Characterization of Deletions of the HBA and HBB Loci by Array Comparative Genomic Hybridization.

Thalassemia is among the most common genetic diseases worldwide. α-Thalassemia is usually caused by deletion of one or more of the duplicated HBA genes on chromosome 16. In contrast, most ß-thalassemia results from point mutations that decrease or eliminate expression of the HBB gene on chromosome 11. Deletions within the HBB locus result in thalassemia or hereditary persistence of fetal Hb. Although routine diagnostic testing cannot distinguish thalassemia deletions from point mutations, deletional hereditary persistence of fetal Hb is notable for having an elevated HbF level with a normal mean corpuscular volume. A small number of deletions accounts for most α-thalassemias; in contrast, there are no predominant HBB deletions causing ß-thalassemia. To facilitate the identification and characterization of deletions of the HBA and HBB globin loci, we performed array-based comparative genomic hybridization using a custom oligonucleotide microarray. We accurately mapped the breakpoints of known and previously uncharacterized HBB deletions defining previously uncharacterized deletion breakpoints by PCR amplification and sequencing. The array also successfully identified the common HBA deletions --(SEA) and --(FIL). In summary, comparative genomic hybridization can be used to characterize deletions of the HBA and HBB loci, allowing high-resolution characterization of novel deletions that are not readily detected by PCR-based methods.

Truncation of the CNS-expressed JNK3 in a patient with a severe developmental epileptic encephalopathy.

We have investigated the breakpoints in a male child with pharmacoresistant epileptic encephalopathy and a de novo balanced translocation t(Y;4)(q11.2;q21). By fluorescence in situ hybridisation, we have identified genomic clones from both chromosome 4 and chromosome Y that span the breakpoints. Precise mapping of the chromosome 4 breakpoint indicated that the c-Jun N-terminal kinase 3 (JNK3) gene is disrupted in the patient. This gene is predominantly expressed in the central nervous system, and it plays an established role in both neuronal differentiation and apoptosis. Expression studies in the patient lymphoblastoid cell line show that the truncated JNK3 protein is expressed, i.e. the disrupted transcript is not immediately subject to nonsense-mediated mRNA decay, as is often the case for truncated mRNAs or those harbouring premature termination codons. Over-expression studies with the mutant protein in various cell lines, including neural cells, indicate that both its solubility and cellular localisation differ from that of the wild-type JNK3. It is plausible, therefore, that the presence of the truncated JNK3 disrupts normal JNK3 signal transduction in neuronal cells. JNK3 is one of the downstream effectors of the GTPase-regulated MAP kinase cascade, several members of which have been implicated in cognitive function. In addition, two known JNK3-interacting proteins, beta-arrestin 2 and JIP3, play established roles in neurite outgrowth and neurological development. These interactions are likely affected by a truncated JNK3 protein, and thereby provide an explanation for the link between alterations in MAP kinase signal transduction and brain disorders.

Identification of C7orf11 (TTDN1) gene mutations and genetic heterogeneity in nonphotosensitive trichothiodystrophy.

We have identified C7orf11, which localizes to the nucleus and is expressed in fetal hair follicles, as the first disease gene for nonphotosensitive trichothiodystrophy (TTD). C7orf11 maps to chromosome 7p14, and the disease locus has been designated "TTDN1" (TTD nonphotosensitive 1). Mutations were found in patients with Amish brittle-hair syndrome and in other nonphotosensititive TTD cases with mental retardation and decreased fertility but not in patients with Sabinas syndrome or Pollitt syndrome. Therefore, genetic heterogeneity in nonphotosensitive TTD is a feature similar to that observed in photosensitive TTD, which is caused by mutations in transcription factor II H (TFIIH) subunit genes. Comparative immunofluorescence analysis, however, suggests that C7orf11 does not influence TFIIH directly. Given the absence of cutaneous photosensitivity in the patients with C7orf11 mutations, together with the protein's nuclear localization, C7orf11 may be involved in transcription but not DNA repair.