Do families have a predisposition to PR3‑positive ANCA‑associated vasculitis (granulomatosis with polyangiitis) given that my brother and sister both have saddle‑nose deformity and are PR3‑positive?

Yes, familial clustering of PR3-positive ANCA vasculitis (GPA) does occur, and your siblings' presentation is consistent with documented familial cases.

Familial association of PR3-positive ANCA-associated vasculitis, particularly granulomatosis with polyangiitis (GPA), is well-documented though rare, and your family's presentation with both siblings having saddle nose deformity and PR3 positivity represents a recognized pattern of genetic susceptibility. 1, 2

Evidence for Familial Predisposition

Strong Familial Clustering Data

Recent case-control studies demonstrate compelling evidence for familial aggregation:

• First-degree relatives of AAV patients are 4 times more likely to develop an immune-mediated inflammatory disease (IMID) compared to relatives of controls (OR 4.03,95% CI 2.46-6.62) 2
• 46.2% of AAV patients report a family history of IMID versus only 16.6% of controls 2
• After adjusting for age and family size, AAV cases are 7 times more likely to have a positive family history (OR 7.25,95% CI 3.27-16.09) 2
• Female first-degree relatives carry double the risk compared to male relatives 2

Genetic Basis

The genetic architecture of PR3-positive AAV involves multiple susceptibility loci:

• PR3-ANCA positive disease has a distinct genetic background from MPO-ANCA disease 3, 4
• Identified susceptibility genes include: CTLA4, PTPN22, COL11A2, SERPINA1, and MHC class II gene clusters 4
• Alpha-1 antitrypsin deficiency (PI*Z allele) was the first identified genetic risk factor specifically for PR3-ANCA vasculitis 5
• PR3 surface expression levels are genetically determined, with elevated expression correlating with increased relapse risk 5
• Specific chromosome 6p haplotypes show strong association with PR3-ANCA+ disease 6

Clinical Implications for Your Family

Shared Phenotypic Features

Your siblings' presentation with saddle nose deformity indicates:

• Significant sinonasal involvement, a hallmark of GPA that can cause cartilage destruction and septal perforation 7, 8
• This ENT-predominant pattern is characteristic of PR3-positive disease 3
• The shared clinical phenotype (both having saddle nose) alongside identical serotype (both PR3-positive) strongly suggests shared genetic susceptibility

Important Caveats

While familial cases are documented:

• They remain rare - most GPA cases are sporadic 1
• Environmental triggers and epigenetic modifications likely interact with genetic predisposition 4
• The exact inheritance pattern is complex and not Mendelian; multiple genes contribute small individual effects 4

Practical Recommendations for Family Members

Given the documented familial risk:

• Other first-degree relatives should be counseled about increased IMID risk (approximately 12% prevalence versus 3.5% in general population) 2
• Maintain high clinical suspicion for AAV symptoms in relatives: chronic sinusitis, epistaxis, hearing loss, pulmonary symptoms, hematuria, or constitutional symptoms
• Consider earlier ANCA testing if suggestive symptoms develop
• Female relatives warrant particular attention given their doubled risk 2

Disease-Specific Monitoring

For your affected siblings, recognize that PR3-positive disease carries higher relapse risk compared to MPO-positive disease 3, 9:

• This genetic predisposition to relapse should inform maintenance therapy duration (18 months to 4 years) 9
• Persistent PR3-ANCA positivity after remission induction further increases relapse risk 9
• Extended maintenance therapy may be particularly warranted in familial cases with PR3 positivity

The convergence of familial clustering data, specific genetic associations with PR3-ANCA disease, and your siblings' identical clinical and serological presentation confirms that your family demonstrates recognized genetic susceptibility to PR3-positive ANCA vasculitis.