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MNS

The MNS blood group system (ISBT 002) is a complex system characterized by antigens located on two major red blood cell sialoglycoproteins: Glycophorin A (GPA) and Glycophorin B (GPB). Unlike ABO or Lewis antigens which are carbohydrates, MNS antigens like the antithetical pairs M/N (on GPA) and S/s (on GPB) are defined by specific amino acid variations in these proteins, encoded by the closely linked GYPA and GYPB genes. The system also includes the high-prevalence U antigen, also on GPB, and numerous variants arising from genetic mutations and rearrangements, with antibodies like anti-S, anti-s, and anti-U holding significant clinical importance

The Players: Glycophorin A (GPA) and Glycophorin B (GPB)

The MNS antigens are located on two important sialoglycoproteins (proteins with lots of sialic acid-containing sugar chains attached) that span the red blood cell membrane:

  • Glycophorin A (GPA)
    • The major sialoglycoprotein on the RBC surface (about 1 million copies per cell)
    • Encoded by the GYPA gene on chromosome 4
    • Carries the M and N antigens
    • Heavily glycosylated, contributing significantly to the RBC’s negative surface charge (due to sialic acid). This charge helps keep RBCs from sticking together
    • May serve as receptors for certain pathogens (like Plasmodium falciparum) and cytokines
  • Glycophorin B (GPB)
    • Structurally similar to the N-terminal portion of GPA but present in fewer copies (about 200,000 per cell)
    • Encoded by the GYPB gene, located very close to GYPA on chromosome 4 (this linkage is important for inheritance)
    • Carries the S, s, and U antigens
    • Also glycosylated and contributes to the negative surface charge

Genetics Recap: Linked Genes

  • GYPA and GYPB are tightly linked on chromosome 4
  • This means they are usually inherited together as a haplotype (e.g., MS, Ms, NS, Ns)

Biochemistry of Antigen Specificity

The differences between the M/N and S/s antigens are determined by variations in the amino acid sequences of GPA and GPB, caused by single nucleotide polymorphisms (SNPs) in the GYPA and GYPB genes

  • M and N Antigens (on GPA)
    • The difference between M and N lies at positions 1 and 5 of the GPA protein sequence (at the N-terminus, sticking out from the cell)
    • M Antigen: Has Serine at position 1 and Glycine at position 5
    • N Antigen: Has Leucine at position 1 and Glutamic Acid at position 5
    • These are antithetical antigens – an individual inherits the genetic information to produce either M or N GPA from each chromosome 4. Genotypes can be MM, MN, or NN
  • S and s Antigens (on GPB)
    • The difference between S and s lies at position 29 of the GPB protein sequence
    • S Antigen: Has Methionine at position 29
    • s Antigen: Has Threonine at position 29
    • These are also antithetical antigens. Genotypes can be SS, Ss, or ss

The U Antigen (on GPB)

  • Location: Located on GPB, closer to the red cell membrane than the S/s antigens. It requires the presence of a significant portion of the GPB protein structure
  • Prevalence: A very high-prevalence antigen, present on the RBCs of over 99.9% of individuals in most populations
  • U-Negative Phenotype
    • Individuals lack the U antigen. This phenotype is rare, found almost exclusively in individuals of African descent (about 1% prevalence in this population)
    • Usually results from deletion or mutations in the GYPB gene, leading to the absence or severe truncation of GPB
    • Because S and s are also on GPB, U-negative individuals are almost always S-s- as well
    • Clinical Significance: U-negative individuals can form anti-U, which is a clinically significant antibody capable of causing severe HTRs and HDFN. Finding U-negative blood for transfusion is extremely challenging

Common Haplotypes and Phenotypes

Due to the linkage of GYPA and GYPB, certain combinations of antigens are inherited together:

  • Common Haplotypes: MS, Ms, NS, Ns
  • Resulting Genotypes: An individual inherits one haplotype from each parent (e.g., MS/Ms, MS/NS, Ns/Ns)
  • Phenotype: The combination of antigens detected on the cells (e.g., M+N+S+s+)

Effect of Proteolytic Enzymes (Ficin, Papain)

Enzyme treatment is a key tool in antibody identification involving the MNS system:

  • M and N Antigens: Located on the outer portion of GPA, which is easily cleaved off by enzymes like ficin and papain. Therefore, M and N antigens are DESTROYED by enzyme treatment
  • S and s Antigens: Located on GPB. Their susceptibility to enzymes is more VARIABLE. S is often more sensitive than s, but both can be weakened or sometimes resistant depending on the specific enzyme and conditions. They are generally considered more resistant than M and N
  • U Antigen: Being closer to the membrane on GPB, the U antigen is generally RESISTANT to enzyme treatment

Dosage Effect

Antibodies in the MNS system (especially anti-M, -N, -S, -s) often show dosage. This means they react more strongly with red cells that are homozygous for the corresponding antigen compared to heterozygous cells

  • Example: Anti-M may react 3+ with M+N- cells (genotype MM) but only 1+ with M+N+ cells (genotype MN)

MNS Antibodies: A Quick Overview

  • Anti-M
    • Commonly encountered “naturally occurring” antibody
    • Often IgM, reactive at room temperature or below (IS phase)
    • Usually clinically insignificant unless reactive at 37°C/AHG
    • Frequently shows dosage
    • pH dependent (enhanced reactivity at pH 6.5)
  • Anti-N
    • Less common than anti-M
    • Usually IgM, cold-reactive, clinically insignificant
    • Often shows dosage
    • A rare “dialysis-associated” anti-N form can be seen (reacts with formaldehyde-treated N+ cells)
  • Anti-S
    • Usually IgG, reactive at 37°C and/or AHG phase
    • Clinically significant: Can cause HTRs and HDFN
    • Can show dosage
  • Anti-s
    • Usually IgG, reactive at 37°C and/or AHG phase
    • Clinically significant: Can cause HTRs and HDFN
    • Can show dosage
  • Anti-U
    • Rare antibody found in U-negative (S-s-) individuals
    • Usually IgG, reactive at 37°C/AHG
    • Clinically significant: Associated with severe HTRs and HDFN. Requires rare U-negative blood for transfusion

Other MNS Variants

Numerous other antigens exist within the MNS system (over 40!), often resulting from:

  • Mutations in GYPA or GYPB
  • Gene rearrangements or unequal crossing-over events between GYPA and GYPB, leading to hybrid glycophorin molecules (e.g., the Miltenberger subsystem antigens)
  • Null phenotypes (e.g., MkMk genotype results in lack of GPA and GPB)

Key Terms

  • Glycophorin A (GPA): Major RBC sialoglycoprotein carrying M and N antigens
  • Glycophorin B (GPB): Minor RBC sialoglycoprotein carrying S, s, and U antigens
  • GYPA & GYPB: Genes encoding GPA and GPB, respectively, located on chromosome 4
  • Sialoglycoprotein: A protein with attached sialic acid-containing carbohydrate chains
  • Antithetical Antigens: Antigens encoded by alleles at the same locus (M/N, S/s)
  • Haplotype: A set of linked genes inherited together (e.g., Ms)
  • U Antigen: High-prevalence antigen on GPB; absence (U-negative) is rare and associated with lack of GPB
  • Dosage Effect: Stronger serological reaction with cells homozygous for an antigen compared to heterozygous cells
  • Enzyme Treatment: Use of proteolytic enzymes (ficin, papain) to modify antigen expression; M/N are destroyed, S/s/U are generally more resistant