P1PK/Globoside(P)

The P1PK (ISBT 003) and Globoside (ISBT 028) blood group systems involve carbohydrate antigens synthesized sequentially on glycosphingolipid precursors found on the red blood cell surface. Specific glycosyltransferase enzymes add sugars, creating the P^k antigen (the precursor for P), the P antigen (Globoside itself), and the independently synthesized P1 antigen. The presence or absence of these antigens, determined by the functionality of the encoding enzymes, defines the various phenotypes within these biochemically linked systems

The Foundation: Glycosphingolipids

Unlike protein-based antigens (like Rh) or antigens primarily on glycoproteins (like MNS), the antigens of these systems are primarily glycosphingolipids. These molecules consist of:

• Ceramide: A lipid molecule embedded in the red cell membrane
• Carbohydrate Chain: Attached to the ceramide base

The basic precursor relevant here is lactosylceramide (LacCer: Galβ1→4Glcβ1→Cer)

The Biosynthetic Pathway: Step-by-Step Antigen Construction

The synthesis involves specific enzymes called glycosyltransferases adding sugar molecules one by one

• Step 1: Synthesis of P^k Antigen (Ceramide Trihexoside, CTH)
  • Enzyme: α1,4-galactosyltransferase
  • Gene: A4GALT (located on chromosome 22)
  • Action: This enzyme transfers a galactose (Gal) molecule from UDP-Galactose onto lactosylceramide in an α1→4 linkage
  • Resulting Structure (P^k): Galα1→4Galβ1→4Glcβ1→Cer
  • Significance: P^k is the essential precursor for the P antigen and is also an antigen within the P1PK system
• Step 2: Synthesis of P Antigen (Globoside, Gb4)
  • Enzyme: β1,3-N-acetylgalactosaminyltransferase 1
  • Gene: B3GALNT1 (located on chromosome 3)
  • Action: This enzyme transfers an N-acetylgalactosamine (GalNAc) molecule from UDP-GalNAc onto the P^k antigen in a β1→3 linkage
  • Resulting Structure (P / Globoside): GalNAcβ1→3Galα1→4Galβ1→4Glcβ1→Cer
  • Significance: P antigen is the defining antigen of the Globoside system. Its synthesis requires the prior synthesis of P^k
• Step 3: Synthesis of P1 Antigen
  • Enzyme: α1,4-galactosyltransferase (functionally distinct from the P^k synthase, often called “P1 synthase”)
  • Gene: The specific gene responsible for P1 synthesis (P1 gene) has not yet been definitively cloned or identified, although its existence is inferred from genetic and phenotypic data. It is thought to be distinct from A4GALT
  • Action: This enzyme transfers a galactose (Gal) molecule onto a different precursor called paragloboside (nLc4Cer: Galβ1→4GlcNAcβ1→3Galβ1→4Glcβ1→Cer) in an α1→4 linkage
  • Resulting Structure (P1): Galα1→4Galβ1→4GlcNAcβ1→3Galβ1→4Glcβ1→Cer
  • Significance: P1 antigen synthesis is independent of P^k and P synthesis but uses a related precursor structure. Its presence defines the P₁ phenotype

The Blood Group Systems & Collection

The classification can seem a bit confusing because the antigens are biochemically linked but defined by different genes/synthetic steps:

• P1PK System (ISBT 003): Includes the P1 and P^k antigens. Defined by the activity of the A4GALT gene (for P^k) and the presumed P1 gene
• Globoside System (ISBT 028): Includes the P antigen (Globoside). Defined by the activity of the B3GALNT1 gene
• Globoside Collection (ISBT 209): Groups antigens that are biochemically related via the globoside pathway but don’t fit neatly into a single system based on current genetic knowledge. This collection currently includes P^k, P, and the rare LKE antigen

Phenotypes: Resulting from Enzyme Presence/Absence

The common and rare phenotypes arise from the inheritance of functional or non-functional alleles of the key enzyme genes:

• P₁ Phenotype: (Approx. 75% Caucasians, >90% Black individuals)
  • Antigens Present: P1, P, P^k
  • Presumed Genetics: Functional A4GALT, B3GALNT1, and P1 genes
  • Biochemistry: Can perform all three synthetic steps described above
• P₂ Phenotype: (Approx. 25% Caucasians, <10% Black individuals)
  • Antigens Present: P, P^k (but NO P1)
  • Presumed Genetics: Functional A4GALT and B3GALNT1, but non-functional P1 gene (or regulatory mechanism preventing P1 synthesis)
  • Biochemistry: Can make P^k and convert it to P, but cannot synthesize P1 from paragloboside
• p Phenotype (Null Phenotype): (Very Rare)
  • Antigens Present: None (Lack P1, P, and P^k)
  • Genetics: Non-functional A4GALT gene (homozygous for null alleles)
  • Biochemistry: Cannot perform Step 1 (P^k synthesis). Without the P^k precursor, P antigen cannot be made (Step 2 fails). P1 antigen is also absent (mechanism unclear - perhaps related precursor issues or linked gene regulation). Red cells only have precursors like lactosylceramide and paragloboside
• P₁^k Phenotype: (Very Rare)
  • Antigens Present: P1, P^k (but NO P)
  • Genetics: Functional A4GALT and P1 genes, but non-functional B3GALNT1 gene
  • Biochemistry: Can make P^k (Step 1) and P1 (Step 3), but cannot convert P^k into P (Step 2 fails). P^k antigen accumulates
• P₂^k Phenotype: (Very Rare)
  • Antigens Present: P^k only (Lack P1 and P)
  • Genetics: Functional A4GALT, but non-functional B3GALNT1 and P1 genes
  • Biochemistry: Can only perform Step 1 (P^k synthesis). Cannot convert P^k to P (Step 2 fails) and cannot make P1 (Step 3 fails)

Location and Expression

• These antigens are found on red blood cells, lymphocytes, platelets, and fibroblasts
• P1 antigen expression strength can vary considerably between individuals and may weaken upon storage
• They exist primarily as glycosphingolipids but can also be found on some glycoproteins

Associated Antibodies (Briefly)

• Anti-P1: Common, naturally occurring IgM found in P₂ individuals. Usually reacts best in the cold and is typically clinically insignificant. Can be neutralized by P1 substance (found in hydatid cyst fluid, pigeon droppings)
• Anti-PP1P^k (formerly Anti-Tj^a): Found in individuals with the rare p phenotype. Reacts with P, P^k, and P1 antigens. A potent mixture of IgM and IgG, clinically significant, causing HTRs and HDFN, and strongly associated with recurrent spontaneous abortions
• Anti-P: Found in individuals with P₁^k or P₂^k phenotypes. Reacts with P antigen. Clinically significant
• Alloanti-P^k: Theoretically possible in individuals lacking P^k (p phenotype), but anti-PP1P^k is the characteristic antibody
• Autoanti-P (Donath-Landsteiner Antibody): The antibody responsible for Paroxysmal Cold Hemoglobinuria (PCH). It’s a biphasic IgG hemolysin – binds to P-positive RBCs in the cold (extremities) and causes complement-mediated lysis when cells warm up (central circulation)

Key Terms

• Glycosphingolipid: A lipid molecule (ceramide) linked to a carbohydrate chain, forming the structural basis for P1PK and Globoside antigens on RBCs
• Glycosyltransferase: An enzyme that catalyzes the addition of specific sugar molecules onto a substrate (e.g., α1,4-galactosyltransferase)
• A4GALT: Gene encoding the enzyme that synthesizes the P^k antigen
• B3GALNT1: Gene encoding the enzyme that synthesizes the P antigen (Globoside) from the P^k precursor
• P^k Antigen: Galα1→4Galβ1→4Glcβ1→Cer; precursor for P antigen
• P Antigen (Globoside): GalNAcβ1→3Galα1→4Galβ1→4Glcβ1→Cer
• P1 Antigen: Galα1→4Galβ1→4GlcNAcβ1→3Galβ1→4Glcβ1→Cer; synthesized on paragloboside
• Paragloboside: The precursor structure (nLc4Cer) onto which P1 antigen is built
• p Phenotype: Rare null phenotype lacking P1, P, and P^k antigens due to non-functional A4GALT
• PCH (Paroxysmal Cold Hemoglobinuria): Autoimmune hemolytic anemia associated with a biphasic IgG autoanti-P (Donath-Landsteiner antibody)