Many large deletions removing the entire α-globin gene cluster on the short arm of the human chromosome 16 (16p13.3) have been described.1–3 At the heterozygous state, the resulting phenotype consists in α-thalassemia (α-thal) for relatively short deletions (100 to 356 kb) while an α-thalassemia mental retardation syndrome (ATR-16 syndrome) is observed for larger deletions (> 1 Mb) which generally include the 16p telomere.4,5 We report here a new large telomeric deletion (~285 kb) associated with the common alpha-thalassemia –α deletion in trans. This genotype led to a phenotypically unusual HbH disease.
The proband was a 14-year old girl (French Caucasian mother and Algerian father) with a marked hypochromic and microcytic anemia (Hb: 9.2 g/dL; MCV: 55.0 fL; MCHC: 29.9 g/dL; MCH: 16.5 pg and reticulocyte count: 1.7%). Physical examination was normal (without hepatosplenomegaly or subicterus) except for a marked scoliosis for which surgery was considered. She presented no developmental delay and had a normal school education. The presence of HbH (~8%) was detected at routine hemoglobin analysis using isoelectric focusing and cation-exchange liquid chromatography (Variant I, Bio-Rad). Unfortunately, a new blood sample to identify Heinz inclusion bodies could not be obtained. The search for the common α-thal deletions was carried out by multiplex PCR6 and the common –α deletion was found at the homozygous state. This result could not be accepted for two reasons: (i) the father carried the –α deletion at the heterozygous state but the mother did not; (ii) a homozygosity for the –α deletion is not associated with Hb disease. We thus performed an MLPA analysis (Salsa MLPA kit P140-B2 HBA, MRC Holland) which identified, for both the proband and her mother, a large deletion of the α-globin gene cluster (Figures 1 and 2). A CGH-array analysis was then carried out to gap the deletion which appeared to be approximately 285 kb in length, spanning from the telomeric region in 5’ to the AXIN1 gene in 3’ (Figure 1). We could finally determine, by semi-quantitative PCR assays,8 that the deletion removes exons 5 to 10 of the AXIN1 gene but leaves exons 1 to 4 intact (the AXIN1 gene is orientated from 3’ to 5’ on the forward strand).
The exact α-globin genotype of our proband (– – / –α) is in total accordance with her HbH disease. According to Horsley et al., monosomy for the 356 kb most telomeric region of the short arm of human chromosome 16 is not associated with the ATR-16 syndrome.3 As the deletion described in the present case report is shorter (~285 kb), it seems logical not to observe major dysmorphic features for our proband, but her very marked scoliosis remains unexplained. The AXIN1 gene has been involved in osteoclasts and osteoblast regulation.9 Thus, the deleted AXIN1 gene could potentially encode a dominant negative protein for bone synthesis. This negative effect would be potentialized by HbH disease, explaining why no scoliosis was observed for the mother. AXIN1 is also a tumor suppressor gene involved in the development of embryo abnormalities and human cancers.10–12 Genetic counseling and a clinical follow-up are thus required for our proband as mutations, loss of heterozygosity or epigenetic inactivation on the unique functional AXINI-1 gene could have severe clinical consequences.
Footnotes
- Funding: this work was partially supported by the French Ministry of Health (DHOS, Plan Maladies Rares) and Hospices Civils de Lyon.
References
- Disorders of Hemoglobin: Genetics, Pathophysiology, and Clinical Management. Cambridge University Press: Cambridge, UK; 2009. Google Scholar
- Daniels RJ, Peden JF, Lloyd C, Horsley SW, Clark K, Tufarelli C. Sequence, structure and pathology of the fully annotated terminal 2 Mb of the short arm of the human chromosome 16. Hum Mol Genet. 2001; 10(4):339-52. Google Scholar
- Horsley SW, Daniels RJ, Anguita E, Raynham HA, Peden JF, Villegas A. Monosomy for the most telomeric, gene-rich region of the short arm of human chromosome 16 cause minimal phenotypic effects. Eur J Hum Genet. 2001; 9(3):217-25. Google Scholar
- Disorders of Hemoglobin: Genetics, Pathophysiology, and Clinical Management. Cambridge University Press: Cambridge, UK; 2009. Google Scholar
- Harteveld CL, Kriek M, Bijlsma EK, Erjavec Z, Balak D, Phylipsen M. Refinement of the genetic cause of ATR-16. Hum Genet. 2007; 122(3–4):283-92. Google Scholar
- Chong SS, Boehm CD, Higgs DR, Cutting GR. Single-tube multiplex-PCR screen for common deletional determinants of α-thalassemia. Blood. 2000; 95(1):360-2. Google Scholar
- Menten B, Maas N, Thienpont B, Buysse K, Vandesompele J, Melotte C. Emerging patterns of cryptic chromosomal imbalance in patients with idiopathic mental retardation and multiple congenital anomalities: a new series of 140 patients and review of published reports. J Med Genet. 2006; 43(8):625-33. Google Scholar
- Joly P, Lacan P, Garcia C, Couprie N, Francina A. Identification and molecular characterization of four new large deletions in the beta-globin gene cluster. Blood Cells Mol Dis. 2009; 43(1):53-7. Google Scholar
- Liu F, Kohlmeier S, Wang CY. Wt signaling and skeletal development. Cell Signal. 2008; 20(6):999-1009. Google Scholar
- Oates NA, Van Vliet J, Duffy DL, Kroes HY, Martin NG, Boomsma DI. Increased DNA methylation at the AXIN1 gene in a monozygotic twin from a pair discordant for a caudal duplication anomaly. Am J Hum Genet. 2006; 79(1):155-62. Google Scholar
- Satoh S, Daigo Y, Furukawa Y, Kato T, Miwa N, Nishiwaki T. AXIN1 mutations in hepatocellular carcinomas, and growth suppression in cancer cells by virus-mediated transfer of AXIN1. Nat Genet. 2000; 24(3):245-50. Google Scholar
- MacDonald BT, Tamai K, He X. Wnt/β-catenin signaling: components, mechanisms, and diseases. Dev Cell. 2009; 17(1):9-26. Google Scholar