Which MTR gene variants should be tested for hereditary megaloblastic anemia?

MTR Gene Variants for Hereditary Megaloblastic Anemia Testing

For hereditary megaloblastic anemia suspected to be caused by cobalamin G deficiency, comprehensive sequencing of the entire MTR gene should be performed to identify pathogenic or likely pathogenic variants, as no specific recurrent mutations predominate and novel variants continue to be discovered.

Key Testing Approach

Primary Recommendation

• Perform whole exome sequencing (WES) or targeted MTR gene sequencing to identify compound heterozygous or homozygous variants 1, 2
• Test for both known pathogenic variants AND variants of uncertain significance (VUS), as functional studies can later reclassify VUS as pathogenic based on clinical response 1

Specific Variant Considerations

Known Pathogenic MTR Variants: The literature demonstrates that MTR variants are highly heterogeneous with no single predominant mutation. Documented pathogenic variants include:

• c.1283T>A (p.Met428Lys) - initially classified as VUS but reclassified as pathogenic based on clinical response 1
• c.2411T>C (p.Ile804Thr) - compound heterozygous variant 1
• Various splice site mutations affecting MTR-001 transcript, leading to expression of only the inactive truncated MTR-201 isoform 3

Critical Testing Nuances:

• MTR encodes two isoforms: full-size methionine synthase (MTR-001) and a 124 kDa truncated form (MTR-201) 3
• In cblG-variant cases (most severe form with undetectable enzyme activity), only the MTR-201 transcript is present 3
• Standard cblG cases retain both isoforms but with reduced function 3

Diagnostic Algorithm

Step 1: Clinical and Biochemical Screening

Look for the characteristic triad:

• Megaloblastic anemia with macrocytosis
• Elevated homocysteine (typically >100 μmol/L)
• Low-normal or decreased methionine levels
• Normal methylmalonic acid (distinguishes from combined disorders)

Step 2: Molecular Testing Strategy

1. First-line: Multigene panel including MTR, MMACHC, MMADHC, and other cobalamin metabolism genes 2
2. If negative but high suspicion: Whole exome sequencing to identify novel variants 1
3. Analyze both coding regions and splice sites - splice variants are clinically significant 3

Step 3: Functional Validation for Novel Variants

When novel MTR variants are identified:

• Measure methionine synthase enzyme activity in fibroblasts 3
• Assess response to therapeutic trial with hydroxocobalamin, betaine, and folinic acid 1, 2
• Rapid biochemical improvement (homocysteine decline within 2 weeks) supports pathogenicity 1

Common Pitfalls to Avoid

Do not limit testing to "hotspot" regions - MTR variants are distributed throughout the gene without clustering 1, 3, 2

Do not dismiss VUS without functional correlation - The case of c.1283T>A demonstrates that variants initially classified as VUS may be pathogenic when clinical response to treatment is dramatic 1

Do not confuse with other cobalamin disorders:

• CblC (MMACHC mutations) presents with combined methylmalonic acidemia AND homocystinuria 3, 2
• CblG (MTR mutations) presents with isolated homocystinuria and normal methylmalonic acid 2
• The biochemical profile must guide genetic testing strategy

Do not overlook protein interaction studies - MTR interacts with MMACHC, and this interaction is disrupted in both cblG and cblC disorders 3. This explains why some cblG-variant cases show overlapping biochemical features with cblC.

Practical Testing Recommendations

Given the rarity of recurrent mutations and ongoing discovery of novel pathogenic variants, comprehensive MTR gene sequencing (all exons and splice junctions) is superior to targeted mutation panels for this condition. The heterogeneous mutation spectrum means that population-specific or ethnicity-based testing strategies are not applicable for MTR, unlike SLC19A2 in thiamine-responsive megaloblastic anemia where founder mutations exist 4, 5.