* 600922

MYOSIN LIGHT CHAIN KINASE; MYLK


Alternative titles; symbols

MYOSIN LIGHT POLYPEPTIDE KINASE; MLCK


Other entities represented in this entry:

KINASE-RELATED PROTEIN, INCLUDED; KRP, INCLUDED
TELOKIN, INCLUDED

HGNC Approved Gene Symbol: MYLK

Cytogenetic location: 3q21.1     Genomic coordinates (GRCh38): 3:123,610,049-123,884,332 (from NCBI)


Gene-Phenotype Relationships
Location Phenotype Phenotype
MIM number
Inheritance Phenotype
mapping key
3q21.1 Aortic aneurysm, familial thoracic 7 613780 AD 3
Megacystis-microcolon-intestinal hypoperistalsis syndrome 1 249210 AR 3

TEXT

Description

The contraction of smooth muscle begins with the phosphorylation of the light chain of myosin (e.g., MYL2; 160781), a reaction catalyzed by myosin light chain kinase that is itself activated by the binding of calcium-calmodulin (see 114180). This key enzyme in muscle contraction, which exists in both nonmuscle and smooth muscle isoforms, has been shown by immunohistology to be present in neurons and glia.


Cloning and Expression

Potier et al. (1995) cloned the cDNA for human myosin light chain kinase, which they symbolized MLCK, from hippocampus and showed that it encodes a protein sequence 95% similar to smooth muscle MLCKs but less than 60% similar to skeletal muscle MLCKs. The cDNA clone detected 2 RNA transcripts in human frontal and entorhinal cortex, in hippocampus, and in jejunum, one corresponding to MLCK and the other probably to telokin (kinase-related protein), the carboxy-terminal 154 codons of MLCK expressed as an independent protein in smooth muscle. The authors found that levels of expression were lower in brain than in smooth muscle. Potier et al. (1995) showed that the protein sequence contains a motif of 28 or 24 residues repeated 5 times, the second repeat ending with the putative methionine start codon. These repeats overlap with the second previously reported module of 12 residues repeated 5 times in the human sequence. In addition, the acidic C terminus of all MLCKs from both brain and smooth muscle resembles the C terminus of tubulins.

Garcia et al. (1997) cloned a human endothelial nonmuscle MLCK cDNA encoding a deduced 1,914-amino acid protein with a calculated molecular mass of 210 kD. The protein contains 9 C2-type immunoglobulin-like homology domains, an SH2-binding domain, and a single tyrosine phosphorylation site in the CaM-binding region.

Lazar and Garcia (1999) reported the cloning of several additional nonmuscle variants of MLCK by RT-PCR from umbilical vein endothelial cell RNA. They noted that the full-length MLCK gene contains at least 2 additional promoters that initiate transcription of 2 shorter isoforms. The shorter isoforms include smooth muscle (SM) MLCK, a 5.8-kb transcript that encodes a deduced 130-kD protein, and a 2.6-kb transcript that encodes the deduced kinase-related protein. KRP contains only the final C2 immunoglobulin-like domain. Northern blot analysis detected the full-length, 8.1-kb nonmuscle MLCK isoform in all tissues examined except skeletal muscle, with highest expression in lung, placenta, liver, and kidney, and intermediate expression in heart, brain, and pancreas. The transcript was also detected in fetal lung and kidney, with lower expression in fetal brain and liver. The 5.8-kb SM-MLCK transcript was detected in all adult tissues except liver, and in fetal lung and kidney, with weaker expression in fetal brain and liver. Lazar and Garcia (1999) also identified several isoforms that resulted from in-frame internal deletions and were widely expressed in adult and fetal tissues. The dominant isoform, which they designated MLCK2, contains a deletion of residues 437-505, causing loss of the tyrosine phosphorylation site and the SH2 binding site.

Watterson et al. (1999) cloned kinase-related protein from a genomic DNA library by PCR using primers based on a previously isolated human placenta KRP sequence. They noted that the human and chicken KRP proteins share 80% sequence identity. Northern blot analysis detected a 2.7-kb KRP transcript in all adult and fetal tissues examined, with highest expression in placenta, brain, heart, colon, small intestine, and fetal small intestine. Probing with a sequence common to the 3 main start site variants, Watterson et al. (1999) identified transcripts of 2.7 and 5.5 kb in heart, and of 2.7, 5.5, and 9.0 kb in placenta. By immunohistochemical analysis of adult and fetal heart sections, they found both full-length MLCK and the shorter KRP in cardiac muscle and in the smooth muscle layer of major blood vessels.

By immunohistochemistry in human embryonic specimens of small intestine and bladder, Halim et al. (2017) observed MYLK localized in smooth muscle cells of all muscular layers of the intestine and bladder, from weeks 9 to 22 of development.


Gene Structure

Watterson et al. (1999) noted that the various isoforms of MLCK are encoded by differential use of 31 coding exons. They also noted that KRP is derived from the last 3 exons spanning approximately 6.0 kb of the MLCK gene, and that the transcription initiation site for KRP lies within the intron preceding exon 29.


Mapping

By PCR and Southern blotting using 2 somatic cell hybrid panels, Potier et al. (1995) localized the MLCK gene to chromosome 3cen-q21.

By analysis of YAC clones, Giorgi et al. (2001) colocalized the MYLK gene with D3S3552 in a greater than 5-Mb region of chromosome 3q21. They confirmed the location of a pseudogene, MYLKP, to chromosome 3p13.


Gene Function

Walker et al. (2001) studied the KRP variant in the rabbit and demonstrated that recombinant rabbit telokin could relax telokin-depleted rabbit ileal smooth muscle in a dose-dependent manner. Mutation analysis revealed that ser13 is the phosphorylation site associated with cyclic nucleotide-induced Ca(2+)-independent relaxation of smooth muscle.

Goeckeler et al. (2000) found that human PAK2 (605022) phosphorylated MLCK on ser439 and ser991, which downregulated MLCK activity and inhibited MLCK-catalyzed phosphorylation of MYL2. Only ser439 of MLCK was phosphorylated by PAK2 in the presence of Ca(2+)/calmodulin. PAK2-catalyzed MLCK phosphorylation limited development of isometric tension in endothelial cells.


Molecular Genetics

Familial Thoracic Aortic Aneurysm 7

Wang et al. (2010) analyzed the MYLK gene in 193 probands from unrelated families in which 2 or more members had thoracic aortic aneurysms or dissections. They identified 2 heterozygous variants (600922.0001 and 600922.0002) that segregated with aortic dissections (AAT7; 613780) in 2 families, respectively, and were not found in 188 ethnically matched controls. Incomplete penetrance was observed in 1 of the families. Three additional MYLK variants were identified in 3 unrelated probands that were not detected in controls, but family members were not available for segregation analysis.

In a large Swedish family segregating autosomal dominant aortic dissection, Hannuksela et al. (2016) identified a 2-bp deletion in the MYLK gene causing a premature termination codon (S1091X) that was present in all 5 affected individuals as well as 9 unaffected family members. In addition, a family member with an intramural hematoma of the descending aorta did not carry the mutation.

Luyckx et al. (2017) screened a cohort of 358 cases of aortic aneurysmal disease for mutations in thoracic aortic aneurysm-associated genes and identified 2 probands who were heterozygous for nonsense mutations in the MYLK gene (Q1458X and R1487X). In the first family, the Q1458X mutation was also identified in the patient's unaffected father, and in the second family, the R1487X mutation was also present in the proband's unaffected half brother.

In a large consanguineous Arab family with thoracic aortic aneurysm and dissection, Shalata et al. (2018) identified a missense mutation in the MYLK gene (A1491S; 600922.0005) that was not found in ethnically matched controls or public variant databases. The mutation was present in homozygosity in 6 severely affected family members and in heterozygosity in 3 patients with a later age of onset; the mutation was also present in heterozygosity in 16 asymptomatic family members. The authors stated that the most appropriate designation for the mode of inheritance in this family was autosomal dominant with incomplete penetrance. Comprehensive medical history and clinical data analysis of homozygous individuals excluded congenital bladder or intestine involvement; there was thus no evidence for megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS; see below) in this family.

Megacystis-Microcolon-Intestinal Hypoperistalsis Syndrome 1

In 2 unrelated consanguineous families in which 5 children had died with megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS1; 249210), Halim et al. (2017) identified homozygosity for mutations in the MYLK gene: a 7-bp duplication (600922.0003) and a splicing variant (600922.0004), respectively. The unaffected parents in each family were heterozygous for the mutation. Noting that heterozygous mutations in MYLK had previously been associated with aortic aneurysm, the authors stated that they were not aware of any cardiac problems in the unaffected parents and suggested that other kinases might salvage smooth muscle contraction of extravisceral organs.


Animal Model

Wang et al. (2010) studied mice with smooth muscle cell-specific knockdown of Mylk and observed increased pools of proteoglycans in the aortic media compared to controls, along with increased expression of lumican (600616) and decorin (125255). Increased collagen staining in the adventitial layer and increased type III collagen (COL3A1; 120180) expression were also identified. In addition, expression of the elastin-degrading metalloproteinase MMP2 (120360) was also increased in the aortas of the mice, although elastic fibers were not degraded in the aortic media.


ALLELIC VARIANTS ( 5 Selected Examples):

.0001 AORTIC ANEURYSM, FAMILIAL THORACIC 7

MYLK, SER1759PRO
  
RCV000023044...

In 3 affected members of a family (family TAA026) with aortic aneurysm and dissection (AAT7; 613780), Wang et al. (2010) identified heterozygosity for a 5275T-C transition in the MYLK gene, resulting in a ser1759-to-pro (S1759P) substitution in the alpha-helix of the calmodulin-binding sequence that was predicted to cause loss of MLCK function by altering calmodulin binding. The mutation segregated with disease in the family and was not found in 188 ethnically matched controls. Transfection studies in COS-7 cells showed minimal endogenous expression of MLCK, and immunoprecipitation studies revealed that binding to calmodulin was abolished with the S1759P mutant. Analysis of kinase activity showed a 6-fold reduction for S1759P compared to wildtype. Wang et al. (2010) noted that affected individuals had acute aortic dissections with little to no aortic enlargement. Examination of ascending aortic tissue from 2 family members showed medial degeneration of the aorta and a significant increase in small arteries in the medial layer.


.0002 AORTIC ANEURYSM, FAMILIAL THORACIC 7

MYLK, ARG1480TER
  
RCV000023045...

In a 51-year-old father and his 18-year-old son (family TAA400) with aortic aneurysm and dissection (AAT7; 613780), Wang et al. (2010) identified heterozygosity for a 4438C-T transition in the MYLK gene, resulting in an arg1480-to-ter (R1480X) substitution that would lead to either nonsense-mediated decay or a truncated protein missing the kinase and calmodulin-binding domains, and was therefore predicted to disrupt kinase activity but not to disturb telokin expression. The father had undergone a type A dissection at 37 years of age, and the son had a type B dissection at 16 years of age. The R1480X mutation was also detected in 5 asymptomatic family members, with ages ranging from 59 years to 76 years. The mutation was not found in 188 ethnically matched controls.


.0003 MEGACYSTIS-MICROCOLON-INTESTINAL HYPOPERISTALSIS SYNDROME 1

MYLK, 7-BP DUP, 3838GAAAGCG
  
RCV000508615...

In a deceased sister and brother of North African origin (family 1) with megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS1; 249210), Halim et al. (2017) identified homozygosity for a 7-bp duplication (c.3838_3844dupGAAAGCG, NM_053025.3) in exon 23 of the MYLK gene,, causing a frameshift predicted to result in a premature termination codon (Glu1282GlyfsTer51). The unaffected consanguineous parents were heterozygous for the mutation, whereas an unaffected younger sister did not carry the mutation; DNA was unavailable from an older sister, who died in utero with a distended bladder. Immunostained specimens of patient small intestine and bladder showed no MYLK signal. The structure and cellular constituents of the bladder and intestine showed no apparent pathologic abnormalities compared to age-matched control samples, suggesting that although MYLK is instrumental for proper functioning of smooth muscle cells, its presence is not required for maintaining the structural architecture of those organs.


.0004 MEGACYSTIS-MICROCOLON-INTESTINAL HYPOPERISTALSIS SYNDROME 1

MYLK, IVS23, C-A, +5
  
RCV000508669...

In a girl of Indian origin (family 2) who died with megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS1; 249210), Halim et al. (2017) identified homozygosity for a splicing mutation (c.3985+5C-A, NM_053025.3) in intron 23 of the MYLK gene, for which her unaffected consanguineous parents were heterozygous. DNA was not available from a younger sister who also died with MMIHS, or from a younger brother who was unaffected. Splicing assays revealed that the mutant construct eliminated all transcription products observed with wildtype MYLK and produced a band with the same size as empty vector, indicating likely skipping of exon 23 with an early stop codon at the beginning of exon 24.


.0005 AORTIC ANEURYSM, FAMILIAL THORACIC 7

MYLK, ALA1491SER
  
RCV000855690

In affected members of a large consanguineous Arab family with thoracic aortic aneurysm and dissection (AAT7; 613780), Shalata et al. (2018) identified a c.4471G-T transversion in exon 27 of the MYLK gene, resulting in an ala1491-to-ser (A1491S) substitution at a conserved residue within the kinase domain. Functional analysis in transfected HeLa cells showed a significant reduction in kinase activity with the A1491S mutant compared to wildtype MYLK. The mutation was not found in 100 ethnically matched controls or in the ExAC or 1000 Genomes Project databases. The mutation was present in homozygosity in 6 severely affected family members with early-onset disease, 5 of whom were deceased, and was found in heterozygosity in 3 individuals with a later age of onset, 1 of whom was deceased; the mutation was also present in heterozygosity in 16 asymptomatic family members. The authors stated that the most appropriate designation for the mode of inheritance in this family was autosomal dominant with incomplete penetrance. In addition, the authors noted that comprehensive medical history and clinical data analysis of homozygotes excluded congenital bladder or intestine involvement in this family.


REFERENCES

  1. Garcia, J. G., Lazar, V., Gilbert-McClain, L. I., Gallagher, P. J., Verin, A. D. Myosin light chain kinase in endothelium: molecular cloning and regulation. Am. J. Resp. Cell Molec. Biol. 16: 489-494, 1997. [PubMed: 9160829, related citations] [Full Text]

  2. Giorgi, D., Brand-Arpon, V., Rouquier, S. The functional myosin light chain kinase (MYLK) gene localizes with marker D3S3552 on human chromosome 3q21 in a greater than 5-Mb yeast artificial chromosome region and is not linked to olfactory receptor genes. Cytogenet. Cell Genet. 92: 85-88, 2001. [PubMed: 11306802, related citations] [Full Text]

  3. Goeckeler, Z. M., Masaracchia, R. A., Zeng, Q., Chew, T.-L., Gallagher, P., Wysolmerski, R. B. Phosphorylation of myosin light chain kinase by p21-activated kinase PAK2. J. Biol. Chem. 275: 18366-18374, 2000. [PubMed: 10748018, related citations] [Full Text]

  4. Halim, D., Brosens, E., Muller, F., Wangler, M. F., Beaudet, A. L., Lupski, J. R., Akdemir, Z. H. C., Doukas, M., Stoop, H. J., de Graaf, B. M., Brouwer, R. W. W., van Ijcken, W. F. J., Oury, J.-F., Rosenblatt, J., Burns, A. J., Tibboel, D., Hofstra, R. M. W., Alves, M. M. Loss-of-function variants in MYLK cause recessive megacystis microcolon intestinal hypoperistalsis syndrome. Am. J. Hum. Genet. 101: 123-129, 2017. [PubMed: 28602422, images, related citations] [Full Text]

  5. Hannuksela, M., Stattin, E.-L., Klar, J., Ameur, A., Johansson, B., Sorensen, K., Carlberg, B. A novel variant in MYLK causes thoracic aortic dissections: genotypic and phenotypic description. BMC Med. Genet. 17: 61, 2016. Note: Electronic Article. [PubMed: 27586135, images, related citations] [Full Text]

  6. Lazar, V., Garcia, J. G. N. A single human myosin light chain kinase gene (MLCK; MYLK) transcribes multiple nonmuscle isoforms. Genomics 57: 256-267, 1999. [PubMed: 10198165, related citations] [Full Text]

  7. Luyckx, I., Proost, D., Hendriks, J. M. H., Saenen, J., Van Craenenbroeck, E. M., Vermeulen, T., Peeters, N., Wuyts, W., Rodrigus, I., Verstraeten, A., Van Laer, L., Loeys, B. L. Two novel MYLK nonsense mutations causing thoracic aortic aneurysms/dissections in patients without apparent family history. Clin. Genet. 92: 444-446, 2017. Note: Erratum: Clin. Genet. 93: 938 only, 2018. [PubMed: 28401540, related citations] [Full Text]

  8. Potier, M.-C., Chelot, E., Pekarsky, Y., Gardiner, K., Rossier, J., Turnell, W. G. The human myosin light chain kinase (MLCK) from hippocampus: cloning, sequencing, expression, and localization to 3cen-q21. Genomics 29: 562-570, 1995. [PubMed: 8575746, related citations] [Full Text]

  9. Shalata, A., Mahroom, M., Milewicz, D. M., Limin, G., Kassum, F., Badarna, K., Tarabeih, N., Assy, N., Fell, R., Cohen, H., Nashashibi, M., Livoff, A., Azab, M., Habib, G., Geiger, D., Weissbrod, O., Nseir, W. Fatal thoracic aortic aneurysm and dissection in a large family with a novel MYLK gene mutation: delineation of the clinical phenotype. Orphanet J. Rare Dis. 13: 41, 2018. Note: Electronic Article. [PubMed: 29544503, images, related citations] [Full Text]

  10. Walker, L. A., MacDonald, J. A., Liu, X., Nakamoto, R. K., Haystead, T. A. J., Somlyo, A. V., Somlyo, A. P. Site-specific phosphorylation and point mutations of telokin modulate its Ca(2+)-desensitizing effect in smooth muscle. J. Biol. Chem. 276: 24519-24524, 2001. [PubMed: 11346659, related citations] [Full Text]

  11. Wang, L., Guo, D., Cao, J., Gong, L., Kamm, K. E., Regalado, E., Li, L., Shete, S., He, W.-Q., Zhu, M.-S., Offermanns, S., Gilchrist, D., Elefteriades, J., Stull, J. T., Milewicz, D. M. Mutations in myosin light chain kinase cause familial aortic dissections. Am. J. Hum. Genet. 87: 701-707, 2010. Note: Erratum: Am. J. Hum. Genet. 88: 516 only, 2011. [PubMed: 21055718, images, related citations] [Full Text]

  12. Watterson, D. M., Schavocky, J. P., Guo, L., Weiss, C., Chlenski, A., Shirinsky, V. P., Van Eldik, L. J., Haiech, J. Analysis of the kinase-related protein gene found at human chromosome 3q21 in a multi-gene cluster: organization, expression, alternative splicing, and polymorphic marker. J. Cell. Biochem. 75: 481-491, 1999. [PubMed: 10536370, related citations]


Marla J. F. O'Neill - updated : 11/07/2019
Bao Lige - updated : 04/18/2019
Marla J. F. O'Neill - updated : 2/24/2011
Patricia A. Hartz - updated : 5/24/2002
Joanna S. Amberger - updated : 6/22/2001
Creation Date:
Victor A. McKusick : 11/7/1995
carol : 09/29/2023
carol : 05/27/2021
carol : 03/17/2020
carol : 12/23/2019
alopez : 11/07/2019
mgross : 04/18/2019
carol : 12/19/2013
wwang : 4/28/2011
wwang : 2/28/2011
terry : 2/24/2011
wwang : 12/20/2005
carol : 5/30/2002
carol : 5/29/2002
carol : 5/29/2002
terry : 5/24/2002
mcapotos : 6/25/2001
joanna : 6/22/2001
alopez : 4/30/1999
alopez : 3/26/1999
jenny : 4/4/1997
mark : 11/9/1995
terry : 11/7/1995

* 600922

MYOSIN LIGHT CHAIN KINASE; MYLK


Alternative titles; symbols

MYOSIN LIGHT POLYPEPTIDE KINASE; MLCK


Other entities represented in this entry:

KINASE-RELATED PROTEIN, INCLUDED; KRP, INCLUDED
TELOKIN, INCLUDED

HGNC Approved Gene Symbol: MYLK

Cytogenetic location: 3q21.1     Genomic coordinates (GRCh38): 3:123,610,049-123,884,332 (from NCBI)


Gene-Phenotype Relationships

Location Phenotype Phenotype
MIM number
Inheritance Phenotype
mapping key
3q21.1 Aortic aneurysm, familial thoracic 7 613780 Autosomal dominant 3
Megacystis-microcolon-intestinal hypoperistalsis syndrome 1 249210 Autosomal recessive 3

TEXT

Description

The contraction of smooth muscle begins with the phosphorylation of the light chain of myosin (e.g., MYL2; 160781), a reaction catalyzed by myosin light chain kinase that is itself activated by the binding of calcium-calmodulin (see 114180). This key enzyme in muscle contraction, which exists in both nonmuscle and smooth muscle isoforms, has been shown by immunohistology to be present in neurons and glia.


Cloning and Expression

Potier et al. (1995) cloned the cDNA for human myosin light chain kinase, which they symbolized MLCK, from hippocampus and showed that it encodes a protein sequence 95% similar to smooth muscle MLCKs but less than 60% similar to skeletal muscle MLCKs. The cDNA clone detected 2 RNA transcripts in human frontal and entorhinal cortex, in hippocampus, and in jejunum, one corresponding to MLCK and the other probably to telokin (kinase-related protein), the carboxy-terminal 154 codons of MLCK expressed as an independent protein in smooth muscle. The authors found that levels of expression were lower in brain than in smooth muscle. Potier et al. (1995) showed that the protein sequence contains a motif of 28 or 24 residues repeated 5 times, the second repeat ending with the putative methionine start codon. These repeats overlap with the second previously reported module of 12 residues repeated 5 times in the human sequence. In addition, the acidic C terminus of all MLCKs from both brain and smooth muscle resembles the C terminus of tubulins.

Garcia et al. (1997) cloned a human endothelial nonmuscle MLCK cDNA encoding a deduced 1,914-amino acid protein with a calculated molecular mass of 210 kD. The protein contains 9 C2-type immunoglobulin-like homology domains, an SH2-binding domain, and a single tyrosine phosphorylation site in the CaM-binding region.

Lazar and Garcia (1999) reported the cloning of several additional nonmuscle variants of MLCK by RT-PCR from umbilical vein endothelial cell RNA. They noted that the full-length MLCK gene contains at least 2 additional promoters that initiate transcription of 2 shorter isoforms. The shorter isoforms include smooth muscle (SM) MLCK, a 5.8-kb transcript that encodes a deduced 130-kD protein, and a 2.6-kb transcript that encodes the deduced kinase-related protein. KRP contains only the final C2 immunoglobulin-like domain. Northern blot analysis detected the full-length, 8.1-kb nonmuscle MLCK isoform in all tissues examined except skeletal muscle, with highest expression in lung, placenta, liver, and kidney, and intermediate expression in heart, brain, and pancreas. The transcript was also detected in fetal lung and kidney, with lower expression in fetal brain and liver. The 5.8-kb SM-MLCK transcript was detected in all adult tissues except liver, and in fetal lung and kidney, with weaker expression in fetal brain and liver. Lazar and Garcia (1999) also identified several isoforms that resulted from in-frame internal deletions and were widely expressed in adult and fetal tissues. The dominant isoform, which they designated MLCK2, contains a deletion of residues 437-505, causing loss of the tyrosine phosphorylation site and the SH2 binding site.

Watterson et al. (1999) cloned kinase-related protein from a genomic DNA library by PCR using primers based on a previously isolated human placenta KRP sequence. They noted that the human and chicken KRP proteins share 80% sequence identity. Northern blot analysis detected a 2.7-kb KRP transcript in all adult and fetal tissues examined, with highest expression in placenta, brain, heart, colon, small intestine, and fetal small intestine. Probing with a sequence common to the 3 main start site variants, Watterson et al. (1999) identified transcripts of 2.7 and 5.5 kb in heart, and of 2.7, 5.5, and 9.0 kb in placenta. By immunohistochemical analysis of adult and fetal heart sections, they found both full-length MLCK and the shorter KRP in cardiac muscle and in the smooth muscle layer of major blood vessels.

By immunohistochemistry in human embryonic specimens of small intestine and bladder, Halim et al. (2017) observed MYLK localized in smooth muscle cells of all muscular layers of the intestine and bladder, from weeks 9 to 22 of development.


Gene Structure

Watterson et al. (1999) noted that the various isoforms of MLCK are encoded by differential use of 31 coding exons. They also noted that KRP is derived from the last 3 exons spanning approximately 6.0 kb of the MLCK gene, and that the transcription initiation site for KRP lies within the intron preceding exon 29.


Mapping

By PCR and Southern blotting using 2 somatic cell hybrid panels, Potier et al. (1995) localized the MLCK gene to chromosome 3cen-q21.

By analysis of YAC clones, Giorgi et al. (2001) colocalized the MYLK gene with D3S3552 in a greater than 5-Mb region of chromosome 3q21. They confirmed the location of a pseudogene, MYLKP, to chromosome 3p13.


Gene Function

Walker et al. (2001) studied the KRP variant in the rabbit and demonstrated that recombinant rabbit telokin could relax telokin-depleted rabbit ileal smooth muscle in a dose-dependent manner. Mutation analysis revealed that ser13 is the phosphorylation site associated with cyclic nucleotide-induced Ca(2+)-independent relaxation of smooth muscle.

Goeckeler et al. (2000) found that human PAK2 (605022) phosphorylated MLCK on ser439 and ser991, which downregulated MLCK activity and inhibited MLCK-catalyzed phosphorylation of MYL2. Only ser439 of MLCK was phosphorylated by PAK2 in the presence of Ca(2+)/calmodulin. PAK2-catalyzed MLCK phosphorylation limited development of isometric tension in endothelial cells.


Molecular Genetics

Familial Thoracic Aortic Aneurysm 7

Wang et al. (2010) analyzed the MYLK gene in 193 probands from unrelated families in which 2 or more members had thoracic aortic aneurysms or dissections. They identified 2 heterozygous variants (600922.0001 and 600922.0002) that segregated with aortic dissections (AAT7; 613780) in 2 families, respectively, and were not found in 188 ethnically matched controls. Incomplete penetrance was observed in 1 of the families. Three additional MYLK variants were identified in 3 unrelated probands that were not detected in controls, but family members were not available for segregation analysis.

In a large Swedish family segregating autosomal dominant aortic dissection, Hannuksela et al. (2016) identified a 2-bp deletion in the MYLK gene causing a premature termination codon (S1091X) that was present in all 5 affected individuals as well as 9 unaffected family members. In addition, a family member with an intramural hematoma of the descending aorta did not carry the mutation.

Luyckx et al. (2017) screened a cohort of 358 cases of aortic aneurysmal disease for mutations in thoracic aortic aneurysm-associated genes and identified 2 probands who were heterozygous for nonsense mutations in the MYLK gene (Q1458X and R1487X). In the first family, the Q1458X mutation was also identified in the patient's unaffected father, and in the second family, the R1487X mutation was also present in the proband's unaffected half brother.

In a large consanguineous Arab family with thoracic aortic aneurysm and dissection, Shalata et al. (2018) identified a missense mutation in the MYLK gene (A1491S; 600922.0005) that was not found in ethnically matched controls or public variant databases. The mutation was present in homozygosity in 6 severely affected family members and in heterozygosity in 3 patients with a later age of onset; the mutation was also present in heterozygosity in 16 asymptomatic family members. The authors stated that the most appropriate designation for the mode of inheritance in this family was autosomal dominant with incomplete penetrance. Comprehensive medical history and clinical data analysis of homozygous individuals excluded congenital bladder or intestine involvement; there was thus no evidence for megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS; see below) in this family.

Megacystis-Microcolon-Intestinal Hypoperistalsis Syndrome 1

In 2 unrelated consanguineous families in which 5 children had died with megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS1; 249210), Halim et al. (2017) identified homozygosity for mutations in the MYLK gene: a 7-bp duplication (600922.0003) and a splicing variant (600922.0004), respectively. The unaffected parents in each family were heterozygous for the mutation. Noting that heterozygous mutations in MYLK had previously been associated with aortic aneurysm, the authors stated that they were not aware of any cardiac problems in the unaffected parents and suggested that other kinases might salvage smooth muscle contraction of extravisceral organs.


Animal Model

Wang et al. (2010) studied mice with smooth muscle cell-specific knockdown of Mylk and observed increased pools of proteoglycans in the aortic media compared to controls, along with increased expression of lumican (600616) and decorin (125255). Increased collagen staining in the adventitial layer and increased type III collagen (COL3A1; 120180) expression were also identified. In addition, expression of the elastin-degrading metalloproteinase MMP2 (120360) was also increased in the aortas of the mice, although elastic fibers were not degraded in the aortic media.


ALLELIC VARIANTS 5 Selected Examples):

.0001   AORTIC ANEURYSM, FAMILIAL THORACIC 7

MYLK, SER1759PRO
SNP: rs387906781, ClinVar: RCV000023044, RCV000603875

In 3 affected members of a family (family TAA026) with aortic aneurysm and dissection (AAT7; 613780), Wang et al. (2010) identified heterozygosity for a 5275T-C transition in the MYLK gene, resulting in a ser1759-to-pro (S1759P) substitution in the alpha-helix of the calmodulin-binding sequence that was predicted to cause loss of MLCK function by altering calmodulin binding. The mutation segregated with disease in the family and was not found in 188 ethnically matched controls. Transfection studies in COS-7 cells showed minimal endogenous expression of MLCK, and immunoprecipitation studies revealed that binding to calmodulin was abolished with the S1759P mutant. Analysis of kinase activity showed a 6-fold reduction for S1759P compared to wildtype. Wang et al. (2010) noted that affected individuals had acute aortic dissections with little to no aortic enlargement. Examination of ascending aortic tissue from 2 family members showed medial degeneration of the aorta and a significant increase in small arteries in the medial layer.


.0002   AORTIC ANEURYSM, FAMILIAL THORACIC 7

MYLK, ARG1480TER
SNP: rs387906782, ClinVar: RCV000023045, RCV001798012

In a 51-year-old father and his 18-year-old son (family TAA400) with aortic aneurysm and dissection (AAT7; 613780), Wang et al. (2010) identified heterozygosity for a 4438C-T transition in the MYLK gene, resulting in an arg1480-to-ter (R1480X) substitution that would lead to either nonsense-mediated decay or a truncated protein missing the kinase and calmodulin-binding domains, and was therefore predicted to disrupt kinase activity but not to disturb telokin expression. The father had undergone a type A dissection at 37 years of age, and the son had a type B dissection at 16 years of age. The R1480X mutation was also detected in 5 asymptomatic family members, with ages ranging from 59 years to 76 years. The mutation was not found in 188 ethnically matched controls.


.0003   MEGACYSTIS-MICROCOLON-INTESTINAL HYPOPERISTALSIS SYNDROME 1

MYLK, 7-BP DUP, 3838GAAAGCG
SNP: rs1553787823, ClinVar: RCV000508615, RCV001804175

In a deceased sister and brother of North African origin (family 1) with megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS1; 249210), Halim et al. (2017) identified homozygosity for a 7-bp duplication (c.3838_3844dupGAAAGCG, NM_053025.3) in exon 23 of the MYLK gene,, causing a frameshift predicted to result in a premature termination codon (Glu1282GlyfsTer51). The unaffected consanguineous parents were heterozygous for the mutation, whereas an unaffected younger sister did not carry the mutation; DNA was unavailable from an older sister, who died in utero with a distended bladder. Immunostained specimens of patient small intestine and bladder showed no MYLK signal. The structure and cellular constituents of the bladder and intestine showed no apparent pathologic abnormalities compared to age-matched control samples, suggesting that although MYLK is instrumental for proper functioning of smooth muscle cells, its presence is not required for maintaining the structural architecture of those organs.


.0004   MEGACYSTIS-MICROCOLON-INTESTINAL HYPOPERISTALSIS SYNDROME 1

MYLK, IVS23, C-A, +5
SNP: rs1553787619, ClinVar: RCV000508669, RCV001804176

In a girl of Indian origin (family 2) who died with megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS1; 249210), Halim et al. (2017) identified homozygosity for a splicing mutation (c.3985+5C-A, NM_053025.3) in intron 23 of the MYLK gene, for which her unaffected consanguineous parents were heterozygous. DNA was not available from a younger sister who also died with MMIHS, or from a younger brother who was unaffected. Splicing assays revealed that the mutant construct eliminated all transcription products observed with wildtype MYLK and produced a band with the same size as empty vector, indicating likely skipping of exon 23 with an early stop codon at the beginning of exon 24.


.0005   AORTIC ANEURYSM, FAMILIAL THORACIC 7

MYLK, ALA1491SER
SNP: rs1576422965, ClinVar: RCV000855690

In affected members of a large consanguineous Arab family with thoracic aortic aneurysm and dissection (AAT7; 613780), Shalata et al. (2018) identified a c.4471G-T transversion in exon 27 of the MYLK gene, resulting in an ala1491-to-ser (A1491S) substitution at a conserved residue within the kinase domain. Functional analysis in transfected HeLa cells showed a significant reduction in kinase activity with the A1491S mutant compared to wildtype MYLK. The mutation was not found in 100 ethnically matched controls or in the ExAC or 1000 Genomes Project databases. The mutation was present in homozygosity in 6 severely affected family members with early-onset disease, 5 of whom were deceased, and was found in heterozygosity in 3 individuals with a later age of onset, 1 of whom was deceased; the mutation was also present in heterozygosity in 16 asymptomatic family members. The authors stated that the most appropriate designation for the mode of inheritance in this family was autosomal dominant with incomplete penetrance. In addition, the authors noted that comprehensive medical history and clinical data analysis of homozygotes excluded congenital bladder or intestine involvement in this family.


REFERENCES

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Contributors:
Marla J. F. O'Neill - updated : 11/07/2019
Bao Lige - updated : 04/18/2019
Marla J. F. O'Neill - updated : 2/24/2011
Patricia A. Hartz - updated : 5/24/2002
Joanna S. Amberger - updated : 6/22/2001

Creation Date:
Victor A. McKusick : 11/7/1995

Edit History:
carol : 09/29/2023
carol : 05/27/2021
carol : 03/17/2020
carol : 12/23/2019
alopez : 11/07/2019
mgross : 04/18/2019
carol : 12/19/2013
wwang : 4/28/2011
wwang : 2/28/2011
terry : 2/24/2011
wwang : 12/20/2005
carol : 5/30/2002
carol : 5/29/2002
carol : 5/29/2002
terry : 5/24/2002
mcapotos : 6/25/2001
joanna : 6/22/2001
alopez : 4/30/1999
alopez : 3/26/1999
jenny : 4/4/1997
mark : 11/9/1995
terry : 11/7/1995