Analysis of the C4 genes in baleen whales using a human cDNA probe

Limited MHC polymorphism in the southern elephant seal: implications for MHC evolution and marine mammal population biology

Genes of the major histocompatibility complex (MHC) are highly polymorphic in most terrestrial mammal populations so far studied. Exceptions to this are typically populations that lack genome-wide diversity. Here I show that two populations of the southern elephant seal (Mirounga leonina) have low DNA restriction fragment length polymorphism at MHC loci when compared with terrestrial mammals. Limited studies on MHC polymorphism in two cetacean species suggest this is a feature of marine mammal populations in general. MHC polymorphism is thought to be maintained by balancing selection, and several types of disease-based and reproductive-based mechanisms have been proposed. For the three marine mammal species examined, the low MHC polymorphism cannot be explained by low genome-wide diversity, or by any reproductive-based selection pressure. It can, however, be explained by diminished exposure to pathogenic selection pressure compared with terrestrial mammals. Reduced exposure to pathogens would also mean that marine mammal populations may be susceptible to occasional pathogen-induced mass mortalities.

Analysis of the duplicated human C4/P450c21/X gene cluster

Gene duplications, deletions and rearrangements occur with an unusually high frequency in the region of the P450c21 genes encoding 21-hydroxylase. In the human genome, the locus contains at least 6 genes, oriented 5' C4A, P450c21A, XA, C4B, P450c21B, XB 3'. Sequence analysis of the XA gene, of the 5' flanking DNA of the C4A gene, and of part of the XB gene revealed that this gene cluster was duplicated by nonhomologous recombination at a CAAG tetranucleotide. The location of this duplication suggests that it may have occurred after mammalian speciation. The XA gene is abundantly expressed in the human adrenal as a stable 2.6 kb RNA, but it is not known if that RNA serves a biological function. Knowledge of the anatomy of the XA gene facilitates genetic analysis of disease-causing lesions in the P450c21B gene. Southern blotting data show that about 76% of disordered P450c21B alleles bear gene microconversions that resemble point mutations; the remaining alleles are equally distributed between gene deletions and large gene conversions.

Mechanism and consequences of the duplication of the human C4/P450c21/gene X locus

The adjacent C4 and P450c21 genes encode the fourth component of serum complement and steroid 21-hydroxylase respectively, and are tandemly duplicated in the human, murine, and bovine genomes. We recently cloned a cDNA for another duplicated gene, operationally termed X, which overlaps the 3' end of human P450c21 and has the opposite transcriptional orientation. Thus, the organization of the locus is 5'-C4A-21A-XA-C4B-21B-XB-3' (Y. Morel, J. Bristow, S. E. Gitelman, and W. L. Miller, Proc. Natl. Acad. Sci. USA 86:6582-6586, 1989). To determine how this locus was duplicated, we sequenced the DNA at the duplication boundaries and the 7 kb between P450c21A and C4B comprising the XA locus. The sequences located the duplication boundaries precisely and indicate that the duplication occurred by nonhomologous recombination. The boundaries are substantially different from those of the corresponding duplication in the mouse genome, suggesting that similar gene duplications may have occurred independently in ancestors of rodents and primates after mammalian speciation. Compared with XB, the XA gene is truncated at its 5' end and bears a 121-bp intragenic deletion causing a frameshift and premature translational stop signal. Nevertheless, XA is transcribed into a stable 2.6-kb polyadenylated RNA that is expressed uniquely in the adrenal gland.

Mechanism and consequences of the duplication of the human C4/P450c21/gene X locus

The adjacent C4 and P450c21 genes encode the fourth component of serum complement and steroid 21-hydroxylase respectively, and are tandemly duplicated in the human, murine, and bovine genomes. We recently cloned a cDNA for another duplicated gene, operationally termed X, which overlaps the 3' end of human P450c21 and has the opposite transcriptional orientation. Thus, the organization of the locus is 5'-C4A-21A-XA-C4B-21B-XB-3' (Y. Morel, J. Bristow, S. E. Gitelman, and W. L. Miller, Proc. Natl. Acad. Sci. USA 86:6582-6586, 1989). To determine how this locus was duplicated, we sequenced the DNA at the duplication boundaries and the 7 kb between P450c21A and C4B comprising the XA locus. The sequences located the duplication boundaries precisely and indicate that the duplication occurred by nonhomologous recombination. The boundaries are substantially different from those of the corresponding duplication in the mouse genome, suggesting that similar gene duplications may have occurred independently in ancestors of rodents and primates after mammalian speciation. Compared with XB, the XA gene is truncated at its 5' end and bears a 121-bp intragenic deletion causing a frameshift and premature translational stop signal. Nevertheless, XA is transcribed into a stable 2.6-kb polyadenylated RNA that is expressed uniquely in the adrenal gland.

Organization of C4 and CYP21 loci in gorilla and orangutan

The standard human haplotype contains two C4 and two CYP21 loci arranged in the order C4A ... CYP21P ... C4B ... CYP21 and intercalated between the class I and class II loci of the HLA complex. The C4A gene is 22 kilobases (kb) long; the C4B gene is either 22 kb or 16 kb long. The CYP21P is a pseudogene characterized by an eight base pair (bp) deletion in exon 3 and other defects; the CYP21 is a functional gene. The standard chimpanzee haplotype is arranged in the same way as the standard human haplotype, except that both C4 genes are of the short variety; like the human gene, the chimpanzee CYP21P gene contains the 8 bp deletion. In the present study we demonstrate that a representative gorilla haplotype also consists of two short C4 genes and two CYP21 genes, neither of which, however, has the characteristic 8 bp deletion. On the other hand, the single characterized orangutan haplotype is organized in the following way: C4A ... CYP21 ... C4A ... CYP21 ... C4B ... CYP21. The first two C4 genes are of the long variety, the third gene is short. None of the defects characterizing the human CYP21P gene is present in any of the three orangutan genes. These conclusions are based on the analysis of overlapping clones isolated from cosmid libraries of the indicated species. The observed haplotype organization of the four primate species can be explained by expansion and contraction of the C4-CYP21 region through unequal homologous crossing-over, which preserves the differentiation of the C4 genes into the A and B categories but otherwise homogenizes these genes, as well as the CYP21 genes, within a given species. The 8 bp deletion in the CYP21P gene is postulated to have occurred before the separation of the lineages that led to modern humans and chimpanzees, but after the separation of these two lineages from the lineage that led to modern gorillas. The 6 kb insertion generating the long C4 gene is postulated to have occurred before the separation of the orangutan, gorilla, chimpanzee, and human lineages.