Expression of the Kell blood group system is dependent on two proteins, Kell and XK, that are linked by a single disulfide bond. Kell, a type II membrane glycoprotein, is a zinc endopeptidase, while XK, which has 10 transmembrane domains, is a putative membrane transporter. A rare phenotype termed Kell null (Ko) is characterized by the absence of Kell protein and Kell antigens from the red cell membrane and diminished amounts of XK protein. We determined the molecular basis of eight unrelated persons with Ko phenotypes by sequencing the coding and the intron-exon splice regions of KEL and, in some cases, analysis of mRNA transcripts and expression of mutants on the cell surface of transfected cells. Six subjects were homozygous: four with premature stop codons, one with a 5 splice site mutation, G to A, in intron 3, and one with an amino acid substitution (S676N) in exon 18. Two Ko persons with premature stop codons had identical mutations in exon 4 (R128Stop), another had a different mutation in exon 4 (C83Stop), and the fourth had a stop codon in exon 9 (Q348Stop). Two Ko persons were heterozygous for two mutations. One had a 5 splice site mutation (G to A) in intron 3 of one allele that caused aberrant splicing and exon skipping, and the other allele had an amino acid substitution in exon 10 (S363N). The other heterozygote had the same amino acid substitution in exon 10 (S363N) in one allele and a premature stop codon in exon 6 (R192Stop) in the other allele. The S363N and S676N mutants, expressed in 293T cells, were retained in a pre-Golgi compartment and were not transported to the cell surface, indicating that these mutations inhibit trafficking. We conclude that several different molecular defects cause the Kell null phenotype.
Although anti-D is still the main cause of HDN, many other antibodies have been implicated. From September 1995 to April 2000, screening for RBC antibodies was performed on samples from 21,730 pregnant women regardless of RhD type. Standard tube and gel methods were used. Anti-D was identified in 254 samples; other antibody specificities were detected in 376 samples, for a total of 630 antibodies. For this study, 522 antibodies were considered clinically significant. The incidence of potentially clinically significant antibodies was 2.4 percent. The majority belonged to the Rh system, followed by anti-M, -Fy a , -S, -Jk a , and -Jk b . Among antibodies of no clinical significance, the most frequent were anti-
Introduction Jr a is a high-frequency antigen belonging to the JR blood group system. Population studies have established that the Jr (a-) phenotype is rare. The clinical significance of anti-Jr a antibodies is controversial. This case report describes a newborn with prolonged jaundice due to alloimmunization against Jr a antigen. Case Outline A female Roma infant, 27 days of age, was admitted to hospital due to prolonged jaundice and failure to thrive. Immunohematological testing determined a blood group type A, D+ C+ E+ c+ e+, K-, and the presence of an antibody direct against a high-prevalence red blood cell antigens. On admission, total bilirubin was 199.6 μmol/l, direct bilirubin 10.3 μmol/l, hemoglobin concentration 132 g/l, hematocrit 41.1%, reticulocytes 1.08%. The newborn was the third child from a third routinely monitored pregnancy. Maternal sensitization to Jr a antigen was detected during the second pregnancy. The titer of anti-Jr a reached the highest value of 1,024 at the 28th week of gestation. Conclusion This is the first description of neonatal hyperbilirubinemia caused by anti-Jr a antibody in the Republic of Serbia. This case report provides new data about the clinical significance of anti-Jr a in pregnancy and the newborn.
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