MMAB rs869320655: What Your Genotype Means
The genetic variant rs869320655 is an extremely rare alteration identified within the MMAB gene. It is clinically cataloged as a pathogenic variant associated with methylmalonic aciduria type cblB, an autosomal recessive metabolic disorder. Because it is inherited in a recessive pattern, carrying a single copy is typically linked to carrier status rather than active clinical disease.
What each genotype means
Typical MMAB gene function
This genotype represents the most common genetic sequence found in the general population for this location in the MMAB gene. It is not associated with the cblB type of methylmalonic aciduria, which is an autosomal recessive condition requiring variants on both copies of the gene.
This is the most common genotype observed across all global populations.
Carrier of MMAB variant
You carry one copy of this variant and one typical copy of the MMAB gene. Because methylmalonic aciduria type cblB is an autosomal recessive condition, carriers typically do not exhibit symptoms of the disorder, though they may pass the variant to their children.
This genotype is very rare, consistent with the overall variant frequency of less than 0.00005 in gnomAD populations.
Potential MMAB variant homozygote
This genotype indicates that both copies of your MMAB gene carry this specific variant. Because the MMAB gene is essential for processing vitamin B12, individuals with two pathogenic variants may be at risk for methylmalonic aciduria type cblB, a condition that can cause metabolic issues like ketoacidosis. Please consult with a medical geneticist or your primary healthcare provider to discuss the clinical significance of this finding in the context of your personal health history.
This genotype is extremely rare and is not expected to be found in the general population at any significant frequency.
What Is rs869320655 and Where Is It Located?
The identifier rs869320655 represents a specific sequence variation located in the MMAB gene on human chromosome 12. In genetic catalogs, this variant is categorized under carrier status due to its relationship with inborn errors of metabolism. Natural sequence variants in coding portions of genes can alter how functional proteins are assembled, folded, or regulated. Public variation repositories such as dbSNP catalog rs869320655 as an ultra-rare single nucleotide variation. For most people taking routine genetic tests, this position is invariant and matches the reference genome. When an alternate allele does appear, clinical curation frameworks like ClinVar track how such changes alter human health phenotypes, focusing primarily on their inheritance patterns.
The Biological Role of the MMAB Gene
The MMAB gene encodes an essential mitochondrial enzyme known as ATP:cob(I)alamin adenosyltransferase. As detailed by [MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/mmab/), this enzyme performs the final enzymatic step in converting vitamin B12 into adenosylcobalamin (AdoCbl). Adenosylcobalamin acts as a required cofactor for the mitochondrial enzyme methylmalonyl-CoA mutase, which is vital for breaking down specific amino acids (such as isoleucine, valine, threonine, and methionine), odd-chain fatty acids, and cholesterol. When functional MMAB protein is deficient, methylmalonyl-CoA mutase cannot function correctly, leading to the toxic accumulation of methylmalonic acid and other metabolic intermediates in tissues and circulating blood.
Associated Health Conditions and Clinical Evidence
Pathogenic variants in MMAB are linked to methylmalonic aciduria type cblB, a serious inborn error of metabolism documented in [OMIM Entry 251110](https://omim.org/entry/251110). According to the [Gene Curation Coalition (GenCC)](https://thegencc.org/submissions/SGC-106497.2), the gene-disease relationship between MMAB and cblB-type methylmalonic aciduria is definitively established. However, the variant-level evidence for rs869320655 itself is classified as limited due to its extreme scarcity in published literature. Individuals who inherit pathogenic variants on both copies of the MMAB gene (either homozygous or compound heterozygous) may experience metabolic crises, developmental delays, and acidemia. Heterozygous individuals carrying only a single copy do not typically develop symptoms but are asymptomatic carriers.
Frequency in General and Ancestral Populations
Population genomics resources like the Genome Aggregation Database (gnomAD) show that rs869320655 is exceptionally rare in the general public. Large-scale sequencing cohorts document an overall allele frequency well below 0.00005, meaning it is absent from the vast majority of individuals across diverse global ancestries. Because pathogenic variants in MMAB are infrequent overall, cblB-type methylmalonic aciduria itself is recognized as a rare condition. Some specific founder mutations in MMAB are enriched in distinct populations—such as certain variants identified in European or Icelandic cohorts—but rs869320655 remains an ultra-rare allele without strong evidence of isolated geographic clustering.
Understanding What You Can and Cannot Do
Receiving genetic data that includes rare variants like rs869320655 requires careful interpretation. Direct-to-consumer genotyping platforms are prone to false positives when reporting ultra-rare variants because common arrays are optimized for frequent polymorphisms. Therefore, this data cannot be used to self-diagnose carrier status or direct medical management. If personal or carrier screening data suggests you carry rs869320655, it should be verified in an accredited diagnostic laboratory. Prospective parents who are confirmed carriers often consult a certified genetic counselor to evaluate reproductive implications, including partner carrier testing to assess the joint likelihood of passing two altered alleles to future biological children.
How common is this variant?
The rs869320655 variant is exceptionally rare across all global ancestries, exhibiting an allele frequency in gnomAD of less than 0.00005.
Frequently asked questions
What is methylmalonic aciduria type cblB?
Methylmalonic aciduria type cblB is an autosomal recessive metabolic disorder caused by defects in the MMAB gene. It impairs the conversion of vitamin B12 into adenosylcobalamin, preventing the body from properly processing specific amino acids and lipids.
Does carrying one copy of rs869320655 mean I have methylmalonic aciduria?
No. Methylmalonic aciduria type cblB requires mutations on both copies of the MMAB gene to cause disease. Carrying a single copy makes someone an asymptomatic carrier who does not exhibit the medical features of the disorder.
Can consumer genetic testing reliably detect rs869320655?
Direct-to-consumer microarray tests have higher error rates when reporting ultra-rare variants like rs869320655. Any unexpected carrier result should be confirmed through a certified clinical genetics laboratory using targeted sequencing before making medical decisions.
What should I do if my carrier screen reports rs869320655?
If a certified carrier screening test confirms this variant, consider discussing the finding with a genetic counselor. They can explain reproductive implications and determine whether carrier testing for your reproductive partner is recommended.
Sources & further reading
Educational information only, last refreshed 9/6/2026. Not medical advice — these associations describe population statistics, not individual predictions.
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This variant is associated with carrier status for methylmalonic aciduria type cblB.
This variant is associated with carrier status for methylmalonic aciduria type cblB.
This variant is identified as a carrier mutation for methylmalonic aciduria type cblB.
This variant is associated with carrier status for methylmalonic aciduria type cblB.