Clinical features and molecular analysis of the alpha thalassemia/mental retardation syndromes. II. Cases without detectable abnormality of the alpha globin complex.

We have identified five unrelated patients, all of north European origin, who have hemoglobin H (Hb H) disease and profound mental handicap. Surprisingly, detailed molecular analysis of the alpha globin complex is normal in these subjects. Clinically, they present with a rather uniform constellation of abnormalities, notably severe mental handicap, microcephaly, relative hypertelorism, unusual facies and genital anomalies. Hematologically, their Hb H disease has subtly but distinctly milder properties than the recognized Mendelian forms of the disease. These common features suggest that these five "nondeletion" patients have a similar underlying mutation, quite distinct from the 16p13.3 deletion associated with alpha thalassemia and mild to moderate mental retardation described in the accompanying paper. We speculate that the locus of this underlying mutation is not closely linked to the alpha globin complex and may encode a trans-acting factor involved in the normal regulation of alpha globin expression.     

Chromosomal 16p microdeletion in Rubinstein-Taybi syndrome detected by oligonucleotide-based array comparative genomic hybridization: a case report

Introduction

Chromosomal aberrations of chromosome 16 are uncommon and submicroscopic deletions have rarely been reported. At present, a cytogenetic or molecular abnormality can only be detected in 55% of Rubinstein-Taybi syndrome patients, leaving the diagnosis in 45% of patients to rest on clinical features only. Interestingly, this microdeletion of 16 p13.3 was found in a young child with an unexplained syndromic condition due to an indistinct etiological diagnosis. To the best of our knowledge, no evidence of a microdeletion of 16 p13.3 with contiguous gene deletion, comprising cyclic adenosine monophosphate-response element-binding protein and tumor necrosis factor receptor-associated protein 1 genes, has been described in typical Rubinstein-Taybi syndrome.

Case presentation

We present the case of a three-year-old Malaysian Chinese girl with a de novo microdeletion on the short arm of chromosome 16, identified by oligonucleotide array-based comparative genomic hybridization. Our patient showed mild to moderate global developmental delay, facial dysmorphism, bilateral broad thumbs and great toes, a moderate size atrial septal defect, hypotonia and feeding difficulties. A routine chromosome analysis on 20 metaphase cells showed a normal 46, XX karyotype. Further investigation by high resolution array-based comparative genomic hybridization revealed a 120 kb microdeletion on chromosomal band 16 p13.3.

Conclusion

A mutation or abnormality in the cyclic adenosine monophosphate-response element-binding protein has previously been determined as a cause of Rubinstein-Taybi syndrome. However, microdeletion of 16 p13.3 comprising cyclic adenosine monophosphate-response element-binding protein and tumor necrosis factor receptor-associated protein 1 genes is a rare scenario in the pathogenesis of Rubinstein-Taybi syndrome. Additionally, due to insufficient coverage of the human genome by conventional techniques, clinically significant genomic imbalances may be undetected in unexplained syndromic conditions of young children. This case report demonstrates the ability of array-based comparative genomic hybridization to offer a genome-wide analysis at high resolution and provide information directly linked to the physical and genetic maps of the human genome. This will contribute to more accurate genetic counseling and provide further insight into the syndrome.     

Molecular characterization of a 130-kb terminal microdeletion at 22q in a child with mild mental retardation.

We have analyzed a recently described 22q13.3 microdeletion in a child with some overlapping features of the cytologically visible 22q13.3 deletion syndrome. Patient NT, who shows mild mental retardation and delay of expressive speech, was previously found to have a paternal microdeletion in the subtelomeric region of 22q. In order to characterize this abnormality further, we have constructed a cosmid/P1 contig covering the terminal 150 kb of 22q, which encompasses the 130-kb microdeletion. The microdeletion breakpoint is within the VNTR locus D22S163. The cloning of the breakpoint sequence revealed that the broken chromosome end was healed by the addition of telomeric repeats, indicating that the microdeletion is terminal. This is the first cloned terminal deletion breakpoint on a human chromosome other than 16p. The cosmid/P1 contig was mapped by pulsed-field gel electrophoresis analysis to within 120 kb of the arylsulfatase A gene, which places the contig in relation to genetic and physical maps of the chromosome. The acrosin gene maps within the microdeletion, approximately 70 kb from the telomere. With the distal end of chromosome 22q cloned, it is now possible to isolate genes that may be involved in the overlapping phenotype of this microdeletion and 22q13.3 deletion syndrome.     

Genetic modifiers in hemoglobinopathies

Hereditary anemias show considerable variation in their clinical presentation. In some cases, the causes of these variations are easily apparent. In thalassemia (or in HbE/thalassemia), genetic variation is primarily caused by the severity of the thalassemia mutation. However, not uncommonly, there is variation unexplained by the globin gene mutations themselves, which may be caused by genetic modifiers. In sickle cell disease, the primary mutation is the same in all patients. Therefore, variations in disease severity generally are due to genetic modifiers. In most genetic diseases involving beta globin, the most clearcut influence on phenotype results from elevated fetal hemoglobin levels. In addition, alpha globin gene number can influence disease phenotype. In thalassemia major or intermedia, reduction in the number of alpha globin genes can ameliorate the disease phenotype; conversely, excess alpha globin genes can convert beta thalassemia trait to a clinical picture of thalassemia intermedia. In sickle cell disease, the number of alpha globin genes has both ameliorating and exacerbating effects, depending on which disease manifestation is being examined. Unlinked genetic factors have substantial effects on the phenotype of hereditary anemias, both on the anemia and other disease manifestations. Recently, studies using genome-wide techniques, particularly studying QTLs causing elevated HbF, or affecting HbE/thalassemia, have revealed other genetic elements whose mechanisms are under study. The elucidation of genetic modifiers will hopefully lead to more rational and effective management of these diseases.     

De novo truncation of chromosome 16p and healing with (TTAGGG)n in the alpha-thalassemia/mental retardation syndrome (ATR-16).

We have previously described a series of patients in whom the deletion of 1-2 megabases (Mb) of DNA from the tip of the short arm of chromosome 16 (band 16p13.3) is associated with alpha-thalassemia/mental retardation syndrome (ATR-16). We now show that one of these patients has a de novo truncation of the terminal 2 Mb of chromosome 16p and that telomeric sequence (TTAGGG)n has been added at the site of breakage. This suggests that the chromosomal break, which is paternal in origin and which probably arose at meiosis, has been stabilized in vivo by the direct addition of the telomeric sequence. Sequence comparisons of this breakpoint with that of a previously described chromosomal truncation (alpha alpha)TI do not reveal extensive sequence homology. However, both breakpoints show minimal complementarity (3-4 bp) to the proposed RNA template of human telomerase at the site at which telomere repeats have been added. Unlike previously characterized individuals with ATR-16, the clinical features of this patient appear to be solely due to monosomy for the terminal portion of 16p13.3. The identification of further patients with "pure" monosomy for the tip of chromosome 16p will be important for defining the loci contributing to the phenotype of this syndrome.     

Genomic regions associated with microdeletion/microduplication syndromes exhibit extreme diversity of structural variation

Segmental duplications (SDs) are a class of long, repetitive DNA elements whose paralogs share a high level of sequence similarity with each other. SDs mediate chromosomal rearrangements that lead to structural variation in the general population as well as genomic disorders associated with multiple congenital anomalies, including the 7q11.23 (Williams–Beuren Syndrome, WBS), 15q13.3, and 16p12.2 microdeletion syndromes. Population-level characterization of SDs has generally been lacking because most techniques used for analyzing these complex regions are both labor and cost intensive. In this study, we have used a high-throughput technique to genotype complex structural variation with a single molecule, long-range optical mapping approach. We characterized SDs and identified novel structural variants (SVs) at 7q11.23, 15q13.3, and 16p12.2 using optical mapping data from 154 phenotypically normal individuals from 26 populations comprising five super-populations. We detected several novel SVs for each locus, some of which had significantly different prevalence between populations. Additionally, we localized the microdeletion breakpoints to specific paralogous duplicons located within complex SDs in two patients with WBS, one patient with 15q13.3, and one patient with 16p12.2 microdeletion syndromes. The population-level data presented here highlights the extreme diversity of large and complex SVs within SD-containing regions. The approach we outline will greatly facilitate the investigation of the role of inter-SD structural variation as a driver of chromosomal rearrangements and genomic disorders.     

Further delineation of novel 1p36 rearrangements by array-CGH analysis: Narrowing the breakpoints and clarifying the "extended" phenotype

High resolution oligonucleotide array Comparative Genome Hybridization technology (array-CGH) has greatly assisted the recognition of the 1p36 contiguous gene deletion syndrome. The 1p36 deletion syndrome is considered to be one of the most common subtelomeric microdeletion syndromes and has an incidence of ~1 in 5000 live births, while respectively the "pure" 1p36 microduplication has not been reported so far. We present seven new patients who were referred for genetic evaluation due to Developmental Delay (DD), Mental Retardation (MR), and distinct dysmorphic features. They all had a wide phenotypic spectrum. In all cases previous standard karyotypes were negative. Array-CGH analysis revealed five patients with interstitial 1p36 microdeletion (four de novo and one maternal) and two patients with de novo reciprocal duplication of different sizes. These were the first reported "pure" 1p36 microduplication cases so far. Three of our patients carrying the 1p36 microdeletion syndrome were also found to have additional pathogenetic aberrations. These findings (del 3q27.1; del 4q21.22-q22.1; del 16p13.3; dup 21q21.2-q21.3; del Xp22.12) might contribute to the patients' severe phenotype, acting as additional modifiers of their clinical manifestations. We review and compare the clinical and array-CGH findings of our patients to previously reported cases with the aim of clearly delineating more accurate genotype-phenotype correlations for the 1p36 syndrome that could allow for a more precise prognosis.     

Partial 5p monosomy or trisomy in 11 patients from a family with a t(5;15)(p13.3;p12) translocation

A family with six alive patients with partial monosomy 5p and five with partial trisomy 5p due to a t(5;15)(p13.3;p12) translocation is reported. The translocation was present in four generations with eight balanced carriers. This is the first molecular-cytogenetic and clinical study with both syndromes present in the same family. Using fluorescence in situ hybridization (FISH) with bacterial artificial chromosome (BAC) probes, the breakpoint was mapped to 5p13.3, in the interval corresponding to the BAC clone RP11-1079N14, thereof resulting a 5pter-5p13.3 deletion or duplication of ~32 Mb. These chromosome imbalances can be considered pure, since the other imbalance produced involving chromosome 15p has no phenotypic effect. The presence of several individuals with 5p monosomy and 5p trisomy in the same family is valuable for a better delineation of both syndromes.     

Characterization of three de novo derivative chromosomes 16 by “Reverse chromosome painting” and molecular analysis

We have analyzed three de novo chromosome 16 rearrangements—two with a 16p+ chromosome and one a 16q+—none of which could be fully characterized by conventional cytogenetics. In each case, flow karyotypes have been produced, and the aberrant chromosome has been isolated by flow sorting. The origin of the additional material has been ascertained by amplifying and labeling the DNA of the abnormal chromosome by degenerate-oligonucleotide-primer–PCR and hybridizing it in situ to normal metaphase spreads (reverse chromosome painting). Both 16p+ chromosomes contain more than 30 Mb of DNA from the short arm of chromosome 9 (9p21.2-pter), while the 16q+ contains approximately 9 Mb of DNA from 2q37. The breakpoints on chromosome 16 have been localized in each case; the two breakpoints on the short arm are at different points within the terminal band, 16p13.3. The breakpoint on the long arm of chromosome 16 is very close to (within 230 kb of) the 16q telomere. Determination of the regions of monosomy and trisomy allowed the observed phenotypes to be compared with other reported cases involving aneuploidy for these regions.     

Deletion at chromosome 16p13.3 as a cause of Rubinstein-Taybi syndrome: clinical aspects.

In the accompanying paper, a chromosomal localization of the Rubinstein-Taybi syndrome by cytogenetic investigations with fluorescence in situ hybridization techniques at chromosome 16p13.3 is described. We investigated 19 of these patients and their parents (a) to ascertain the parental origin of the chromosome with the deletion in families where such a deletion was detected, (b) to disclose whether uniparental disomy plays a role in etiology, and (c) to compare clinical features in patients with a deletion to those in individuals in whom deletions were not detectable. Molecular studies showed a copy of chromosome 16 from each parent in all 19 patients. Uniparental disomy was also excluded for five other chromosome arms known to be imprinted in mice. None of the probes used for determining the origin of the deleted chromosome proved to be informative. The clinical features were essentially the same in patients with and without visible deletion, with a possible exception for the incidence of microcephaly, angulation of thumbs and halluces, and partial duplication of the halluces. A small deletion at 16p13.3 may be found in some patients with Rubinstein-Taybi syndrome. Cytogenetically undetectable deletions, point mutations, mosaicism, heterogeneity, or phenocopy by a nongenetic cause are the most probable explanations for the absence of cytogenetic or molecular abnormalities in other patients with Rubinstein-Taybi syndrome.     

Localization of the alpha-like globin gene cluster to region q12 of rabbit chromosome 6 by in situ hybridization

The rabbit (Oryctolagus cuniculus) alpha-like globin gene cluster (HBAC) contains several block duplications of zeta-, alpha and theta-globin genes. Using in situ hybridizations to metaphase chromosome spreads, the gene cluster has been mapped to region q12 of chromosome 6. Given that human HBAC maps to the short arm of chromosome 16, the mapping of rabbit HBAC to 6q12 confirms the assignment of homology between OCU6q and HSA16p based on similarities of chromosomal banding patterns. In both species, HBAC is in a very G + C-rich region within the most distal band of the chromosome.     

Detection of deletions and cryptic translocations in Miller-Dieker syndrome by in situ hybridization.

Fluorescence in situ hybridization (FISH) using two cosmid probes (41A and P13) from the Miller-Dieker syndrome (MDS) critical region in 17p13.3 was performed in a blinded comparison of three MDS patients with submicroscopic deletions and in four normal relatives used as controls. The controls showed both chromosome 17 homologues labeled in 85%-95% of cells, while each patient showed only one homologue labeled in 75%-80% of cells. Two MDS patients with cryptic translocations were also studied. In one case, a patient and her mother had the same der(17) (p+), but the reciprocal product of the translocation could not be identified in the mother by G-banding (i.e., it was a "half-cryptic" translocation). FISH revealed a 3q;17p translocation. The other case involved a patient with apparently normal karyotype. Because a large molecular deletion was found, a translocation involving two G-negative telomeres (i.e., a "full-cryptic" translocation) was postulated. FISH studies on her father and normal brother showed an 8q;17p translocation. These studies demonstrate that in situ hybridization is an efficient method for deletion detection in Miller-Dieker syndrome. More important, parental studies by FISH on patients demonstrating molecular deletions and a normal karyotype may identify cryptic translocation events, which cannot be detected by other molecular genetic strategies. Similar in situ strategies for deletion detection can be developed for other microdeletion syndromes, such as Prader-Willi/Angelman syndrome or DiGeorge syndrome.     

应用实时定量RT-PCR技术检测b地中海贫血 珠蛋白基因的表达

为了定量检测 b 地中海贫血(b 地贫)的 a、b 和γ珠蛋白基因表达水平, 提取正常成人对照组、正常胎儿对照组和b 地贫患者组组成的样本 DNA,采用反向点杂交法（RDB）分析b 地贫各种突变类型;提取样本RNA用于进行针对a、b 和γ珠蛋白基因的荧光实时定量RT-PCR（FQ RT-PCR）。根据FQ RT-PCR原理,设计合成分别对应于a、b 和γ珠蛋白基因的3对引物和3条荧光探针,FQ RT-PCR在ABI 7700系统进行。用SPSS 10.0对实验数据进行统计学分析,分别计算正常对照组 (bA/bA,aa/aa),脐带血组(bA/bA,aa/aa),轻型b 地贫组(bT/bA,aa/aa),重型b地贫组(bT/bT,aa/aa)的a、b 和γmRNA比值,其中a/b分别为4.62±1.20、7.81±2.89、13.51±5.12、188.24±374.04;a/(b +γ)分别为4.43±1.17、0.56±0.49、9.62±4.37、2.14±1.58;γ/(b+γ) 分别为0.04±0.03、0.92±0.06、0.28±0.18、0.95±0.04。由于组与组之间均值变异范围较大,将其进行对数转换后再进行方差分析。结果表明： a/b与a/(b+γ)在所有组与组之间均有显著性差异。γ/(b+γ)除了在脐带血组和重型b地贫组之间无显著性差异外,在其他组与组之间均有显著性差异。实验说明,人类b珠蛋白基因的表达水平从正常对照组到重型b地贫组急剧下降且以重型b地贫组为最低;相反γ珠蛋白基因表达却明显升高,以重型b地贫组为最高。与正常成人对照组相比,胎儿期b mRNA水平较低但γmRNA 水平较高。因此,正常个体不同时期和不同类型b 地贫之间a、b与γ珠蛋白基因表达不同而且互相影响。 Abstract：whole blood samples were collected from 100 normal healthy adults, from umbilical cord of 33 newborn infants, 111 individuals with b-thalassemia minor (bT/bA,aa/aa) and 39 with b-thalassemia major (bT/bT,aa/aa). Prior to quantitative analysis of globin gene expression, DNA was extracted from all blood samples and used for b-thalassemia genotype analysis. Different types of b globin gene mutations were analyzed using reverse dot blotting (RDB) method. Total RNA were extracted and subjected to real-time RT-PCR for quantitative measurement of a, b andγglobin mRNA using three sets of primers and fluorescent-labeled probes, designed according to the sequences of a, b andγhuman globin gene. Real-time RT-PCR was performed in ABI 7700 system. Following the real-time RT-PCR, the mean values of a, b andγglobin mRNA were calculated and the ratios of a/b, a/(b + γ) andγ/(b + γ) were determined to characterize the relative expression levels of different globin genes among normal adult, infant, b-thalassemia minor and b-thalassemia major patients. The resultant data were analyzed using SPSS 10.0 software to determine statistical significance of human globin gene expression among normal controls and b-thalassemia patients. Due to vast variations of the mean globin gene mRNA levels among different groups, log conversion of a/b + 1, a/(b + γ) + 1 andγ/(b + γ) +1 was used for statistical analyses and intergroup comparison. The a/b globin gene mRNA ratios were determined to be 4.62±1.20, 7.81±2.89, 13.51±5.12, and 188.24±374.04 for normal healthy adult (bA/bA,aa/aa), infant (bA/bA,aa/aa), b- thalassemia minor (bT/bA,aa/aa) and b-thalassemia major(bT/bT,aa/aa) respectively. The a/(b+γ) ratios were 4.43±1.17, 0.56±0.49, 9.62±4.37, and 2.14±1.58 for normal healthy adult (bA/bA,aa/aa), infant (bA/bA,aa/aa), b- thalassemia minor (bT/bA,aa/aa) and b- thalassemia major(bT/bT,aa/aa) respectively. Theγ/(b+γ) ratios were 0.04±0.03, 0.92±0.06, 0.28±0.18, and 0.95±0.04 for normal healthy adult (bA/bA,aa/aa), infant (bA/bA,aa/aa), b- thalassemia minor (bT/bA,aa/aa) and b- thalassemia major(bT/bT,aa/aa) respectively. Following statistical analyses, the a/b and a/(b+γ) globin gene mRNA ratios were significantly different among four different groups (normal adult, normal infant, b- thalassemia minor and b- thalassemia major). The γ/(b + γ) globin gene mRNA ratio was significantly different among all groups except for between infant and b- thalassemia major patients. Human b globin gene mRNA levels decrease progressively and dramatically from normal adults to b-thalassemia patients with b-thalassemia major having the lowest levels. On the other hand, the γglobin gene mRNA levels increase progressively from normal adult to b-thalassemia patients with b-thalassemia major having the highest levels. Infants have relatively lower levels of b but higher levels of γglobin gene mRNA as compared to those in normal adults. Thus, the relative expression levels of a, b or γglobin genes varied but inter-related among different ages of normal individuals and different b-thalassemia genotypes.     

Mapping of autosomal dominant osteopetrosis type II (Albers-Schönberg disease) to chromosome 16p13.3

The osteopetroses are a heterogeneous group of conditions characterized by a bone-density increase due to impaired bone resorption. As well as the two or more autosomal recessive types, two autosomal dominant forms of osteopetrosis, differentiated by clinical and radiological signs, are described. Autosomal dominant osteopetrosis (ADO) type II, also known as "Albers-Sch?nberg disease," is characterized by sclerosis, predominantly involving the spine (vertebral end-plate thickening, or Rugger-Jersey spine), the pelvis ("bone-within-bone" structures), and the skull base. An increased fracture rate can be observed in these patients. By linkage analysis, the presence, on chromosome 1p21, of a gene causing ADO type II was previously suggested. However, analysis of further families with ADO type II indicated genetic heterogeneity within ADO type II, with the chromosome 1p21 locus being only a minor locus. We now perform a genomewide linkage scan of a French extended family with ADO type II, which allows us to localize an ADO type II gene on chromosome 16p13.3. Analysis of microsatellite markers in five further families with ADO type II could not exclude this chromosomal region. A summed maximum LOD score of 12.70 was generated with marker D16S3027, at a recombination fraction (straight theta) of 0. On the basis of the key recombinants in the families, a candidate region of 8.4 cM could be delineated, flanked by marker D16S521, on distal side, and marker D16S423, on the proximal side. Surprisingly, one of the families analyzed is the Danish family previously suggested to have linkage to chromosome 1p21. Linkage to chromosome 16p13.3 clearly cannot be excluded in this family, since a maximum LOD score of 4.21 at theta=0 is generated with marker D16S3027. Because at present no other family with ADO type II has proved to have linkage to chromosome 1p21, we consider the most likely localization of the disease-causing gene in this family to be to chromosome 16p13.3. This thus reopens the possibility that ADO type II is genetically homogeneous because of a single gene on chromosome 16p13.3.     

Evidence for a new contiguous gene syndrome, the chromosome 16p13.3 deletion syndrome alias severe Rubinstein–Taybi syndrome

Rubinstein–Taybi syndrome (RSTS) is a well-known autosomal dominant mental retardation syndrome with typical facial and skeletal abnormalities. Previously, we have reported two patients presenting with RSTS and additional clinical features including failure to thrive, seizures, and intractable infections (Bartsch et al. in Eur J Hum Genet 7:748–756, 1999). Recently we identified a third patient with this condition, termed here severe RSTS, or chromosome 16p13.3 deletion syndrome. The three patients died in infancy, and all displayed a specific mutation, a chromosomal microdeletion including the 3′-end of the CREBBP gene. Using fluorescence in situ hybridization and closely spaced DNA probes, we characterized the deletion intervals in these patients and in three individuals with a deletion of CREBBP and typical RSTS. The deleted DNA segments were found to greatly vary in size, spanning from ∼40 kb to >3 Mb. Four individuals, including the patients with severe RSTS, exhibited deletions containing gene/s in addition to CREBBP. The patients with severe RSTS all had deletions comprising telomeric neighbor genes of CREBBP, including DNASE1, a dominant gene encoding a nuclease that has been associated with systemic lupus erythematodes. Our findings suggest that severe RSTS is distinct from RSTS and represents a novel true contiguous gene syndrome (chromosome 16p13.3 deletion syndrome). Because of the risk of critical infections and high mortality rate, we recommend that the size of the deletion interval should be determined in CREBBP deletion-positive patients with RSTS, especially in young children. Further studies are needed to delineate the clinical spectrum of the new disorder and to clarify the role of DNASE1.     

Clinical and molecular diagnosis of Miller-Dieker syndrome.

We report results of clinical, cytogenetic, and molecular studies in 27 patients with Miller-Dieker syndrome (MDS) from 25 families. All had severe type I lissencephaly with grossly normal cerebellum and a distinctive facial appearance consisting of prominent forehead, bitemporal hollowing, short nose with upturned nares, protuberant upper lip, thin vermilion border, and small jaw. Several other abnormalities, especially growth deficiency, were frequent but not constant. Chromosome analysis showed deletion of band 17p13 in 14 of 25 MDS probands. RFLP and somatic cell hybrid studies using probes from the 17p13.3 region including pYNZ22 (D17S5), pYNH37 (D17S28), and p144-D6 (D17S34) detected deletions in 19 of 25 probands tested including seven in whom chromosome analysis was normal. When the cytogenetic and molecular data are combined, deletions were detected in 21 of 25 probands. Parental origin of de novo deletions was determined in 11 patients. Paternal origin occurred in seven and maternal origin in four. Our demonstration of cytogenetic or molecular deletions in 21 of 25 MDS probands proves that deletion of a "critical region" comprising two or more genetic loci within band 17p13.3 is the cause of the MDS phenotype. We suspect that the remaining patients have smaller deletions involving the proposed critical region which are not detected with currently available probes.     

Mouse beta thalassemia, a model for the membrane defects of erythrocytes in the human disease

The present study describes the pathophysiology, at the cellular level, of the mouse beta thalassemia and shows the pertinence of this model for the human disease. The homozygous state of mouse beta thalassemia is characterized by a clinical syndrome similar to the human beta thalassemia intermedia, but it cannot be explained by the small deficiency in beta chain synthesis. The small pool of unpaired and soluble alpha chains present in mouse reticulocytes contrasts with the large amount of insoluble alpha chains in erythrocytes which is induced by the high instability of mouse alpha chains and the absence of significant proteolysis. The amount of insoluble alpha chains associated with red cell ghosts is similar in human and mouse disease of similar severity. The study of membrane protein defects showed a decreased amount of spectrin (alpha and beta chains) and dramatic changes in the distribution of the most reactive thiol groups of membrane proteins. These results were similar to that previously described in the human disease (Rouyer-Fessard, P., Garel, M. C., Domenget, C., Guetarni, D., Bachir, D., Colonna, P., and Beuzard, Y. (1989) J. Biol. Chem. 264, 19092-19098). Abnormal density distribution curves of erythrocytes and oxidant-induced lysis of red blood cells used as functional tests were similar in the human and mouse beta thalessemia. We conclude from the present study that 1) mouse beta thalassemia is an excellent model for the membrane defects occurring in the human disease; 2) disease expression is not the reflection of the globin chain unbalance only nor of the soluble pool of alpha hemoglobin chain but mainly is a reflection of insoluble alpha chains; and 3) the rate of proteolysis and instability of alpha chains are important factors which must be taken into consideration in the pathophysiology and the clinical heterogeneity of the disease.