ALMS1 Mutations: Understanding Your Genetic Test Report

The text on a genetic test report can feel like another language. Terms like "c.10775delC," "p.Pro3592Glnfs4," "exon 16," and "compound heterozygous" land in a parent's lap with no key. This article translates the most common terms found in ALMS1* test reports, explains the spectrum of variant types, and addresses the question many parents have: does the specific variant affect how severe Alström is?

A typical ALMS1 report includes:

• Patient information and lab details
• Test method — which sequencing technology was used, which regions were covered
• Variants identified — the specific changes found, with their HGVS nomenclature
• Variant interpretation — pathogenic, likely pathogenic, VUS, etc.
• Zygosity — heterozygous (one copy), homozygous (two copies of the same), compound heterozygous (two different variants)
• Inheritance information — autosomal recessive
• Clinical correlation — interpretation in the context of the patient's clinical findings
• Recommendations — for family testing or further investigation¹

Variants are described in a standard format called HGVS (Human Genome Variation Society). For ALMS1, you'll see:

• c. (something) — change at the DNA level (in the coding sequence)
• p. (something) — change at the protein level
• del — deletion
• ins — insertion
• dup — duplication
• fs — frameshift
• * (asterisk) — stop codon

Examples:

• c.10775delC — a single nucleotide (C) is deleted at coding position 10775
• *p.Pro3592Glnfs4** — protein change beginning with proline at position 3592, becoming glutamine, then a frameshift, with a stop codon 4 amino acids later
• *c.5584C>T (p.Arg1862)** — coding nucleotide 5584 changes from C to T, resulting in a stop codon at protein position 1862 (a nonsense mutation)

You don't need to memorize this — your geneticist or genetic counselor will translate.

A 2025 large cohort study analyzing 127 Chinese Alström patients found the following variant distribution:²

• Nonsense mutations — about 47%
• Frameshift mutations — about 45%
• Splice-site mutations — about 3%
• Missense mutations — about 2%
• Exon-level deletions — about 2%
• Larger segmental deletions — about 0.4%

Earlier studies in European, North American, and other populations have shown similar patterns. The dominance of nonsense and frameshift mutations is striking — both result in a truncated, non-functional ALMS1 protein.

Most pathogenic ALMS1 variants cluster in three regions:

• Exon 8 — a long exon with many described mutations
• Exon 10 — the longest exon and the largest single hotspot
• Exon 16 — also enriched for pathogenic changes

Some clinical genetics labs offer a tiered testing approach that starts with these three exons before sequencing the entire gene.³

Most affected individuals carry compound heterozygous variants — two different mutations, one inherited from each parent. For example, a child might have a nonsense mutation on one chromosome and a frameshift on the other. This is the most common pattern.

Homozygous variants — two copies of the same mutation — are more common in consanguineous families (parents who are related). The same variant gets inherited from both parents because it's been present in the shared family lineage.

Both patterns produce the syndrome. The clinical picture isn't reliably different between compound heterozygotes and homozygotes.

A common question after diagnosis is whether the specific variants predict how severe the syndrome will be. The honest answer is: mostly no, with limited exceptions.

What's been observed:

• The location of mutations within the gene shows weak correlation with severity
• Some studies have suggested mutations near the C-terminus may correlate with milder cardiac phenotype, but findings have not been consistently replicated
• Severe insulin resistance, cardiomyopathy, retinal degeneration, and hearing loss develop in nearly all patients regardless of which specific variants they carry
• The age and severity of complications vary widely even among siblings with identical variants

This means knowing the specific variant is useful for diagnostic confirmation, family planning, and (eventually) personalized therapy — but doesn't predict your child's specific clinical course.⁴

Several scenarios prompt further analysis:

Variant of uncertain significance (VUS)

A VUS is a change that may or may not be disease-causing. Reasons it might be classified as VUS:

• Not seen before in databases
• Some computational predictions suggest harm but evidence is limited
• Not segregating clearly in the family

VUSs are sometimes reclassified as pathogenic or benign as more data accumulates. We cover this in Variants of Uncertain Significance in Alström Testing.

Only one pathogenic variant found

Alström is autosomal recessive — two pathogenic variants are needed for a clinical diagnosis. Finding only one means:

• The second variant may be in a region not covered by sequencing (deep intronic, regulatory, large deletion)
• Your child may have a different condition

Next steps include deletion/duplication analysis, intronic sequencing, or whole-exome sequencing.

Heterozygous parent with affected child

If only the affected child carries a particular variant and the parent who supposedly transmitted it doesn't, possibilities include:

• A new (de novo) variant
• Non-paternity (rare but possible)
• Lab error (uncommon with modern testing)
• Mosaicism in a parent (the variant present in some cells but not others, including not in the cells tested)

The genetic counselor walks through these.

• Alström Syndrome Genetics: ALMS1 Gene Explained
• What Is the ALMS1 Gene?
• Genetic Testing for Alström Syndrome: A Parent's Guide
• Variants of Uncertain Significance in Alström Testing
• The Genetics of Alström Syndrome for Healthcare Providers

April 30, 2026.