The sequence in the first hypervariable segment (HVS-I) of the control region has been used as a source of evolutionary information in most phylogenetic analyses of mtDNA. Population genetic inference would benefit from a better understanding of the variation in the mtDNA coding region, but, thus far, complete mtDNA sequences have been rare. We determined the nucleotide sequence in the coding region of mtDNA from 121 Finns, by conformation-sensitive gel electrophoresis and subsequent sequencing and by direct sequencing of the D loop. Furthermore, 71 sequences from our previous reports were included, so that the samples represented all the mtDNA haplogroups present in the Finnish population. We found a total of 297 variable sites in the coding region, which allowed the compilation of unambiguous phylogenetic networks. The D loop harbored 104 variable sites, and, in most cases, these could be localized within the coding-region networks, without discrepancies. Interestingly, many homoplasies were detected in the coding region. Nucleotide variation in the rRNA and tRNA genes was 6%, and that in the third nucleotide positions of structural genes amounted to 22% of that in the HVS-I. The complete networks enabled the relationships between the mtDNA haplogroups to be analyzed. Phylogenetic networks based on the entire coding-region sequence in mtDNA provide a rich source for further population genetic studies, and complete sequences make it easier to differentiate between disease-causing mutations and rare polymorphisms.

Mutations in mtDNA have accumulated sequentially, and maternal lineages have diverged to form population-specific genotypes. Classification of the genotypes has been made based on differences found in restriction fragment analysis of the coding region or in the sequence of the hypervariable segment I. Both methods have shortcomings, as the former may not detect all the important polymorphisms and the latter makes use of a segment containing hypervariable nucleotide positions. Here, we have used conformation-sensitive gel electrophoresis (CSGE) to detect polymorphisms within the coding region of mtDNA from 22 Finns belonging to haplogroup U. Sixty-three overlapping PCR fragments covering the entire coding region were analyzed by CSGE, and the fragments that differed in their migration pattern were sequenced. CSGE proved to be a sensitive and specific method for identifying mtDNA substitutions. The phylogenetic network of the 22 coding-region sequences constituted a perfect tree, free of homoplasy, and provided several previously unidentified common polymorphisms characterizing subgroups of U. After contrasting this data with that of hypervariable segment I, we concluded that position 16192 seems to be prone to recurrent mutations and that position 16270 has experienced a back mutation. Interestingly, all 22 samples were found to belong to subcluster U5, suggesting that this subcluster is more frequent in Finns than in other European populations. Complete sequence data of the mtDNA yield a more reliable phylogenetic network and a more accurate classification of the haplogroups than previous ones. In medical genetics, such networks may help to decide between a rare polymorphism and a pathogenic mutation; in population genetics, the networks may enable more detailed analyses of population history and mtDNA evolution.

Objectives: The Finns, and to a more extreme extent the Saami, are genetic outliers in Europe. Despite the close geographical contact between these populations, no major contribution of Saami mtDNA haplotypes to the Finnish population has been detected. Methods: To examine the extent of maternal gene flow from the Saami into Finnish populations, we determined the mtDNA variation in 403 persons living in four provinces in central and northern Finland. For all of these samples, we assessed the frequencies of mtDNA haplogroups and examined sequence variation in the hypervariable segment I (HVS-I). The resulting data were compared with published information for Saami populations. Results: The frequencies of the mtDNA haplogroups differed between the populations of the four provinces, suggesting a distinction between northern and central Finland. Analysis of molecular variance suggested that the Saami deviated less from the population of northern Finland than from that of central Finland. Five HVS-I haplotypes, including that harboring the Saami motif and the Asian-specific haplogroup Z, were shared between the Finns and the Saami and allowed comparisons between the populations. Their frequency was highest in the Saami and decreased towards central Finland. Conclusions: The high frequency of certain mtDNA haplotypes considered to be Saami specific in the Finnish population suggests a genetic admixture, which appears to be more pronounced in northern Finland. Furthermore, the presence of haplogroup Z in the Finns and the Saami indicates that traces of Asian mtDNA genotypes have survived in the contemporary populations.

Hydrolethalus syndrome (HLS) is an autosomal recessive lethal malformation syndrome characterized by multiple developmental defects of fetus. We have earlier mapped and restricted the HLS region to a critical 1 cM interval on 11q23-25. The linkage disequilibrium (LD) and haplotype analyses of single nucleotide polymorphism (SNP) markers helped to further restrict the HLS locus to 476 kb between genes PKNOX2 and DDX25. An HLS associated mutation was identified in a novel regional transcript (GenBank accession no. FLJ32915), referred to here as the HYLS1 gene. The identified A to G transition results in a D211G change in the 299 amino acid polypeptide with unknown function. The HYLS1 gene shows alternative splicing and the transcript is found in multiple tissues during fetal development. In situ hybridization shows spatial and temporal distributions of transcripts in good agreement with the tissue phenotype of HLS patients. Immunostaining of in vitro expressed polypeptides from wild-type (WT) cDNA revealed cytoplasmic staining, whereas mutant polypeptides became localized in distinct nuclear structures, implying a disturbed cellular localization of the mutant protein. The Drosophila melanogaster model confirmed these findings and provides evidence for the significance of the mutation both in vitro and in vivo.

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