) Close genetic linkage between X-linked retinitis pigmentosa and a restriction length polymorphism identified by recombinant DNA probe L1.28. Nature 309: 253-255

Bunker CH, Berson EL, Bromley WC, Hayes RP, Roderick TH (1984) Prevalence of retinitis pigmentosa in Maine. Am J Ophthalmol 97 : 357-365

Clayton JF, Wright AF, Jay M, McKeown CME, Depster M, Jay BS, Bird AC, Bhattacharya SS (1988) Genetic linkage between X- linked retinitis pigmentosa and DNA probe DXS7 (LI.28): fur- ther linkage data, heterogeneity testing and risk estimation. Hum Genet 74 : 168-171

202

Denton M J, Chen J-D, Serravalle S, Colley P, Halliday FB, Donald J (1988) Analysis of linkage relationship of X-linked retinits pig- mentosa with the following Xp loci: L1.28, OTC, 754, XJ-I.1, pERT87, and C7. Hum Genet 78 : 60-64

Friedrich U, Warburg M, Wieacker P, Wienker TF, Gal A, Ropers H-H (1985) X-linked retinitis pigmentosa: linkage with the centro- mere and a cloned DNA sequence from the proximal short arm of the X chromosome. Hum Genet 71 : 93-99

Human Gene Mapping 8 (1985) Helsinki Conference. (8th Interna- tional Workshop on Human Gene Mapping) Cytogenet Cell Genet 40: 308-329

Nussbaum RL, Lewis RA, Lesko JG, Ferrell R (1985) Mapping X- linked ophthalmic diseases. II. Linkage relationship of X-linked retinitis pigmentosa to X chromosomal short arm markers. Hum Genet 70: 45-50

Ott J (1974) Estimation of the recombination fraction in human pedi- grees: efficient computation of the likelihood for human linkage studies. Am J Hum Genet 26 : 588-597

Ott J (1985) Analysis of human genetic linkage. Johns Hopkins Uni- versity Press, Baltimore London

Wright AF, Bhattacharya SS, Clayton JF, Dempster M, Tippett P, McKeown CME, Jay M, Jay B, Bird AC (1987) Linkage relation- ships between X-linked retinitis pigmentosa and nine short arm markers: exclusion of the disease locus from Xp21 and localization to between DXS7 and DXS14. Am J Hum Genet 41:635-644

Received May 1, 1988 ! Revised August 24, 1988