Early Clinical Diagnosis of PC1/3 Deficiency in a Patient With a Novel Homozygous PCSK1 Splice-Site Mutation

Medical semiology of patients with monogenic obesity: A systematic review

Patients with monogenic obesity display numerous medical features on top of hyperphagic obesity, but no study to date has provided an exhaustive description of their semiology. Two reviewers independently conducted a systematic review of MEDLINE, Embase, and Web of Science Core Collection databases from inception to January 2022 to identify studies that described symptoms of patients carrying pathogenic mutations in at least one of eight monogenic obesity genes (ADCY3, LEP, LEPR, MC3R, MC4R, MRAP2, PCSK1, and POMC). Of 5207 identified references, 269 were deemed eligible after title and abstract screening, full-text reading, and risk of bias and quality assessment. Data extraction included mutation spectrum and mode of inheritance, clinical presentation (e.g., anthropometry, energy intake and eating behaviors, digestive function, puberty and fertility, cognitive features, infectious diseases, morphological characteristics, chronic respiratory disease, and cardiovascular disease), biological characteristics (metabolic profile, endocrinology, hematology), radiological features, and treatments. The review provides an exhaustive description of mandatory, non-mandatory, and unique symptoms in heterozygous and homozygous carriers of mutation in eight monogenic obesity genes. This information is critical to help clinicians to orient genetic testing in subsets of patients with suspected monogenic obesity and provide actionable treatments (e.g., recombinant leptin and MC4R agonist).

Rare Heterozygous PCSK1 Variants in Human Obesity: The Contribution of the p.Y181H Variant and a Literature Review

Recently, it was reported that heterozygous PCSK1 variants, causing partial PC1/3 deficiency, result in a significant increased risk for obesity. This effect was almost exclusively generated by the rare p.Y181H (rs145592525, GRCh38.p13 NM_000439.5:c.541T>C) variant, which affects PC1/3 maturation but not enzymatic capacity. As most of the identified individuals with the heterozygous p.Y181H variant were of Belgian origin, we performed a follow-up study in a population of 481 children and adolescents with obesity, and 486 lean individuals. We identified three obese (0.62%) and four lean (0.82%) p.Y181H carriers (p = 0.506) through sanger sequencing and high resulting melting curve analysis, indicating no association with obesity. Haplotype analysis was performed in 13 p.Y181H carriers, 20 non-carriers (10 with obesity and 10 lean), and two p.Y181H families, and showed identical haplotypes for all heterozygous carriers (p < 0.001). Likewise, state-of-the-art literature concerning the role of rare heterozygous PCSK1 variants implies them to be rarely associated with monogenic obesity, as first-degree carrier relatives of patients with PC1/3 deficiency are mostly not reported to be obese. Furthermore, recent meta-analyses have only indicated a robust association for scarce disruptive heterozygous PCSK1 variants with obesity, while clinical significance is less or sometimes lacking for most nonsynonymous variants.

Modulation of PC1/3 activity by a rare double-site homozygous mutation

OBJECTIVES

Preprotein convertase 1/3 deficiency is a rare autosomal recessive disorder in which patients present with malabsorptive diarrhea and a series of symptoms of endocrine disorders such as polydipsia, reactive hypoglycemia, growth hormone deficiency, hypothyroidism, adrenal insufficiency, and early onset obesity. In its essence, pituitary hormone deficiency is caused by insufficient cleavage of pituitary prohormones. Here, we describe a female child with a rare double-site homozygous mutation in PCSK1 (Proprotein convertase subtilisin/kexin-type 1) gene, and thereby intend to investigate the relationship between these novel mutation sites and changes in protein synthesis and function.

METHODS

We tested this patient's blood and urine fecal indicators of infection, blood electrolytes, and relevant endocrine hormone levels in the laboratory. Next Generation Sequencing was applied to screen the patient's DNA. Western Blot was performed to evaluate the mutant protein's expression. The enzymatic activity was measured as the rate of cleavage of a synthetic fluorogenic substrate in a specific solution.

RESULTS

We found that this patient presented shortly after birth with uncorrectable diarrhea and symptoms of metabolic acidosis with hypothyroidism. Next Generation Sequencing revealed that a rare double-site homozygous missense mutation, c.763G > A (p.G255R) and c.758C > T (p.S253L), were detected in exon 7 of PCSK1 (Proprotein convertase subtilisin/kexin-type 1) gene on chromosome 5 of the patient. Western blotting revealed that there was no significant decrease in protein synthesis levels in the mutant phenotype compared to the wild type. Compared with WT type, the proteins expressed by the mutations showed a significant decrease in the enzyme activity towards the fluorescent substrates. However, neither the single site mutation p.S253L or p.G255R, nor the double-site mutation of both, all showed no significant differences from each other.

CONCLUSIONS

These two missense mutations have not been reported before, and it is even rarer to find homozygous variation of two sites in one patient. This study identifies two novel mutations for the first time and further investigates the changes in protein synthesis and enzyme activity, providing a new pathway to continue to explore the pathogenesis of diseases associated with the function of PC1/3.

A novel mutation of PCSK1 responsible for PC1/3 deficiency in two siblings

Proprotein convertase 1 (PCSK1, PC1/3) deficiency is an uncommon cause of neonatal malabsorptive diarrhoea associated with endocrinopathies that are due to the disrupted processing of a large number of prohormones, including proinsulin. To date, only 26 cases have been reported. Herein, we describe two siblings with typical features including severe congenital diarrhoea, central diabetes insipidus, growth hormone deficiency, and hypoadrenalism. Next generation sequencing found a homozygous missense mutation in exon 5 of PCSK1 gene, c.500A>C (p.Asp167Ala), located within the catalytic domain. Both patients presented a high level of proinsulin. In the first years of life they required parenteral nutrition and hormone replacement therapy. The patients, aged 3 and 1.5 years, experienced several infectious episodes associated with septic shocks. While the mechanism underlying intestinal failure remains poorly investigated, parenteral nutrition is essential in order to ensure normal growth in early childhood.

Enteroendocrine Dysfunction in Two Saudi Sisters

Proprotein convertase (PC) deficiency is a rare autosomal recessive disorder caused by mutations in proprotein convertase subtilisin/kexin type 1 (PCSK1). It is characterized by severe malabsorptive early-onset diarrhea, obesity, and systemic endocrinopathies. Only few cases have been reported in the literature; we have add two female sisters with some difference in clinical progress. Herein, we describe two sisters with congenital osmotic diarrhea diagnosed with PC1/3 deficiency, causing malabsorptive diarrhea and enteroendocrine dysfunction, who presented with chronic enteropathy with hypernatremia but with different expressivity. PC1/3 deficiency presents with symptoms and signs that mimic glucose-galactose malabsorption. Because of the clinical paucity and heterogeneity of congenital enteropathies, whole-exome sequencing may be of great help towards early diagnosis and effective treatment.

Molecular mechanisms of β-cell dysfunction and death in monogenic forms of diabetes

Monogenetic forms of diabetes represent 1%-5% of all diabetes cases and are caused by mutations in a single gene. These mutations, that affect genes involved in pancreatic β-cell development, function and survival, or insulin regulation, may be dominant or recessive, inherited or de novo. Most patients with monogenic diabetes are very commonly misdiagnosed as having type 1 or type 2 diabetes. The severity of their symptoms depends on the nature of the mutation, the function of the affected gene and, in some cases, the influence of additional genetic or environmental factors that modulate severity and penetrance. In some patients, diabetes is accompanied by other syndromic features such as deafness, blindness, microcephaly, liver and intestinal defects, among others. The age of diabetes onset may also vary from neonatal until early adulthood manifestations. Since the different mutations result in diverse clinical presentations, patients usually need different treatments that range from just diet and exercise, to the requirement of exogenous insulin or other hypoglycemic drugs, e.g., sulfonylureas or glucagon-like peptide 1 analogs to control their glycemia. As a consequence, awareness and correct diagnosis are crucial for the proper management and treatment of monogenic diabetes patients. In this chapter, we describe mutations causing different monogenic forms of diabetes associated with inadequate pancreas development or impaired β-cell function and survival, and discuss the molecular mechanisms involved in β-cell demise.

Association of PCSK1 gene polymorphisms with abdominal fat content in broilers

Protein proteolytic enzymes (Proprotein Convertase, PC) is a Ca²⁺ -dependent serine protease family, whose main function is to cleave precursors of biologically inactive proteins or peptide chains into active functional molecules. Proprotein convertase subtilisin/kexin type 1 (PCSK1) gene is mainly expressed in nerve and endocrine tissues. In this study, PCSK1 was selected as an important candidate gene for abdominal fat content in broilers. We cloned the exon region of chicken PCSK1 gene and found six single-nucleotide polymorphisms (SNPs). Association analysis was carried out and we found that the polymorphisms of these six SNPs were significantly associated with abdominal fat content in G19 and G20 populations. Five of these SNPs were significantly associated with abdominal fat content in G19 and G20 combined population. The polymorphism of these five SNPs was significantly correlated with the abdominal fat content of AA broilers. Together, our study demonstrated that c.927T>C, c.1880C>T, c.*900G>A, and c.*1164C>T were significantly associated with abdominal fat content in populations used in this study, which means that these SNPs in PCSK1 gene could be used as candidate markers to select lean broiler lines.

A New Case of PCSK1 Pathogenic Variant With Congenital Proprotein Convertase 1/3 Deficiency and Literature Review

ISSUE

To report a homozygous pathogenic variant in PCSK1 in a boy affected with proprotein convertase 1/3 (PC1/3) deficiency.

CASE DESCRIPTION AND LITERATURE REVIEW

A male infant born to consanguineous Turkish parents presented in the first week of life with transient central diabetes insipidus, watery diarrhea, micropenis due to hypogonadotropic hypogonadism and GH deficiency, and transient asymptomatic hypoglycemia. Further endocrine defects gradually appeared, including central hypothyroidism and mild central hypocortisolism (at 1 year), central diabetes insipidus that reappeared progressively (at 2.5 years), and obesity (at 2 years). Whole-exome sequencing revealed a homozygous nonsense pathogenic variant (NM_000439.4) c. 595 C>T in exon 5 of PCSK1, not yet reported in cases of PC1/3 deficiency. To date, 26 cases of PC1/3 deficiency have been reported in the literature. All individuals had early and severe malabsorptive diarrhea and 83% had polyuria-polydipsia syndrome (before 5 years). Most (79%) had early onset obesity. Various endocrine disorders were present, including GH deficiency (44%), mild central hypothyroidism (56%), central hypogonadism (44%), central hypocortisolism (57%), and postprandial hypoglycemia (52%). When described (n = 15), proinsulin levels were consistently high: between 8 and 154 times the upper limit of normal (mean 74).

CONCLUSION

We described a homozygous nonsense pathogenic variant (NM_000439.4) c. 595 C>T in exon 5 of PCSK1 in a boy with congenital PC1/3 deficiency. Elevated proinsulin could be useful in the diagnosis of this condition.

Genetics of Obesity in Consanguineous Populations: Toward Precision Medicine and the Discovery of Novel Obesity Genes

OBJECTIVE

Consanguinity has been instrumental in the elucidation of many Mendelian genetic diseases. Here, the unique advantage of consanguineous populations was considered in the quest for genes causing obesity.

METHODS

PubMed was searched for articles relevant to consanguinity and obesity published between 1995 and 2016. Some earlier articles of interest were also consulted.

RESULTS

Although obesity is the most heritable disorder, even in outbred populations, only 2% to 5% of severe obesity cases have so far been proven to be caused by single gene mutations. In some highly consanguineous populations, a remarkably higher proportion of obesity cases because of known and novel monogenic variants has been identified (up to 30%).

CONCLUSIONS

Combining the power conferred by consanguinity with current large-capacity sequencing techniques should bring new genetic factors and molecular mechanisms to the fore, unveiling a large part of the yet-elusive neurohumoral circuitry involved in the regulation of energy homeostasis and appetite. Importantly, the undertaking of such initiatives is destined to unfold novel targets for the development of precision medicine relevant to different forms of obesity.

Ethnic and population differences in the genetic predisposition to human obesity

Obesity rates have escalated to the point of a global pandemic with varying prevalence across ethnic groups. These differences are partially explained by lifestyle factors in addition to genetic predisposition to obesity. This review provides a comprehensive examination of the ethnic differences in the genetic architecture of obesity. Using examples from evolution, heritability, admixture, monogenic and polygenic studies of obesity, we provide explanations for ethnic differences in the prevalence of obesity. The debate over definitions of race and ethnicity, the advantages and limitations of multi-ethnic studies and future directions of research are also discussed. Multi-ethnic studies have great potential to provide a better understanding of ethnic differences in the prevalence of obesity that may result in more targeted and personalized obesity treatments.

Leveraging Rules of Nonsense-Mediated mRNA Decay for Genome Engineering and Personalized Medicine

Nonsense-mediated mRNA decay (NMD) is a eukaryotic mRNA quality control and regulatory process that plays direct roles in human health and disease. In this Minireview, we discuss how understanding the molecular events that trigger NMD can facilitate strategic targeting of genes via CRISPR/Cas9 technologies and also inform disease diagnostics and treatments.

Functional and clinical relevance of novel and known PCSK1 variants for childhood obesity and glucose metabolism

Objective

Variants in Proprotein Convertase Subtilisin/Kexin Type 1 (PCSK1) may be causative for obesity as suggested by monogenic cases and association studies. Here we assessed the functional relevance in experimental studies and the clinical relevance through detailed metabolic phenotyping of newly identified and known PCSK1 variants in children.

Results

In 52 obese children selected for elevated proinsulin levels and/or impaired glucose tolerance, we found eight known variants and two novel heterozygous variants (c.1095 + 1G > A and p.S24C) by sequencing the PCSK1 gene. Patients with the new variants presented with extreme obesity, impaired glucose tolerance, and PCOS. Functionally, c.1095 + 1G > A caused skipping of exon8 translation and a complete loss of enzymatic activity. The protein was retained within the endoplasmic reticulum (ER) causing ER stress. The p.S24C variant had no functional effect on protein size, cell trafficking, or enzymatic activity. The known variants rs6230, rs35753085, and rs725522 in the 5′ end did not affect PCSK1 promoter activity.

In clinical association studies in 1673 lean and obese children, we confirmed associations of rs6232 and rs6234 with BMI-SDS and of rs725522 with glucose stimulated insulin secretion and Matsuda index. We did not find the new variants in any other subjects.

Conclusions

We identified and functionally characterized two rare novel PCSK1 variants of which c.1095 + 1G > A caused complete loss of protein function. In addition to confirming rs6232 and rs6234 in PCSK1 as polygenic risk variants for childhood obesity, we describe an association of rs725522 with insulin metabolism. Our results support the contribution of PCSK1 variants to obesity predisposition in children.

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We identified two novel variants in PCSK1 in severely obese adolescents.

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The phenotype of these two heterozygous carriers is more severe than in “common childhood obesity”.

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The ΔEx8 variant leads to a truncated protein with a complete loss of function, which is retained within the ER.

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For common variant rs725522 detailed metabolic phenotyping revealed impaired glucose dynamics.

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Overall, variants in PCSK1 are not only associated with childhood obesity, but a more severe phenotype than in BMI-matched controls.

PCSK1 Mutations and Human Endocrinopathies: From Obesity to Gastrointestinal Disorders

Prohormone convertase 1/3, encoded by the PCSK1 gene, is a serine endoprotease that is involved in the processing of a variety of proneuropeptides and prohormones. Humans who are homozygous or compound heterozygous for loss-of-function mutations in PCSK1 exhibit a variable and pleiotropic syndrome consisting of some or all of the following: obesity, malabsorptive diarrhea, hypogonadotropic hypogonadism, altered thyroid and adrenal function, and impaired regulation of plasma glucose levels in association with elevated circulating proinsulin-to-insulin ratio. Recently, more common variants in the PCSK1 gene have been found to be associated with alterations in body mass index, increased circulating proinsulin levels, and defects in glucose homeostasis. This review provides an overview of the endocrinopathies and other disorders observed in prohormone convertase 1/3-deficient patients, discusses the possible biochemical basis for these manifestations of the disease, and proposes a model whereby certain missense mutations in PCSK1 may result in proteins with a dominant negative action.