Human milk oligosaccharides

Abundancy and variation of human milk oligosaccharides

About 12 human milk oligosaccharides (HMO) are the most abundant isoforms in human milk. HMO composition differs individually because of genetic regulation, based on gene activation of the Lewis positive/negative (Le gene) and secretor/non‑secretor (Se gene) status. Women with Lewis positive gene express the enzyme fucosyltransferase‑3 producing the isoform 3'‑fucosyllactose whereas Lewis negative women do not. Women with secretor status express the enzyme fucosyltransferase‑3 and can produce 2'‑fucosyllactose (2'‑FL) in high abundancy, while non-secretor women do not.

The most abundant HMO

The most abundant human milk oligosaccharide (HMO) isoforms are shown in table 534_02-01.  However, the HMO cluster () composition varies highly between individuals and is genetically regulated (see below). Therefore, it is not possible to define one true standard or concentration for human milk composition (Kunz et al., 2017, Thurl et al., 2017). 

Enzymes define HMO isoforms

Based on glycan biology, more than 1000 HMO structures could theoretically be in existence but “only” 200 isoforms are present in human milk. This is because only a subset of known HMO synthesizing enzymes are present. They limit the possibilities of what building blocks are combined or added at specific positions within the core glycan body of an HMO (figures 534_01-03) (Kobata. 2010⁠; Zivkovic et al. 2010). About 13 core structures have been identified in human milk that are then further elongated or branched (Urashima et al. 2012). 

The four HMO milk groups

Two blood group antigens regulating key enzyzymes in HMO synthesis are responsible for our ability to group milks according to their HMO composition. These milk groups differ mostly in type, presence or concentration of fucosylated HMO (534_01-07) (Kumazaki and Yoshida. 1984⁠; Thurl et al. 1997). In contrast, sialylated and neutral non-fucosylated HMO (534_01-08 and 534_01-09) are abundant with relatively stable concentrations in the four groups (Kunz et al. 2017⁠; Thurl et al. 2017). The Secretor (Se) and Lewis (Le) genes are known  blood group antigens (Dotz and Wuhrer. 2016⁠; Shen et al. 1968).

The Se gene encrodes for the enzyme fucosyltransferase (FUC)-2 Kumazaki and Yoshida. 1984) whereas the Le gene encodes for FUC-3 (Xu et al. 1996). Le and Se gene activity depends on maternal genotype (Kumazaki and Yoshida. 1984⁠; Shen et al. 1968). Individuals with active Se gene are identified as “secretors” or "Se+" Kumazaki and Yoshida. 1984) and individuals with active Le genes are “Le‑positive” or "Le+" (Xu et al. 1996).

Secretors (milk groups 1 and 2)

Le-positive secretors (Le+/Se+) express high oligosaccharide concentrations and all fucosylated HMO isoforms from both FUC-2 and ‑3. Le-negative secretors (Le-/Se+) express similar oligosaccharide concentrations overall but are lacking HMO from FUC‑3 such as lacto-N-fucopentaose‑II and ‑III. Presence of 3'-fucosyllactose (3’-FL) in this milk group is under debate because some reports state its absence (Kunz et al. 2017) whereas other and sometimes older reports stated presence of 3’-FL (Ayechu-Muruzabal et al. 2018⁠; Thurl et al. 2010⁠; Thurl et al. 1997). If FUC‑3 is indeed responsible for 3’-FL synthesis it would be logical to expect its absence from this group.

Non-secretors (milk groups 3 and 4)

Le-positive non-secretors (Le+/Se-) express lower total oligosaccharide concentrations compared to the other milk groups. FUC‑2 is absent. Consequenty, this group lacks 2'-fucosyllactose (2’-FL), which is the most abundant HMO isoform in the oterh milk groups (Kunz et al. 2017). Less than 1% of women are Le-negative non‑secretors (Le-/Se-) and compositional data of their milks are scarce. Total HMO concentrations have not been reported to date but are estimated to be comparable to Le-positive non-secretors (Thurl et al. 1997). Fucosylated HMO are barely traceable and sialylated or acidic HMO take higher precedence in the total oligosaccharide fraction compared to the other milk groups. Although both enzymes are absent in Le-negative non‑secretors, traces of fucosylated HMO are detectable. This indicates presence of other enzymes with very low efficacy that are expressed indepdentendly of Le and Se-gene activation (Erney et al. 2000⁠; Thurl et al. 1997). Their identification and biological relevance remains to be determined.


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