Genetic Heterogeneity of Congenital Dyserythropoietic Anemia Type I

To the Editor:

Congenital dyserythropoietic anemia type I (CDA type I) is an uncommon disorder of unknown etiology characterized by a variable degree of anemia, macrocytosis, internuclear chromatin bridges between incompletely separated erythroblasts, a striking ultrastructural abnormality (spongy or ‘Swiss-cheese’ appearance) of erythroblast heterochromatin, grossly ineffective erythropoiesis, and autosomal-recessive inheritance.1,2 Some patients have dysmorphic features such as syndactyly, absent phalanges, and absent or dysplastic nails. The disease course may be complicated by iron overload. Recently, Tamary et al3 4 reported the localization of the CDA type I disease gene (CDAN1) by homozygosity mapping and linkage-disequilibrium and haplotype analysis to chromosome 15q15.1-15.3. They studied 25 Bedouins with CDA type I living in Southern Israel from 4 large consanguineous families, and the existence of a founder mutation arising more than 400 years ago was suggested. Haplotype analysis localized the gene to a 0.5-cM region between microsatellite markers D15S778 and D15S779 corresponding to 1.3 to 1.6 Mb. Within this region the haplotypes of all the cases were identical, with allele homozygosity at both D15S780 and D15S515. Furthermore, 88% of affected individuals were homozygous at D15S778 and D15S779, with only 3 members from 2 different families showing heterozygosity at one of these markers.

In the present study, we used the 4 haplotype markers that were most closely associated with the disease gene in this highly inbred Bedouin family to determine whether a similar haplotype was found in 6 English and 2 Lebanese patients with CDA type I. The diagnosis of CDA type I was based on macrocytosis; various dysplastic changes in erythroblasts, including internuclear chromatin strands, and the presence of the ‘Swiss-cheese’ ultrastructural abnormality in the heterochromatin of a substantial proportion of the erythroblasts. One CDA type I family with 2 affected individuals were among the 8 patients studied. Haplotype analysis for markers D15S778, D15S779, D15S780, and D15S515 was performed using fluorescently labeled polymerase chain reaction primers (5′ FAM or HEX) and ABI 310 GeneScanner (PE Applied Biosystems, Foster City, CA) capillary electrophoresis analysis. Genotyper software (PE Applied Biosystems) was used to call the different alleles.

The results from the haplotype analysis are given in Table1. The alleles were numbered according to size (bp) starting with the highest number of dinucleotide repeats. D15S779 alleles, numbered 1 to 6, were 94, 90, 88, 84, 82, and 80 bp, respectively. Similarly, at marker D15S780, alleles 1, 2, and 3 were 132, 130, and 128 bp, respectively. The single allele at D15S515 was 191 bp and the 5 alleles at marker D15S778, numbered 1 to 5, were 181, 179, 177, 175, and 173 bp, respectively. Allele heterogeneity was observed in marker D15S778 for 6 of the 8 CDA type I patients and both unaffected relatives. The 2 related CDA type I patients (cases 7 and 8) are homozygous for allele 2 at this marker. Similarly, for marker D15S779 the 2 related CDA type I patients are homozygous for allele 4; cases 1 and 10 are homozygous for allele 6 and case 2 is homozygous for allele 1. All other cases show heterozygosity at this locus. At D15S780, 4 CDA type 1 patients are homozygous for allele 1 and all other cases are heterozygous at this locus. Marker D15S515 shows homozygosity for allele 1 in all 10 cases. The results indicate that, unlike in the Israeli Bedouins, a high degree of heterogeneity exists at these markers in our cases of CDA type I. Each of the 6 English patients had a different haplotype, with only the 2 related Lebanese CDA type I patients showing identical haplotypes with allele homozygosity at all 4 markers. Although the allele numbers at these markers were different from those reported for the Israeli CDA type I cases, in the absence of experimental detail on how the alleles were assigned in Tamary’s study, it cannot be ruled out that the Lebanese patients may have the same haplotype as the Israeli Bedouins. The data on the English patients suggest either that mutations in 1 or more genes other than CDAN1 can cause CDA type I, that the same mutation or different mutations in the CDAN1 gene arose independently on a number of occasions on completely different genetic backgrounds, or that there was a single mutation in the CDAN1 gene that occurred on a particular haplotype background and that recombinations have occurred to change the haplotypes. Interestingly, genetic heterogeneity has also been reported in CDA type II. Linkage analysis, performed mainly in Italian families where a founder mutation is suggested, localized the CDAN2 gene to the long arm of chromosome 20 (20q11.2).5 Recently, however, 2 unrelated families where the CDA type II disease was not linked to the CDAN2 locus have been described.6 Haplotype analysis performed in our study indicates that CDA type I is also a heterogeneous disorder.