Scientific Background¶

The MUC1 Gene¶

The MUC1 gene, located on chromosome 1q22, encodes mucin-1, a transmembrane glycoprotein expressed on the apical surface of epithelial cells, including renal tubular cells (Kirby et al., Nat Genet 2013). A defining feature of MUC1 is a large coding Variable Number Tandem Repeat (VNTR) in exon 2.

Variable Number Tandem Repeats¶

VNTRs are genomic regions where short sequence motifs repeat in tandem, with copy numbers varying across individuals. In MUC1, each repeat unit spans 60 bp (encoding 20 amino acids), and alleles carry between 20 and over 125 repeat units (Saei et al., iScience 2023; Kirby et al., Nat Genet 2013).

ADTKD-MUC1¶

Autosomal Dominant Tubulointerstitial Kidney Disease caused by MUC1 mutations (ADTKD-MUC1) produces progressive tubular damage and renal fibrosis, leading to end-stage kidney disease in adulthood (Bleyer et al., Kidney Int 2014). Causative mutations are frameshift insertions or deletions within the VNTR coding sequence that generate a truncated, toxic protein termed MUC1-fs (Dvela-Levitt et al., Cell 2019). The most common mutation is a single cytosine duplication (dupC) in the homopolymer tract at position 67 of the repeat unit (Kirby et al., Nat Genet 2013).

The Genotyping Challenge¶

Standard short-read aligners (BWA, Bowtie2) depend on unique mapping locations. Within the repetitive, GC-rich VNTR, reads map ambiguously across multiple repeat units with zero mapping quality. Conventional variant callers (such as GATK HaplotypeCaller or DeepVariant) routinely miss pathogenic frameshift insertions in this region (Saei et al., iScience 2023).

VNtyper 2 Approach¶

VNtyper 2 incorporates the Kestrel variant caller (Audano et al., Bioinformatics 2018), which reconstructs local haplotypes directly from k-mer frequency spectra without reference alignment. This design avoids mapping bias in repetitive sequence. VNtyper 2 refines raw Kestrel outputs through empirical depth-score thresholds, quality metric checks, and confidence classifications, with optional orthogonal validation via the alignment-based adVNTR caller (Park et al., iScience 2022).