Sequencing of genomic data is increasingly becoming a valuable tool in identifying disease etiology in diagnostically challenging cases, as one research team discovered while evaluating a family who had multiple females with macrocytic anemia. While multiple etiologies have been implicated for the development of hematopoietic disorders, many cases of abnormal blood cell production have no discernable cause.
Using genetic sequencing, Vijay G. Sankaran, MD, and his team were able to identify a dominant negative mutation in the aminolevulinic acid synthase (ALAS2) allele on one X chromosome of the proband and other affected individuals in the family. In this case, the proband, or the first affected individual in the family who prompted the genetic study, was a 32-year-old woman with a history of atrial septal defect and anemia with an unclear etiology since childhood. Dominant negative mutations usually result in an altered, often inactive, gene product.
Researchers believe this report, which was published in The Journal of Clinical Investigation, supports the use of whole-exome sequencing as a tool for characterizing disease etiology.
Macrocytic anemia, in which the red blood cells are larger than normal, and dyserythropoiesis, a morphological hallmark of disordered maturation of blood cells, can be found in a variety of inherited and acquired hematopoietic disorders, including: congenital dyserythropoietic anemias (CDAs), specific mitochondrial disorders, myelodysplastic syndromes (MDS), and megaloblastic anemias. However, in some cases the cause of the disorder is unclear.
Researchers hoped to provide more insight into mechanisms of abnormal blood cell production in patients by developing a greater understanding of the origin of these diseases.
After identifying the proband, Dr. Sankaran and colleagues believed that the 32-year-old woman, who was also found to have dyserythropoiesis and iron overload, likely had a congenital syndrome. However, the hematopathology evaluation and genetic analysis of the most likely candidates did not yield a diagnosis.
After discovering that the patient’s mother and sister also had minor hematologic abnormalities, the investigators suspected a genetic cause may be the culprit.
“We performed whole-exome sequencing to try to better understand her disorder and found a dominant negative mutation in ALAS2, a gene that is closely associated with X-linked sideroblastic anemia; however, this patient did not have ring sideroblasts,” says David P. Steensma, MD, a co-author of the study.
Because only female carriers were found to be affected, the researchers also wondered whether there may have been highly skewed X-inactivation toward the mutant allele; however, they were surprised to learn that the severity of the anemia phenotype was not associated with severe X-inactivation skewing. Instead, they found that cells expressing the mutant ALAS2 allele exhibited an early block in red blood cell production – leading to perturbed erythropoiesis in cells expressing the normal allele. This was most significant in the proband, who had 82 percent skewing toward the mutant allele.
“In addition to providing an answer for the mechanism of anemia in this family, the analysis provided a good example of how genome sequencing can help provide clarity in a clinically ambiguous case,” Dr. Steensma said.
Sankaran V, Ulirsch J, Tchaikovskii V, et al. X-linked macrocytic dyserythropoietic anemia in females with an ALAS2 mutation. J Clin Invest. 2015 February 23. [Epub ahead of print]