Heterozygous carriers of pathogenic/likely pathogenic variants in autosomal recessive disorders seem to be asymptomatic. However, in recent years, an increasing number of case reports have suggested that mild and unspecific symptoms can occur in some heterozygotes, as symptomatic heterozygotes have been identified across different disease types, including neurological, neuromuscular, hematological, and pulmonary diseases. The symptoms are usually milder in heterozygotes than in biallelic variants and occur “later in life”. The status of symptomatic heterozygotes as separate entities is often disputed, and alternative diagnoses are considered. Indeed, often only a thin line exists between dual, dominant, and recessive modes of inheritance and symptomatic heterozygosity. Interestingly, recent population studies have found global disease effects in heterozygous carriers of some genetic variants. What makes the few heterozygotes symptomatic, while the majority show no symptoms? The molecular basis of this phenomenon is still unknown. Possible explanations include undiscovered deep-splicing variants, genetic and environmental modifiers, digenic/oligogenic inheritance, skewed methylation patterns, and mutational burden. Symptomatic heterozygotes are rarely reported in the literature, mainly because most did not undergo the complete diagnostic procedure, so alternative diagnoses could not be conclusively excluded. However, despite the increasing accessibility to high-throughput technologies, there still seems to be a small group of patients with mild symptoms and just one variant of autosomes in biallelic diseases. Here, we present some examples, the current state of knowledge, and possible explanations for this phenomenon, and thus argue against the existing dominant/recessive classification.

Introduction: Biallelic pathogenic variants in the LPL gene are associated with familial lipoprotein lipase (LPL) deficiency. Homozygotes exhibit very severe hypertriglyceridemia (HTG) already in childhood with phenotypic features, such as pancreatitis, abdominal pain and xanthomata. Recent studies showed that HTG levels varied greatly between monoallelic LPL pathogenic/likely pathogenic variant carriers. The aim of our study is to investigate whether heterozygotes for pathogenic variants in the LPL gene in the Polish population may have clinical symptoms and to what extent.Material and methods: Genetic data derived from the Polish cohort of 5623 whole exome sequenced patients. In 52 cases the indication for WES genetic testing was “hypertriglyceridemia '' and for 5571 there was another clinical indication, mainly autism spectrum disorder, dysmorphia and neurodegenerative diseases.Results: We present 22 heterozygous and 2 homozygous/compound heterozygotes individuals for the pathogenic/likely pathogenic LPL variant and describe HTG levels, phenotypic manifestations and age of onset in the context of molecular findings where available. We report for the first time heterozygous LPL individuals with a very severe TG (TG ≥ 22.6 mmol/L; >2000 mg/dL) and additional symptoms such as pancreatitis and recurrent abdominal pain.Conclusions: We argue that although the individuals carrying the single LPL pathogenic/likely pathogenic variant display the whole disease spectrum, the severe phenotype of heterozygotes dominant LPL-related HTG may also exists.