Skipping of multiple CFTR exons is not a result of single exon omissions

Characterization of a novel HMG-CoA lyase enzyme with a dual location in endoplasmic reticulum and cytosol

A novel lyase activity enzyme is characterized for the first time: HMG-CoA lyase-like1 (er-cHL), which is a close homolog of mitochondrial HMG-CoA lyase (mHL). Initial data show that there are nine mature transcripts for the novel gene HMGCLL1, although none of them has all its exons. The most abundant transcript is called “variant b,” and it lacks exons 2 and 3. Moreover, a three-dimensional model of the novel enzyme is proposed. Colocalization studies show a dual location of the er-cHL in the endoplasmic reticulum (ER) and cytosol, but not in mitochondria or peroxisomes. Furthermore, the dissociation experiment suggests that it is a nonendoplasmic reticulum integral membrane protein. The kinetic parameters of er-cHL indicate that it has a lower V_max and a higher substrate affinity than mHL. Protein expression and lyase activity were found in several tissues, and were particularly strong in lung and kidney. The occurrence of er-cHL in brain is surprising, as mHL has not been found there. Although mHL activity is clearly associated with energy metabolism, the results suggest that er-cHL is more closely related to another metabolic function, mostly at the pulmonary and brain level.

Characterization of splice variants of the genes encoding human mitochondrial HMG-CoA lyase and HMG-CoA synthase, the main enzymes of the ketogenesis pathway

The genes HMGCS2 and HMGCL encode the two main enzymes for ketone-body synthesis, mitochondrial HMG-CoA synthase and HMG-CoA lyase. Here, we identify and describe possible splice variants of these genes in human tissues. We detected an alternative transcript of HMGCS2 carrying a deletion of exon 4, and two alternative transcripts of HMGCL with deletions of exons 5 and 6, and exons 5, 6 and 7, respectively. All splice variants maintained the reading frame. However, Western blot studies and overexpression measurements in eukaryotic or prokaryotic cell models did not reveal HL or mHS protein variants. Both genes showed a similar distribution of the inactive variants in different tissues. Surprisingly, the highest percentages were found in tissues where almost no ketone bodies are synthesized: heart, skeletal muscle and brain. Our results suggest that alternative splicing might coordinately block the two main enzymes of ketogenesis in specific human tissues.

Two new genes from the human ATP-binding cassette transporter superfamily, ABCC11 and ABCC12, tandemly duplicated on chromosome 16q12

Several years ago, we initiated a long-term project of cloning new human ATP-binding cassette (ABC) transporters and linking them to various disease phenotypes. As one of the results of this project, we present two new members of the human ABCC subfamily, ABCC11 and ABCC12. These two new human ABC transporters were fully characterized and mapped to the human chromosome 16q12. With the addition of these two genes, the complete human ABCC subfamily has 12 identified members (ABCC1-12), nine from the multidrug resistance-like subgroup, two from the sulfonylurea receptor subgroup, and the CFTR gene. Phylogenetic analysis determined that ABCC11 and ABCC12 are derived by duplication, and are most closely related to the ABCC5 gene. Genetic variation in some ABCC subfamily members is associated with human inherited diseases, including cystic fibrosis (CFTR/ABCC7), Dubin-Johnson syndrome (ABCC2), pseudoxanthoma elasticum (ABCC6) and familial persistent hyperinsulinemic hypoglycemia of infancy (ABCC8). Since ABCC11 and ABCC12 were mapped to a region harboring gene(s) for paroxysmal kinesigenic choreoathetosis, the two genes represent positional candidates for this disorder.