The congenital dyserythropoietic anemias (CDAs) are a heterogeneous group of genetic disorders of erythropoiesis characterized by anemia secondary to ineffective erythropoiesis and striking morphologic abnormalities of erythroblasts (reviewed in Wickramasinghe and Wood1 ). The autosomal recessive inheritance of CDA type I has been well documented and linkage analysis in Israeli Arab families has localized the gene to chromosome 15q15.2  Analysis of genes in this region revealed mutations in a gene designated CDAN1 that were associated with CDA type I disease in both an Israeli Arab family and in sporadic cases, mostly of European origin.3-5  In the present study, we confirmed several of these mutations of the CDAN1 gene in affected patients, report a new mutation in this gene, and provide convincing evidence that not all affected patients show linkage of CDA I to 15q15, thereby demonstrating genetic heterogeneity in this condition.

In 3 unrelated consanguineous families, 13 affected CDA I individuals (diagnosed by demonstrating the Swiss-cheese appearance of erythroblast heterochromatin on electron microscopy of bone marrow from at least 1 affected member) showed mild to moderate anemia and a high or high-normal mean corpuscular volume (MCV) (Table 1). Patients from 2 unrelated consanguineous families (families A and B, both of Lebanese descent) demonstrated homozygosity for microsatellite markers in the CDA I locus across 45 members from 2 generations, and mutational analysis revealed homozygosity for the same missense mutation (3238C > T) that characterized the Israeli Arab family. It therefore seems likely that a founder effect is responsible for the disease in this population.

Table 1.

Essential clinical and laboratory data in the 3 families (A-C) and 6 sporadic patients with CDA type I


Patient/sex

Year of birth

Hb, g/L

MCV, fL

Retics, % or 109/L

Bilirubin, mM

sTK, U/L

sTfR, U/L

Ferritin, ng/mL

Genotype
A1/F   1944   90   109   —   46   —   —   1341   R1041W homozygote  
A2/M   1956   112   98   5.0%   32   99   9.1   812   R1041W homozygote  
A3*/M   1967   140   95   —   —   17   12.8   —   R1041W homozygote  
A4/M   1979   124   101   —   15   179   13.7   650   R1041W homozygote  
A5/F   1987   107   93   2.3%   39   111   14   359   R1041W homozygote  
A6/F   1992   121   97   —   —   130   12.4   —   R1041W homozygote  
A7/M   1946   116   100   —   —   92   12.2   —   R1041W homozygote  
A8/F   1950   97   97   —   —   161   11.6   —   R1041W homozygote  
A9/M   1986   100   97   —   —   86   12.5   —   R1041W homozygote  
B3/F   1960   79   103   67   42   187   59.7   1184   R1041W homozygote  
B7/M   1974   88   105   54   110   222   57.1   1592   R1041W homozygote  
C3/F   1980   103   97   106   —   78   25.2   232   WT/WT  
C5/F   1973   77   95   61   63   88   43.5   260   WT/WT  
E/M   1929   112   104   27   77   44   35.2   766   D1042/WT  
F/M   1953   120   102   73   38   117   42.6   735   P671L/WT  
G/F   1950   82   104   15   27   —   11.4   1755   N598S/R724W  
H/M   1950   99   99   146   34   144   60.9   1190   DeIV372/WT  
I/M   1977   110   92   93   32   167   59.6   382   WT/WT  
J/F
 
1927
 
105
 
111
 
43
 
25
 

 
33.1
 
766
 
P671L/WT
 

Patient/sex

Year of birth

Hb, g/L

MCV, fL

Retics, % or 109/L

Bilirubin, mM

sTK, U/L

sTfR, U/L

Ferritin, ng/mL

Genotype
A1/F   1944   90   109   —   46   —   —   1341   R1041W homozygote  
A2/M   1956   112   98   5.0%   32   99   9.1   812   R1041W homozygote  
A3*/M   1967   140   95   —   —   17   12.8   —   R1041W homozygote  
A4/M   1979   124   101   —   15   179   13.7   650   R1041W homozygote  
A5/F   1987   107   93   2.3%   39   111   14   359   R1041W homozygote  
A6/F   1992   121   97   —   —   130   12.4   —   R1041W homozygote  
A7/M   1946   116   100   —   —   92   12.2   —   R1041W homozygote  
A8/F   1950   97   97   —   —   161   11.6   —   R1041W homozygote  
A9/M   1986   100   97   —   —   86   12.5   —   R1041W homozygote  
B3/F   1960   79   103   67   42   187   59.7   1184   R1041W homozygote  
B7/M   1974   88   105   54   110   222   57.1   1592   R1041W homozygote  
C3/F   1980   103   97   106   —   78   25.2   232   WT/WT  
C5/F   1973   77   95   61   63   88   43.5   260   WT/WT  
E/M   1929   112   104   27   77   44   35.2   766   D1042/WT  
F/M   1953   120   102   73   38   117   42.6   735   P671L/WT  
G/F   1950   82   104   15   27   —   11.4   1755   N598S/R724W  
H/M   1950   99   99   146   34   144   60.9   1190   DeIV372/WT  
I/M   1977   110   92   93   32   167   59.6   382   WT/WT  
J/F
 
1927
 
105
 
111
 
43
 
25
 

 
33.1
 
766
 
P671L/WT
 

M indicates male; F, female; Hb, hemoglobin; sTK, serum thymidine kinase; sTfR, serum transferrin receptor; Retics, reticulocytes; and —, not done. Reference ranges: retics, 0.5%-3.8% (20-130 × 109/L); bilirubin, < 17 mM; sTK, 4.2-6.6 U/L; sTfR, 2.3-8.4 U/L; and ferritin, 22-402 ng/mL.

*

patient on interferon therapy.

Figure 1.

Haplotype analysis using microsatellite markers in consanguineous family C with familial CDA1. A total of 19 markers spanning the 8-Mb region of the CDAN1 locus were used for linkage analysis; only informative markers are shown. The index patient C5 lacks homozygosity over this region.

Figure 1.

Haplotype analysis using microsatellite markers in consanguineous family C with familial CDA1. A total of 19 markers spanning the 8-Mb region of the CDAN1 locus were used for linkage analysis; only informative markers are shown. The index patient C5 lacks homozygosity over this region.

In affected individuals of the other consanguineous CDA I family (C; Pakistani), with hematologic results typical of CDA I and a substantial proportion of the erythroblasts showing the “Swiss-cheese” abnormality on electron microscopy, there was no homozygosity for the microsatellite markers across the extended CDAN1 locus and sequence analysis detected no mutations in the CDAN1 gene (Figure 1). Clearly the disease is not linked to chromosome 15q15 in this family, strongly suggesting that there is at least one alternative genetic locus for CDA I.

We also investigated 6 sporadic cases with CDA I for CDAN1 gene mutations by bidirectional genomic sequencing. Six mutant alleles were found in these cases, 5 previously reported (3242A > T, 2129C > T [2 patients], 1910A > G, and 2287C > T) together with a previously unreported 3 base pairs in frame deletion. resulting in deletion of valine 372. In 4 of the 6 sporadic cases, only 1 mutation in the CDAN1 gene could be demonstrated, a situation also seen in half of the sporadic cases described previously,3-5  and in 1 case, with multiple dysmorphic features, no mutations were detected. Mutations in regions of the CDAN1 gene that were not sequenced (promoter, introns) or in regulatory sequences may explain these findings. Given the genetic heterogeneity of this condition, however, it is plausible that a mutation in a second gene could interact with a CDAN1 mutation to result in the CDA I phenotype as a result of digenic inheritance.

A more comprehensive understanding of CDA I will require a better knowledge of the function of CDAN1 protein, its cellular distribution and intracellular localization, and the existence of any putative binding partners. The genetic heterogeneity demonstrated here indicates that this should be a fruitful field for the study of erythropoiesis.

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