Somatic ATRX mutations: ATMDS
Although aberrant templates of hemoglobin synthesis are almost always inherited, sporadically individuals with previously normal hematology may develop abnormal hemoglobin synthesis as an acquired abnormality . There are a number of reports describing α thalassemia as newly acquired traits in the background of hematologic malignancy . This syndrome was characterized by a marked hypochromic and microcytic anemia with the presence of HbH (β4 tetramers) and named α-thalassemia myelodysplastic syndrome or ATMDS . In the light of these findings, an acquired α thalassemia patients registry was established in the early 1980s . ATMDS predominantly occurs in male (male-to-female ratio greater than 6:1) within the 7th decade of life . Acquired α thalassemia is not limited to the geographical regions in which the inherited forms of α thalassemia are common (e.g., the Mediterranean basin, Southeast Asia, Africa, and Melanesia). Most of ATMDS patients have been of Northern European descent and few Mediterranean and Asian patient have been reported to date. It is feasible that this distribution represents ascertainment bias. If microcytic, hypochromic red cell indices or other signs of thalassemia were apperceived in a patient with hematologic malignancy originating from an area in which inherited forms of thalassemia are common, it is likely that these findings would be related to a previously unrecognized inherited thalassemia, and such a hypothesis would usually be correct. However, rare cases of acquired α thalassemia in patients with these areas of the world origin may have been missed. On the other hand, thalassemia red cell indices in an individual with hematologic malignancy who originate from outside of the malaria belt (e.g., from Northern Europe) are unanticipated and should operate further evaluations [12, 13].
At least 2 molecular mechanisms for acquired α-thalassemia are presented today: cis-acting defects including acquired deletion of the α-globin gene cluster limited to the neoplastic clone and, more commonly, inactivating somatic mutations of the trans-acting regulator of globin gene expression ATRX, which cause significant down regulation of α-globin gene expression [11, 14]. It is now demonstrated that most patients with ATMDS have an acquired somatic splicing abnormality or point mutation involving ATRX .
The rare association of α-thalassemia and mental retardation (MR) was presented over 36 years ago in northern European origin patients by Weatherall and colleagues . It is now distinct that this association may occur as a result of two quite distinguished mechanisms; one resulting from large deletions in telomeric region of chromosome 16 (α thalassemia with retardation on chromosome 16, ATR-16 syndrome OMIM catalog #141750); the other, caused by mutations in ATRX gene (ATR-X syndrome) . The main clinical features of ATR-X syndrome include severe psychomotor delay, abnormal facial appearance, microcephaly, urogenital abnormalities and a variable degree of α thalassemia, a condition caused by deficient α-globin expression [17, 18]. Patients are characterized during early childhood . ATR-X syndrome is predominant in males, and almost all female carriers have a normal appearance and intellectual ability, although approximately one in four carriers has subtle signs of a-thalassemia, which show an skewed pattern of X-inactivation that lead to the expression of the mutant allele [17, 19]. The molecular cause of ATR-X is mainly from point mutations in the ATRX gene . More over in some cases the disease appears to raise de novo. In fact, a number of families have been reported in which some or all of the affected members with mutations of ATRX, and the characteristic manifestations described previously, have no signs of α-thalassemia [17, 20, 21]. In ATR-X patients which have abnormal development of the genitalia (e.g., male pseudohermaphroditism), testis characteristics occurs but the cellular components maturation is failed. It is possible that many of the genes whose regulation is perturbed by ATRX mutations have critical rule in terminal differentiation. It is noteworthy that ATR-X patients do not have an increased incidence of cancer .
ATRX gene and protein
The ATRX gene is located on the Xq13.3; contains 35 exons (Reference sequence NG_008838.3; https://www.ncbi.nlm.nih.gov/). ATRX is a relatively large protein consists of 2492 amino acids (283 kDa). It is a chromatin- associated protein with ATRX-DNMT3-DNMT3L (ADD) domain (encoded by exons 8–10) containing GATA-like zinc finger at the N-terminus, and a long C-terminal that pack together to form a single globular domain containing a helicase/ATP domain (encoded by exons 18–31) [23, 24]. The previous domain formed by seven conserved “helicase” motifs found in DNA-stimulated ATPases and DNA helicases of the SNF2/SWI2 (Switching defective/Sucrose nonfermenting) protein family. The SWI/SNF complexes act as global gene regulators, changing the chromatin structure and altering the accessibility of transcriptin factor to DNA in a subset of specific genes . The ADD and helicase/ATPase are extremely conserved domains. These proteins frequently exist in multicomponent complexes that remodel chromatin and thereby influence multiple epigenetic nuclear processes (e.g., DNA replication, DNA repair, DNA methylation, recombination, transcription). ATRX is widely expressed throughout development [26, 27].
The CpG methylation at heterochromatic loci is disturbed in patients with inherited mutations in ATRX gene. The reality that ATRX mutations affect α but apparently not β globin expression may be informative. These two gene clusters are embedded in exactly different chromosomal environments, which supports the vision that ATRX influences gene expression via one or more of the epigenetic aspects that distinguish these different regions .
Various types of mutations including missense, nonsense, in/del, duplication and splice site have now been reported in ATRX gene in related to ATR-X syndrome (Fi. 2, 3). It has been hypothesized that in ATR-X syndrome, both copies of gene are inactivated in the patient, one by mutation and the other by X inactivation . Considering the monogenic nature of ATR-X syndrome, it is important to recognize the type of mutations involved and their clinical severity. For example, the ADD domain amino acids mutations lead to more severe psychomotor phenotypes than the helicase domain mutations. Interestingly, a nonsense mutation at residue 37 of ATRX is associated with a milder phenotype than the phenotype created by missense mutations in the ADD and helicase domains . The nonsense mutation at residue 37 is spliced out of a proper subset of transcripts, which partially compensate ATRX protein function. Interestingly, mutations in genes that encode proteins cooperate with ATRX, such as DAXX, have not been identified in patients with ATR-X syndrome .
The hematologic phenotype in patients with ATMDS is in general, more severe than that seen in boys with congenital ATR-X syndrome. For example, α/β globin synthesis ratios are usually very low (< 0.2 in 52% of patients) and the amounts of HbH are large (median 30%) in a patients with ATMDS, compared to boys with inherited ATR-X syndrome, who commonly have only mildly reduction in α/β synthesis ratios with low and sometimes undetectable amounts of HbH . The suggestion is that the different in hematologic effects of ATRX mutations in erythroid cells depends on mutation occurrence time during development or, more likely, on the cellular context in which the mutation occurs [4, 31].
The patients with the same mutations may have very different degrees of α-thalassemia, suggesting that the effect of the ATRX protein on α-globin expression may be modified by other genetic and epigenetic factors. This is most clearly illustrated by comparing the hematology phenotype of cases with identical mutations .
Here we report a male case with ATRX hereditary mutation manifested mild α-thalassemia and no sign of mental retardation, facial dysmorphism, and urogenital abnormalities. His mother was carrier and has a normal appearance and intellect, and no signs of α-thalassemia. Our patient has shown neither the severe hematologic signs expected in ATMDS nor the widespread manifestations of the ATR-X syndrome, this may be due to the fact that the mutation in this patient occurs outside of highly conserved domains including ADD and helicase. The other possibility is that this mutation is hypomorphic or modifier genetic and epigenetic factors contributing to modulating the effects of this mutation. Moreover the symptoms of the disease did not occur at birth, as expected in the ATR-X syndrome, and he showed disease symptoms earlier than the age expected foe ATMDS disease.